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554 lines
27 KiB
Plaintext
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This describes the "RSA-based mutable files" which were shipped in Tahoe v0.8.0.
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= Mutable Files =
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Mutable File Slots are places with a stable identifier that can hold data
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that changes over time. In contrast to CHK slots, for which the
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URI/identifier is derived from the contents themselves, the Mutable File Slot
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URI remains fixed for the life of the slot, regardless of what data is placed
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inside it.
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Each mutable slot is referenced by two different URIs. The "read-write" URI
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grants read-write access to its holder, allowing them to put whatever
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contents they like into the slot. The "read-only" URI is less powerful, only
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granting read access, and not enabling modification of the data. The
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read-write URI can be turned into the read-only URI, but not the other way
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around.
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The data in these slots is distributed over a number of servers, using the
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same erasure coding that CHK files use, with 3-of-10 being a typical choice
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of encoding parameters. The data is encrypted and signed in such a way that
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only the holders of the read-write URI will be able to set the contents of
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the slot, and only the holders of the read-only URI will be able to read
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those contents. Holders of either URI will be able to validate the contents
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as being written by someone with the read-write URI. The servers who hold the
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shares cannot read or modify them: the worst they can do is deny service (by
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deleting or corrupting the shares), or attempt a rollback attack (which can
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only succeed with the cooperation of at least k servers).
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== Consistency vs Availability ==
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There is an age-old battle between consistency and availability. Epic papers
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have been written, elaborate proofs have been established, and generations of
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theorists have learned that you cannot simultaneously achieve guaranteed
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consistency with guaranteed reliability. In addition, the closer to 0 you get
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on either axis, the cost and complexity of the design goes up.
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Tahoe's design goals are to largely favor design simplicity, then slightly
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favor read availability, over the other criteria.
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As we develop more sophisticated mutable slots, the API may expose multiple
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read versions to the application layer. The tahoe philosophy is to defer most
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consistency recovery logic to the higher layers. Some applications have
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effective ways to merge multiple versions, so inconsistency is not
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necessarily a problem (i.e. directory nodes can usually merge multiple "add
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child" operations).
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== The Prime Coordination Directive: "Don't Do That" ==
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The current rule for applications which run on top of Tahoe is "do not
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perform simultaneous uncoordinated writes". That means you need non-tahoe
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means to make sure that two parties are not trying to modify the same mutable
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slot at the same time. For example:
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* don't give the read-write URI to anyone else. Dirnodes in a private
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directory generally satisfy this case, as long as you don't use two
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clients on the same account at the same time
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* if you give a read-write URI to someone else, stop using it yourself. An
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inbox would be a good example of this.
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* if you give a read-write URI to someone else, call them on the phone
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before you write into it
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* build an automated mechanism to have your agents coordinate writes.
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For example, we expect a future release to include a FURL for a
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"coordination server" in the dirnodes. The rule can be that you must
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contact the coordination server and obtain a lock/lease on the file
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before you're allowed to modify it.
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If you do not follow this rule, Bad Things will happen. The worst-case Bad
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Thing is that the entire file will be lost. A less-bad Bad Thing is that one
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or more of the simultaneous writers will lose their changes. An observer of
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the file may not see monotonically-increasing changes to the file, i.e. they
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may see version 1, then version 2, then 3, then 2 again.
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Tahoe takes some amount of care to reduce the badness of these Bad Things.
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One way you can help nudge it from the "lose your file" case into the "lose
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some changes" case is to reduce the number of competing versions: multiple
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versions of the file that different parties are trying to establish as the
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one true current contents. Each simultaneous writer counts as a "competing
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version", as does the previous version of the file. If the count "S" of these
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competing versions is larger than N/k, then the file runs the risk of being
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lost completely. If at least one of the writers remains running after the
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collision is detected, it will attempt to recover, but if S>(N/k) and all
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writers crash after writing a few shares, the file will be lost.
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== Small Distributed Mutable Files ==
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SDMF slots are suitable for small (<1MB) files that are editing by rewriting
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the entire file. The three operations are:
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* allocate (with initial contents)
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* set (with new contents)
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* get (old contents)
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The first use of SDMF slots will be to hold directories (dirnodes), which map
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encrypted child names to rw-URI/ro-URI pairs.
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=== SDMF slots overview ===
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Each SDMF slot is created with a public/private key pair. The public key is
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known as the "verification key", while the private key is called the
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"signature key". The private key is hashed and truncated to 16 bytes to form
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the "write key" (an AES symmetric key). The write key is then hashed and
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truncated to form the "read key". The read key is hashed and truncated to
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form the 16-byte "storage index" (a unique string used as an index to locate
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stored data).
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The public key is hashed by itself to form the "verification key hash".
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The write key is hashed a different way to form the "write enabler master".
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For each storage server on which a share is kept, the write enabler master is
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concatenated with the server's nodeid and hashed, and the result is called
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the "write enabler" for that particular server. Note that multiple shares of
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the same slot stored on the same server will all get the same write enabler,
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i.e. the write enabler is associated with the "bucket", rather than the
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individual shares.
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The private key is encrypted (using AES in counter mode) by the write key,
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and the resulting crypttext is stored on the servers. so it will be
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retrievable by anyone who knows the write key. The write key is not used to
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encrypt anything else, and the private key never changes, so we do not need
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an IV for this purpose.
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The actual data is encrypted (using AES in counter mode) with a key derived
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by concatenating the readkey with the IV, the hashing the results and
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truncating to 16 bytes. The IV is randomly generated each time the slot is
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updated, and stored next to the encrypted data.
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The read-write URI consists of the write key and the verification key hash.
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The read-only URI contains the read key and the verification key hash. The
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verify-only URI contains the storage index and the verification key hash.
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URI:SSK-RW:b2a(writekey):b2a(verification_key_hash)
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URI:SSK-RO:b2a(readkey):b2a(verification_key_hash)
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URI:SSK-Verify:b2a(storage_index):b2a(verification_key_hash)
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Note that this allows the read-only and verify-only URIs to be derived from
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the read-write URI without actually retrieving the public keys. Also note
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that it means the read-write agent must validate both the private key and the
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public key when they are first fetched. All users validate the public key in
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exactly the same way.
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The SDMF slot is allocated by sending a request to the storage server with a
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desired size, the storage index, and the write enabler for that server's
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nodeid. If granted, the write enabler is stashed inside the slot's backing
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store file. All further write requests must be accompanied by the write
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enabler or they will not be honored. The storage server does not share the
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write enabler with anyone else.
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The SDMF slot structure will be described in more detail below. The important
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pieces are:
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* a sequence number
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* a root hash "R"
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* the encoding parameters (including k, N, file size, segment size)
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* a signed copy of [seqnum,R,encoding_params], using the signature key
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* the verification key (not encrypted)
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* the share hash chain (part of a Merkle tree over the share hashes)
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* the block hash tree (Merkle tree over blocks of share data)
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* the share data itself (erasure-coding of read-key-encrypted file data)
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* the signature key, encrypted with the write key
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The access pattern for read is:
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* hash read-key to get storage index
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* use storage index to locate 'k' shares with identical 'R' values
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* either get one share, read 'k' from it, then read k-1 shares
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* or read, say, 5 shares, discover k, either get more or be finished
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* or copy k into the URIs
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* read verification key
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* hash verification key, compare against verification key hash
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* read seqnum, R, encoding parameters, signature
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* verify signature against verification key
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* read share data, compute block-hash Merkle tree and root "r"
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* read share hash chain (leading from "r" to "R")
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* validate share hash chain up to the root "R"
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* submit share data to erasure decoding
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* decrypt decoded data with read-key
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* submit plaintext to application
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The access pattern for write is:
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* hash write-key to get read-key, hash read-key to get storage index
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* use the storage index to locate at least one share
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* read verification key and encrypted signature key
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* decrypt signature key using write-key
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* hash signature key, compare against write-key
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* hash verification key, compare against verification key hash
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* encrypt plaintext from application with read-key
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* application can encrypt some data with the write-key to make it only
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available to writers (use this for transitive read-onlyness of dirnodes)
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* erasure-code crypttext to form shares
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* split shares into blocks
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* compute Merkle tree of blocks, giving root "r" for each share
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* compute Merkle tree of shares, find root "R" for the file as a whole
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* create share data structures, one per server:
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* use seqnum which is one higher than the old version
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* share hash chain has log(N) hashes, different for each server
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* signed data is the same for each server
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* now we have N shares and need homes for them
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* walk through peers
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* if share is not already present, allocate-and-set
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* otherwise, try to modify existing share:
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* send testv_and_writev operation to each one
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* testv says to accept share if their(seqnum+R) <= our(seqnum+R)
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* count how many servers wind up with which versions (histogram over R)
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* keep going until N servers have the same version, or we run out of servers
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* if any servers wound up with a different version, report error to
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application
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* if we ran out of servers, initiate recovery process (described below)
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=== Server Storage Protocol ===
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The storage servers will provide a mutable slot container which is oblivious
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to the details of the data being contained inside it. Each storage index
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refers to a "bucket", and each bucket has one or more shares inside it. (In a
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well-provisioned network, each bucket will have only one share). The bucket
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is stored as a directory, using the base32-encoded storage index as the
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directory name. Each share is stored in a single file, using the share number
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as the filename.
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The container holds space for a container magic number (for versioning), the
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write enabler, the nodeid which accepted the write enabler (used for share
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migration, described below), a small number of lease structures, the embedded
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data itself, and expansion space for additional lease structures.
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# offset size name
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1 0 32 magic verstr "tahoe mutable container v1" plus binary
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2 32 20 write enabler's nodeid
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3 52 32 write enabler
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4 84 8 data size (actual share data present) (a)
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5 92 8 offset of (8) count of extra leases (after data)
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6 100 368 four leases, 92 bytes each
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0 4 ownerid (0 means "no lease here")
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4 4 expiration timestamp
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8 32 renewal token
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40 32 cancel token
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72 20 nodeid which accepted the tokens
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7 468 (a) data
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8 ?? 4 count of extra leases
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9 ?? n*92 extra leases
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The "extra leases" field must be copied and rewritten each time the size of
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the enclosed data changes. The hope is that most buckets will have four or
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fewer leases and this extra copying will not usually be necessary.
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The (4) "data size" field contains the actual number of bytes of data present
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in field (7), such that a client request to read beyond 504+(a) will result
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in an error. This allows the client to (one day) read relative to the end of
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the file. The container size (that is, (8)-(7)) might be larger, especially
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if extra size was pre-allocated in anticipation of filling the container with
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a lot of data.
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The offset in (5) points at the *count* of extra leases, at (8). The actual
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leases (at (9)) begin 4 bytes later. If the container size changes, both (8)
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and (9) must be relocated by copying.
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The server will honor any write commands that provide the write token and do
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not exceed the server-wide storage size limitations. Read and write commands
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MUST be restricted to the 'data' portion of the container: the implementation
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of those commands MUST perform correct bounds-checking to make sure other
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portions of the container are inaccessible to the clients.
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The two methods provided by the storage server on these "MutableSlot" share
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objects are:
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* readv(ListOf(offset=int, length=int))
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* returns a list of bytestrings, of the various requested lengths
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* offset < 0 is interpreted relative to the end of the data
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* spans which hit the end of the data will return truncated data
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* testv_and_writev(write_enabler, test_vector, write_vector)
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* this is a test-and-set operation which performs the given tests and only
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applies the desired writes if all tests succeed. This is used to detect
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simultaneous writers, and to reduce the chance that an update will lose
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data recently written by some other party (written after the last time
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this slot was read).
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* test_vector=ListOf(TupleOf(offset, length, opcode, specimen))
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* the opcode is a string, from the set [gt, ge, eq, le, lt, ne]
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* each element of the test vector is read from the slot's data and
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compared against the specimen using the desired (in)equality. If all
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tests evaluate True, the write is performed
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* write_vector=ListOf(TupleOf(offset, newdata))
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* offset < 0 is not yet defined, it probably means relative to the
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end of the data, which probably means append, but we haven't nailed
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it down quite yet
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* write vectors are executed in order, which specifies the results of
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overlapping writes
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* return value:
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* error: OutOfSpace
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* error: something else (io error, out of memory, whatever)
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* (True, old_test_data): the write was accepted (test_vector passed)
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* (False, old_test_data): the write was rejected (test_vector failed)
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* both 'accepted' and 'rejected' return the old data that was used
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for the test_vector comparison. This can be used by the client
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to detect write collisions, including collisions for which the
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desired behavior was to overwrite the old version.
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In addition, the storage server provides several methods to access these
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share objects:
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* allocate_mutable_slot(storage_index, sharenums=SetOf(int))
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* returns DictOf(int, MutableSlot)
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* get_mutable_slot(storage_index)
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* returns DictOf(int, MutableSlot)
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* or raises KeyError
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We intend to add an interface which allows small slots to allocate-and-write
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in a single call, as well as do update or read in a single call. The goal is
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to allow a reasonably-sized dirnode to be created (or updated, or read) in
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just one round trip (to all N shareholders in parallel).
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==== migrating shares ====
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If a share must be migrated from one server to another, two values become
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invalid: the write enabler (since it was computed for the old server), and
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the lease renew/cancel tokens.
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Suppose that a slot was first created on nodeA, and was thus initialized with
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WE(nodeA) (= H(WEM+nodeA)). Later, for provisioning reasons, the share is
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moved from nodeA to nodeB.
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Readers may still be able to find the share in its new home, depending upon
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how many servers are present in the grid, where the new nodeid lands in the
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permuted index for this particular storage index, and how many servers the
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reading client is willing to contact.
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When a client attempts to write to this migrated share, it will get a "bad
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write enabler" error, since the WE it computes for nodeB will not match the
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WE(nodeA) that was embedded in the share. When this occurs, the "bad write
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enabler" message must include the old nodeid (e.g. nodeA) that was in the
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share.
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The client then computes H(nodeB+H(WEM+nodeA)), which is the same as
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H(nodeB+WE(nodeA)). The client sends this along with the new WE(nodeB), which
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is H(WEM+nodeB). Note that the client only sends WE(nodeB) to nodeB, never to
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anyone else. Also note that the client does not send a value to nodeB that
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would allow the node to impersonate the client to a third node: everything
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sent to nodeB will include something specific to nodeB in it.
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The server locally computes H(nodeB+WE(nodeA)), using its own node id and the
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old write enabler from the share. It compares this against the value supplied
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by the client. If they match, this serves as proof that the client was able
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to compute the old write enabler. The server then accepts the client's new
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WE(nodeB) and writes it into the container.
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This WE-fixup process requires an extra round trip, and requires the error
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message to include the old nodeid, but does not require any public key
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operations on either client or server.
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Migrating the leases will require a similar protocol. This protocol will be
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defined concretely at a later date.
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=== Code Details ===
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The current FileNode class will be renamed ImmutableFileNode, and a new
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MutableFileNode class will be created. Instances of this class will contain a
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URI and a reference to the client (for peer selection and connection). The
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methods of MutableFileNode are:
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* replace(newdata) -> OK, ConsistencyError, NotEnoughPeersError
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* get() -> [deferred] newdata, NotEnoughPeersError
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* if there are multiple retrieveable versions in the grid, get() returns
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the first version it can reconstruct, and silently ignores the others.
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In the future, a more advanced API will signal and provide access to
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the multiple heads.
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The peer-selection and data-structure manipulation (and signing/verification)
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steps will be implemented in a separate class in allmydata/mutable.py .
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=== SMDF Slot Format ===
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This SMDF data lives inside a server-side MutableSlot container. The server
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is oblivious to this format.
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This data is tightly packed. In particular, the share data is defined to run
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all the way to the beginning of the encrypted private key (the encprivkey
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offset is used both to terminate the share data and to begin the encprivkey).
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# offset size name
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1 0 1 version byte, \x00 for this format
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2 1 8 sequence number. 2^64-1 must be handled specially, TBD
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3 9 32 "R" (root of share hash Merkle tree)
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4 41 16 IV (share data is AES(H(readkey+IV)) )
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5 57 18 encoding parameters:
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57 1 k
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58 1 N
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59 8 segment size
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67 8 data length (of original plaintext)
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6 75 32 offset table:
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75 4 (8) signature
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79 4 (9) share hash chain
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83 4 (10) block hash tree
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87 4 (11) share data
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91 8 (12) encrypted private key
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99 8 (13) EOF
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7 107 436ish verification key (2048 RSA key)
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8 543ish 256ish signature=RSAenc(sigkey, H(version+seqnum+r+IV+encparm))
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9 799ish (a) share hash chain, encoded as:
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"".join([pack(">H32s", shnum, hash)
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for (shnum,hash) in needed_hashes])
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10 (927ish) (b) block hash tree, encoded as:
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"".join([pack(">32s",hash) for hash in block_hash_tree])
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11 (935ish) LEN share data (no gap between this and encprivkey)
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12 ?? 1216ish encrypted private key= AESenc(write-key, RSA-key)
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13 ?? -- EOF
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(a) The share hash chain contains ceil(log(N)) hashes, each 32 bytes long.
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This is the set of hashes necessary to validate this share's leaf in the
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share Merkle tree. For N=10, this is 4 hashes, i.e. 128 bytes.
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(b) The block hash tree contains ceil(length/segsize) hashes, each 32 bytes
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long. This is the set of hashes necessary to validate any given block of
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share data up to the per-share root "r". Each "r" is a leaf of the share
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has tree (with root "R"), from which a minimal subset of hashes is put in
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the share hash chain in (8).
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=== Recovery ===
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The first line of defense against damage caused by colliding writes is the
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Prime Coordination Directive: "Don't Do That".
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The second line of defense is to keep "S" (the number of competing versions)
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lower than N/k. If this holds true, at least one competing version will have
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k shares and thus be recoverable. Note that server unavailability counts
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against us here: the old version stored on the unavailable server must be
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included in the value of S.
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|
The third line of defense is our use of testv_and_writev() (described below),
|
|
which increases the convergence of simultaneous writes: one of the writers
|
|
will be favored (the one with the highest "R"), and that version is more
|
|
likely to be accepted than the others. This defense is least effective in the
|
|
pathological situation where S simultaneous writers are active, the one with
|
|
the lowest "R" writes to N-k+1 of the shares and then dies, then the one with
|
|
the next-lowest "R" writes to N-2k+1 of the shares and dies, etc, until the
|
|
one with the highest "R" writes to k-1 shares and dies. Any other sequencing
|
|
will allow the highest "R" to write to at least k shares and establish a new
|
|
revision.
|
|
|
|
The fourth line of defense is the fact that each client keeps writing until
|
|
at least one version has N shares. This uses additional servers, if
|
|
necessary, to make sure that either the client's version or some
|
|
newer/overriding version is highly available.
|
|
|
|
The fifth line of defense is the recovery algorithm, which seeks to make sure
|
|
that at least *one* version is highly available, even if that version is
|
|
somebody else's.
|
|
|
|
The write-shares-to-peers algorithm is as follows:
|
|
|
|
* permute peers according to storage index
|
|
* walk through peers, trying to assign one share per peer
|
|
* for each peer:
|
|
* send testv_and_writev, using "old(seqnum+R) <= our(seqnum+R)" as the test
|
|
* this means that we will overwrite any old versions, and we will
|
|
overwrite simultaenous writers of the same version if our R is higher.
|
|
We will not overwrite writers using a higher seqnum.
|
|
* record the version that each share winds up with. If the write was
|
|
accepted, this is our own version. If it was rejected, read the
|
|
old_test_data to find out what version was retained.
|
|
* if old_test_data indicates the seqnum was equal or greater than our
|
|
own, mark the "Simultanous Writes Detected" flag, which will eventually
|
|
result in an error being reported to the writer (in their close() call).
|
|
* build a histogram of "R" values
|
|
* repeat until the histogram indicate that some version (possibly ours)
|
|
has N shares. Use new servers if necessary.
|
|
* If we run out of servers:
|
|
* if there are at least shares-of-happiness of any one version, we're
|
|
happy, so return. (the close() might still get an error)
|
|
* not happy, need to reinforce something, goto RECOVERY
|
|
|
|
RECOVERY:
|
|
* read all shares, count the versions, identify the recoverable ones,
|
|
discard the unrecoverable ones.
|
|
* sort versions: locate max(seqnums), put all versions with that seqnum
|
|
in the list, sort by number of outstanding shares. Then put our own
|
|
version. (TODO: put versions with seqnum <max but >us ahead of us?).
|
|
* for each version:
|
|
* attempt to recover that version
|
|
* if not possible, remove it from the list, go to next one
|
|
* if recovered, start at beginning of peer list, push that version,
|
|
continue until N shares are placed
|
|
* if pushing our own version, bump up the seqnum to one higher than
|
|
the max seqnum we saw
|
|
* if we run out of servers:
|
|
* schedule retry and exponential backoff to repeat RECOVERY
|
|
* admit defeat after some period? presumeably the client will be shut down
|
|
eventually, maybe keep trying (once per hour?) until then.
|
|
|
|
|
|
|
|
|
|
== Medium Distributed Mutable Files ==
|
|
|
|
These are just like the SDMF case, but:
|
|
|
|
* we actually take advantage of the Merkle hash tree over the blocks, by
|
|
reading a single segment of data at a time (and its necessary hashes), to
|
|
reduce the read-time alacrity
|
|
* we allow arbitrary writes to the file (i.e. seek() is provided, and
|
|
O_TRUNC is no longer required)
|
|
* we write more code on the client side (in the MutableFileNode class), to
|
|
first read each segment that a write must modify. This looks exactly like
|
|
the way a normal filesystem uses a block device, or how a CPU must perform
|
|
a cache-line fill before modifying a single word.
|
|
* we might implement some sort of copy-based atomic update server call,
|
|
to allow multiple writev() calls to appear atomic to any readers.
|
|
|
|
MDMF slots provide fairly efficient in-place edits of very large files (a few
|
|
GB). Appending data is also fairly efficient, although each time a power of 2
|
|
boundary is crossed, the entire file must effectively be re-uploaded (because
|
|
the size of the block hash tree changes), so if the filesize is known in
|
|
advance, that space ought to be pre-allocated (by leaving extra space between
|
|
the block hash tree and the actual data).
|
|
|
|
MDMF1 uses the Merkle tree to enable low-alacrity random-access reads. MDMF2
|
|
adds cache-line reads to allow random-access writes.
|
|
|
|
== Large Distributed Mutable Files ==
|
|
|
|
LDMF slots use a fundamentally different way to store the file, inspired by
|
|
Mercurial's "revlog" format. They enable very efficient insert/remove/replace
|
|
editing of arbitrary spans. Multiple versions of the file can be retained, in
|
|
a revision graph that can have multiple heads. Each revision can be
|
|
referenced by a cryptographic identifier. There are two forms of the URI, one
|
|
that means "most recent version", and a longer one that points to a specific
|
|
revision.
|
|
|
|
Metadata can be attached to the revisions, like timestamps, to enable rolling
|
|
back an entire tree to a specific point in history.
|
|
|
|
LDMF1 provides deltas but tries to avoid dealing with multiple heads. LDMF2
|
|
provides explicit support for revision identifiers and branching.
|
|
|
|
== TODO ==
|
|
|
|
improve allocate-and-write or get-writer-buckets API to allow one-call (or
|
|
maybe two-call) updates. The challenge is in figuring out which shares are on
|
|
which machines. First cut will have lots of round trips.
|
|
|
|
(eventually) define behavior when seqnum wraps. At the very least make sure
|
|
it can't cause a security problem. "the slot is worn out" is acceptable.
|
|
|
|
(eventually) define share-migration lease update protocol. Including the
|
|
nodeid who accepted the lease is useful, we can use the same protocol as we
|
|
do for updating the write enabler. However we need to know which lease to
|
|
update.. maybe send back a list of all old nodeids that we find, then try all
|
|
of them when we accept the update?
|
|
|
|
We now do this in a specially-formatted IndexError exception:
|
|
"UNABLE to renew non-existent lease. I have leases accepted by " +
|
|
"nodeids: '12345','abcde','44221' ."
|
|
|
|
confirm that a repairer can regenerate shares without the private key. Hmm,
|
|
without the write-enabler they won't be able to write those shares to the
|
|
servers.. although they could add immutable new shares to new servers.
|