mirror of
https://github.com/tahoe-lafs/tahoe-lafs.git
synced 2024-12-23 14:52:26 +00:00
649 lines
32 KiB
Plaintext
649 lines
32 KiB
Plaintext
|
|
This describes the "RSA-based mutable files" which were shipped in Tahoe v0.8.0.
|
|
|
|
= Mutable Files =
|
|
|
|
Mutable File Slots are places with a stable identifier that can hold data
|
|
that changes over time. In contrast to CHK slots, for which the
|
|
URI/identifier is derived from the contents themselves, the Mutable File Slot
|
|
URI remains fixed for the life of the slot, regardless of what data is placed
|
|
inside it.
|
|
|
|
Each mutable slot is referenced by two different URIs. The "read-write" URI
|
|
grants read-write access to its holder, allowing them to put whatever
|
|
contents they like into the slot. The "read-only" URI is less powerful, only
|
|
granting read access, and not enabling modification of the data. The
|
|
read-write URI can be turned into the read-only URI, but not the other way
|
|
around.
|
|
|
|
The data in these slots is distributed over a number of servers, using the
|
|
same erasure coding that CHK files use, with 3-of-10 being a typical choice
|
|
of encoding parameters. The data is encrypted and signed in such a way that
|
|
only the holders of the read-write URI will be able to set the contents of
|
|
the slot, and only the holders of the read-only URI will be able to read
|
|
those contents. Holders of either URI will be able to validate the contents
|
|
as being written by someone with the read-write URI. The servers who hold the
|
|
shares cannot read or modify them: the worst they can do is deny service (by
|
|
deleting or corrupting the shares), or attempt a rollback attack (which can
|
|
only succeed with the cooperation of at least k servers).
|
|
|
|
== Consistency vs Availability ==
|
|
|
|
There is an age-old battle between consistency and availability. Epic papers
|
|
have been written, elaborate proofs have been established, and generations of
|
|
theorists have learned that you cannot simultaneously achieve guaranteed
|
|
consistency with guaranteed reliability. In addition, the closer to 0 you get
|
|
on either axis, the cost and complexity of the design goes up.
|
|
|
|
Tahoe's design goals are to largely favor design simplicity, then slightly
|
|
favor read availability, over the other criteria.
|
|
|
|
As we develop more sophisticated mutable slots, the API may expose multiple
|
|
read versions to the application layer. The tahoe philosophy is to defer most
|
|
consistency recovery logic to the higher layers. Some applications have
|
|
effective ways to merge multiple versions, so inconsistency is not
|
|
necessarily a problem (i.e. directory nodes can usually merge multiple "add
|
|
child" operations).
|
|
|
|
== The Prime Coordination Directive: "Don't Do That" ==
|
|
|
|
The current rule for applications which run on top of Tahoe is "do not
|
|
perform simultaneous uncoordinated writes". That means you need non-tahoe
|
|
means to make sure that two parties are not trying to modify the same mutable
|
|
slot at the same time. For example:
|
|
|
|
* don't give the read-write URI to anyone else. Dirnodes in a private
|
|
directory generally satisfy this case, as long as you don't use two
|
|
clients on the same account at the same time
|
|
* if you give a read-write URI to someone else, stop using it yourself. An
|
|
inbox would be a good example of this.
|
|
* if you give a read-write URI to someone else, call them on the phone
|
|
before you write into it
|
|
* build an automated mechanism to have your agents coordinate writes.
|
|
For example, we expect a future release to include a FURL for a
|
|
"coordination server" in the dirnodes. The rule can be that you must
|
|
contact the coordination server and obtain a lock/lease on the file
|
|
before you're allowed to modify it.
|
|
|
|
If you do not follow this rule, Bad Things will happen. The worst-case Bad
|
|
Thing is that the entire file will be lost. A less-bad Bad Thing is that one
|
|
or more of the simultaneous writers will lose their changes. An observer of
|
|
the file may not see monotonically-increasing changes to the file, i.e. they
|
|
may see version 1, then version 2, then 3, then 2 again.
|
|
|
|
Tahoe takes some amount of care to reduce the badness of these Bad Things.
|
|
One way you can help nudge it from the "lose your file" case into the "lose
|
|
some changes" case is to reduce the number of competing versions: multiple
|
|
versions of the file that different parties are trying to establish as the
|
|
one true current contents. Each simultaneous writer counts as a "competing
|
|
version", as does the previous version of the file. If the count "S" of these
|
|
competing versions is larger than N/k, then the file runs the risk of being
|
|
lost completely. [TODO] If at least one of the writers remains running after
|
|
the collision is detected, it will attempt to recover, but if S>(N/k) and all
|
|
writers crash after writing a few shares, the file will be lost.
|
|
|
|
Note that Tahoe uses serialization internally to make sure that a single
|
|
Tahoe node will not perform simultaneous modifications to a mutable file. It
|
|
accomplishes this by using a weakref cache of the MutableFileNode (so that
|
|
there will never be two distinct MutableFileNodes for the same file), and by
|
|
forcing all mutable file operations to obtain a per-node lock before they
|
|
run. The Prime Coordination Directive therefore applies to inter-node
|
|
conflicts, not intra-node ones.
|
|
|
|
|
|
== Small Distributed Mutable Files ==
|
|
|
|
SDMF slots are suitable for small (<1MB) files that are editing by rewriting
|
|
the entire file. The three operations are:
|
|
|
|
* allocate (with initial contents)
|
|
* set (with new contents)
|
|
* get (old contents)
|
|
|
|
The first use of SDMF slots will be to hold directories (dirnodes), which map
|
|
encrypted child names to rw-URI/ro-URI pairs.
|
|
|
|
=== SDMF slots overview ===
|
|
|
|
Each SDMF slot is created with a public/private key pair. The public key is
|
|
known as the "verification key", while the private key is called the
|
|
"signature key". The private key is hashed and truncated to 16 bytes to form
|
|
the "write key" (an AES symmetric key). The write key is then hashed and
|
|
truncated to form the "read key". The read key is hashed and truncated to
|
|
form the 16-byte "storage index" (a unique string used as an index to locate
|
|
stored data).
|
|
|
|
The public key is hashed by itself to form the "verification key hash".
|
|
|
|
The write key is hashed a different way to form the "write enabler master".
|
|
For each storage server on which a share is kept, the write enabler master is
|
|
concatenated with the server's nodeid and hashed, and the result is called
|
|
the "write enabler" for that particular server. Note that multiple shares of
|
|
the same slot stored on the same server will all get the same write enabler,
|
|
i.e. the write enabler is associated with the "bucket", rather than the
|
|
individual shares.
|
|
|
|
The private key is encrypted (using AES in counter mode) by the write key,
|
|
and the resulting crypttext is stored on the servers. so it will be
|
|
retrievable by anyone who knows the write key. The write key is not used to
|
|
encrypt anything else, and the private key never changes, so we do not need
|
|
an IV for this purpose.
|
|
|
|
The actual data is encrypted (using AES in counter mode) with a key derived
|
|
by concatenating the readkey with the IV, the hashing the results and
|
|
truncating to 16 bytes. The IV is randomly generated each time the slot is
|
|
updated, and stored next to the encrypted data.
|
|
|
|
The read-write URI consists of the write key and the verification key hash.
|
|
The read-only URI contains the read key and the verification key hash. The
|
|
verify-only URI contains the storage index and the verification key hash.
|
|
|
|
URI:SSK-RW:b2a(writekey):b2a(verification_key_hash)
|
|
URI:SSK-RO:b2a(readkey):b2a(verification_key_hash)
|
|
URI:SSK-Verify:b2a(storage_index):b2a(verification_key_hash)
|
|
|
|
Note that this allows the read-only and verify-only URIs to be derived from
|
|
the read-write URI without actually retrieving the public keys. Also note
|
|
that it means the read-write agent must validate both the private key and the
|
|
public key when they are first fetched. All users validate the public key in
|
|
exactly the same way.
|
|
|
|
The SDMF slot is allocated by sending a request to the storage server with a
|
|
desired size, the storage index, and the write enabler for that server's
|
|
nodeid. If granted, the write enabler is stashed inside the slot's backing
|
|
store file. All further write requests must be accompanied by the write
|
|
enabler or they will not be honored. The storage server does not share the
|
|
write enabler with anyone else.
|
|
|
|
The SDMF slot structure will be described in more detail below. The important
|
|
pieces are:
|
|
|
|
* a sequence number
|
|
* a root hash "R"
|
|
* the encoding parameters (including k, N, file size, segment size)
|
|
* a signed copy of [seqnum,R,encoding_params], using the signature key
|
|
* the verification key (not encrypted)
|
|
* the share hash chain (part of a Merkle tree over the share hashes)
|
|
* the block hash tree (Merkle tree over blocks of share data)
|
|
* the share data itself (erasure-coding of read-key-encrypted file data)
|
|
* the signature key, encrypted with the write key
|
|
|
|
The access pattern for read is:
|
|
* hash read-key to get storage index
|
|
* use storage index to locate 'k' shares with identical 'R' values
|
|
* either get one share, read 'k' from it, then read k-1 shares
|
|
* or read, say, 5 shares, discover k, either get more or be finished
|
|
* or copy k into the URIs
|
|
* read verification key
|
|
* hash verification key, compare against verification key hash
|
|
* read seqnum, R, encoding parameters, signature
|
|
* verify signature against verification key
|
|
* read share data, compute block-hash Merkle tree and root "r"
|
|
* read share hash chain (leading from "r" to "R")
|
|
* validate share hash chain up to the root "R"
|
|
* submit share data to erasure decoding
|
|
* decrypt decoded data with read-key
|
|
* submit plaintext to application
|
|
|
|
The access pattern for write is:
|
|
* hash write-key to get read-key, hash read-key to get storage index
|
|
* use the storage index to locate at least one share
|
|
* read verification key and encrypted signature key
|
|
* decrypt signature key using write-key
|
|
* hash signature key, compare against write-key
|
|
* hash verification key, compare against verification key hash
|
|
* encrypt plaintext from application with read-key
|
|
* application can encrypt some data with the write-key to make it only
|
|
available to writers (use this for transitive read-onlyness of dirnodes)
|
|
* erasure-code crypttext to form shares
|
|
* split shares into blocks
|
|
* compute Merkle tree of blocks, giving root "r" for each share
|
|
* compute Merkle tree of shares, find root "R" for the file as a whole
|
|
* create share data structures, one per server:
|
|
* use seqnum which is one higher than the old version
|
|
* share hash chain has log(N) hashes, different for each server
|
|
* signed data is the same for each server
|
|
* now we have N shares and need homes for them
|
|
* walk through peers
|
|
* if share is not already present, allocate-and-set
|
|
* otherwise, try to modify existing share:
|
|
* send testv_and_writev operation to each one
|
|
* testv says to accept share if their(seqnum+R) <= our(seqnum+R)
|
|
* count how many servers wind up with which versions (histogram over R)
|
|
* keep going until N servers have the same version, or we run out of servers
|
|
* if any servers wound up with a different version, report error to
|
|
application
|
|
* if we ran out of servers, initiate recovery process (described below)
|
|
|
|
=== Server Storage Protocol ===
|
|
|
|
The storage servers will provide a mutable slot container which is oblivious
|
|
to the details of the data being contained inside it. Each storage index
|
|
refers to a "bucket", and each bucket has one or more shares inside it. (In a
|
|
well-provisioned network, each bucket will have only one share). The bucket
|
|
is stored as a directory, using the base32-encoded storage index as the
|
|
directory name. Each share is stored in a single file, using the share number
|
|
as the filename.
|
|
|
|
The container holds space for a container magic number (for versioning), the
|
|
write enabler, the nodeid which accepted the write enabler (used for share
|
|
migration, described below), a small number of lease structures, the embedded
|
|
data itself, and expansion space for additional lease structures.
|
|
|
|
# offset size name
|
|
1 0 32 magic verstr "tahoe mutable container v1" plus binary
|
|
2 32 20 write enabler's nodeid
|
|
3 52 32 write enabler
|
|
4 84 8 data size (actual share data present) (a)
|
|
5 92 8 offset of (8) count of extra leases (after data)
|
|
6 100 368 four leases, 92 bytes each
|
|
0 4 ownerid (0 means "no lease here")
|
|
4 4 expiration timestamp
|
|
8 32 renewal token
|
|
40 32 cancel token
|
|
72 20 nodeid which accepted the tokens
|
|
7 468 (a) data
|
|
8 ?? 4 count of extra leases
|
|
9 ?? n*92 extra leases
|
|
|
|
The "extra leases" field must be copied and rewritten each time the size of
|
|
the enclosed data changes. The hope is that most buckets will have four or
|
|
fewer leases and this extra copying will not usually be necessary.
|
|
|
|
The (4) "data size" field contains the actual number of bytes of data present
|
|
in field (7), such that a client request to read beyond 504+(a) will result
|
|
in an error. This allows the client to (one day) read relative to the end of
|
|
the file. The container size (that is, (8)-(7)) might be larger, especially
|
|
if extra size was pre-allocated in anticipation of filling the container with
|
|
a lot of data.
|
|
|
|
The offset in (5) points at the *count* of extra leases, at (8). The actual
|
|
leases (at (9)) begin 4 bytes later. If the container size changes, both (8)
|
|
and (9) must be relocated by copying.
|
|
|
|
The server will honor any write commands that provide the write token and do
|
|
not exceed the server-wide storage size limitations. Read and write commands
|
|
MUST be restricted to the 'data' portion of the container: the implementation
|
|
of those commands MUST perform correct bounds-checking to make sure other
|
|
portions of the container are inaccessible to the clients.
|
|
|
|
The two methods provided by the storage server on these "MutableSlot" share
|
|
objects are:
|
|
|
|
* readv(ListOf(offset=int, length=int))
|
|
* returns a list of bytestrings, of the various requested lengths
|
|
* offset < 0 is interpreted relative to the end of the data
|
|
* spans which hit the end of the data will return truncated data
|
|
|
|
* testv_and_writev(write_enabler, test_vector, write_vector)
|
|
* this is a test-and-set operation which performs the given tests and only
|
|
applies the desired writes if all tests succeed. This is used to detect
|
|
simultaneous writers, and to reduce the chance that an update will lose
|
|
data recently written by some other party (written after the last time
|
|
this slot was read).
|
|
* test_vector=ListOf(TupleOf(offset, length, opcode, specimen))
|
|
* the opcode is a string, from the set [gt, ge, eq, le, lt, ne]
|
|
* each element of the test vector is read from the slot's data and
|
|
compared against the specimen using the desired (in)equality. If all
|
|
tests evaluate True, the write is performed
|
|
* write_vector=ListOf(TupleOf(offset, newdata))
|
|
* offset < 0 is not yet defined, it probably means relative to the
|
|
end of the data, which probably means append, but we haven't nailed
|
|
it down quite yet
|
|
* write vectors are executed in order, which specifies the results of
|
|
overlapping writes
|
|
* return value:
|
|
* error: OutOfSpace
|
|
* error: something else (io error, out of memory, whatever)
|
|
* (True, old_test_data): the write was accepted (test_vector passed)
|
|
* (False, old_test_data): the write was rejected (test_vector failed)
|
|
* both 'accepted' and 'rejected' return the old data that was used
|
|
for the test_vector comparison. This can be used by the client
|
|
to detect write collisions, including collisions for which the
|
|
desired behavior was to overwrite the old version.
|
|
|
|
In addition, the storage server provides several methods to access these
|
|
share objects:
|
|
|
|
* allocate_mutable_slot(storage_index, sharenums=SetOf(int))
|
|
* returns DictOf(int, MutableSlot)
|
|
* get_mutable_slot(storage_index)
|
|
* returns DictOf(int, MutableSlot)
|
|
* or raises KeyError
|
|
|
|
We intend to add an interface which allows small slots to allocate-and-write
|
|
in a single call, as well as do update or read in a single call. The goal is
|
|
to allow a reasonably-sized dirnode to be created (or updated, or read) in
|
|
just one round trip (to all N shareholders in parallel).
|
|
|
|
==== migrating shares ====
|
|
|
|
If a share must be migrated from one server to another, two values become
|
|
invalid: the write enabler (since it was computed for the old server), and
|
|
the lease renew/cancel tokens.
|
|
|
|
Suppose that a slot was first created on nodeA, and was thus initialized with
|
|
WE(nodeA) (= H(WEM+nodeA)). Later, for provisioning reasons, the share is
|
|
moved from nodeA to nodeB.
|
|
|
|
Readers may still be able to find the share in its new home, depending upon
|
|
how many servers are present in the grid, where the new nodeid lands in the
|
|
permuted index for this particular storage index, and how many servers the
|
|
reading client is willing to contact.
|
|
|
|
When a client attempts to write to this migrated share, it will get a "bad
|
|
write enabler" error, since the WE it computes for nodeB will not match the
|
|
WE(nodeA) that was embedded in the share. When this occurs, the "bad write
|
|
enabler" message must include the old nodeid (e.g. nodeA) that was in the
|
|
share.
|
|
|
|
The client then computes H(nodeB+H(WEM+nodeA)), which is the same as
|
|
H(nodeB+WE(nodeA)). The client sends this along with the new WE(nodeB), which
|
|
is H(WEM+nodeB). Note that the client only sends WE(nodeB) to nodeB, never to
|
|
anyone else. Also note that the client does not send a value to nodeB that
|
|
would allow the node to impersonate the client to a third node: everything
|
|
sent to nodeB will include something specific to nodeB in it.
|
|
|
|
The server locally computes H(nodeB+WE(nodeA)), using its own node id and the
|
|
old write enabler from the share. It compares this against the value supplied
|
|
by the client. If they match, this serves as proof that the client was able
|
|
to compute the old write enabler. The server then accepts the client's new
|
|
WE(nodeB) and writes it into the container.
|
|
|
|
This WE-fixup process requires an extra round trip, and requires the error
|
|
message to include the old nodeid, but does not require any public key
|
|
operations on either client or server.
|
|
|
|
Migrating the leases will require a similar protocol. This protocol will be
|
|
defined concretely at a later date.
|
|
|
|
=== Code Details ===
|
|
|
|
The MutableFileNode class is used to manipulate mutable files (as opposed to
|
|
ImmutableFileNodes). These are initially generated with
|
|
client.create_mutable_file(), and later recreated from URIs with
|
|
client.create_node_from_uri(). Instances of this class will contain a URI and
|
|
a reference to the client (for peer selection and connection).
|
|
|
|
NOTE: this section is out of date. Please see src/allmydata/interfaces.py
|
|
(the section on IMutableFilesystemNode) for more accurate information.
|
|
|
|
The methods of MutableFileNode are:
|
|
|
|
* download_to_data() -> [deferred] newdata, NotEnoughSharesError
|
|
* if there are multiple retrieveable versions in the grid, get() returns
|
|
the first version it can reconstruct, and silently ignores the others.
|
|
In the future, a more advanced API will signal and provide access to
|
|
the multiple heads.
|
|
* update(newdata) -> OK, UncoordinatedWriteError, NotEnoughSharesError
|
|
* overwrite(newdata) -> OK, UncoordinatedWriteError, NotEnoughSharesError
|
|
|
|
download_to_data() causes a new retrieval to occur, pulling the current
|
|
contents from the grid and returning them to the caller. At the same time,
|
|
this call caches information about the current version of the file. This
|
|
information will be used in a subsequent call to update(), and if another
|
|
change has occured between the two, this information will be out of date,
|
|
triggering the UncoordinatedWriteError.
|
|
|
|
update() is therefore intended to be used just after a download_to_data(), in
|
|
the following pattern:
|
|
|
|
d = mfn.download_to_data()
|
|
d.addCallback(apply_delta)
|
|
d.addCallback(mfn.update)
|
|
|
|
If the update() call raises UCW, then the application can simply return an
|
|
error to the user ("you violated the Prime Coordination Directive"), and they
|
|
can try again later. Alternatively, the application can attempt to retry on
|
|
its own. To accomplish this, the app needs to pause, download the new
|
|
(post-collision and post-recovery) form of the file, reapply their delta,
|
|
then submit the update request again. A randomized pause is necessary to
|
|
reduce the chances of colliding a second time with another client that is
|
|
doing exactly the same thing:
|
|
|
|
d = mfn.download_to_data()
|
|
d.addCallback(apply_delta)
|
|
d.addCallback(mfn.update)
|
|
def _retry(f):
|
|
f.trap(UncoordinatedWriteError)
|
|
d1 = pause(random.uniform(5, 20))
|
|
d1.addCallback(lambda res: mfn.download_to_data())
|
|
d1.addCallback(apply_delta)
|
|
d1.addCallback(mfn.update)
|
|
return d1
|
|
d.addErrback(_retry)
|
|
|
|
Enthusiastic applications can retry multiple times, using a randomized
|
|
exponential backoff between each. A particularly enthusiastic application can
|
|
retry forever, but such apps are encouraged to provide a means to the user of
|
|
giving up after a while.
|
|
|
|
UCW does not mean that the update was not applied, so it is also a good idea
|
|
to skip the retry-update step if the delta was already applied:
|
|
|
|
d = mfn.download_to_data()
|
|
d.addCallback(apply_delta)
|
|
d.addCallback(mfn.update)
|
|
def _retry(f):
|
|
f.trap(UncoordinatedWriteError)
|
|
d1 = pause(random.uniform(5, 20))
|
|
d1.addCallback(lambda res: mfn.download_to_data())
|
|
def _maybe_apply_delta(contents):
|
|
new_contents = apply_delta(contents)
|
|
if new_contents != contents:
|
|
return mfn.update(new_contents)
|
|
d1.addCallback(_maybe_apply_delta)
|
|
return d1
|
|
d.addErrback(_retry)
|
|
|
|
update() is the right interface to use for delta-application situations, like
|
|
directory nodes (in which apply_delta might be adding or removing child
|
|
entries from a serialized table).
|
|
|
|
Note that any uncoordinated write has the potential to lose data. We must do
|
|
more analysis to be sure, but it appears that two clients who write to the
|
|
same mutable file at the same time (even if both eventually retry) will, with
|
|
high probability, result in one client observing UCW and the other silently
|
|
losing their changes. It is also possible for both clients to observe UCW.
|
|
The moral of the story is that the Prime Coordination Directive is there for
|
|
a reason, and that recovery/UCW/retry is not a subsitute for write
|
|
coordination.
|
|
|
|
overwrite() tells the client to ignore this cached version information, and
|
|
to unconditionally replace the mutable file's contents with the new data.
|
|
This should not be used in delta application, but rather in situations where
|
|
you want to replace the file's contents with completely unrelated ones. When
|
|
raw files are uploaded into a mutable slot through the tahoe webapi (using
|
|
POST and the ?mutable=true argument), they are put in place with overwrite().
|
|
|
|
|
|
|
|
The peer-selection and data-structure manipulation (and signing/verification)
|
|
steps will be implemented in a separate class in allmydata/mutable.py .
|
|
|
|
=== SMDF Slot Format ===
|
|
|
|
This SMDF data lives inside a server-side MutableSlot container. The server
|
|
is oblivious to this format.
|
|
|
|
This data is tightly packed. In particular, the share data is defined to run
|
|
all the way to the beginning of the encrypted private key (the encprivkey
|
|
offset is used both to terminate the share data and to begin the encprivkey).
|
|
|
|
# offset size name
|
|
1 0 1 version byte, \x00 for this format
|
|
2 1 8 sequence number. 2^64-1 must be handled specially, TBD
|
|
3 9 32 "R" (root of share hash Merkle tree)
|
|
4 41 16 IV (share data is AES(H(readkey+IV)) )
|
|
5 57 18 encoding parameters:
|
|
57 1 k
|
|
58 1 N
|
|
59 8 segment size
|
|
67 8 data length (of original plaintext)
|
|
6 75 32 offset table:
|
|
75 4 (8) signature
|
|
79 4 (9) share hash chain
|
|
83 4 (10) block hash tree
|
|
87 4 (11) share data
|
|
91 8 (12) encrypted private key
|
|
99 8 (13) EOF
|
|
7 107 436ish verification key (2048 RSA key)
|
|
8 543ish 256ish signature=RSAenc(sigkey, H(version+seqnum+r+IV+encparm))
|
|
9 799ish (a) share hash chain, encoded as:
|
|
"".join([pack(">H32s", shnum, hash)
|
|
for (shnum,hash) in needed_hashes])
|
|
10 (927ish) (b) block hash tree, encoded as:
|
|
"".join([pack(">32s",hash) for hash in block_hash_tree])
|
|
11 (935ish) LEN share data (no gap between this and encprivkey)
|
|
12 ?? 1216ish encrypted private key= AESenc(write-key, RSA-key)
|
|
13 ?? -- EOF
|
|
|
|
(a) The share hash chain contains ceil(log(N)) hashes, each 32 bytes long.
|
|
This is the set of hashes necessary to validate this share's leaf in the
|
|
share Merkle tree. For N=10, this is 4 hashes, i.e. 128 bytes.
|
|
(b) The block hash tree contains ceil(length/segsize) hashes, each 32 bytes
|
|
long. This is the set of hashes necessary to validate any given block of
|
|
share data up to the per-share root "r". Each "r" is a leaf of the share
|
|
has tree (with root "R"), from which a minimal subset of hashes is put in
|
|
the share hash chain in (8).
|
|
|
|
=== Recovery ===
|
|
|
|
The first line of defense against damage caused by colliding writes is the
|
|
Prime Coordination Directive: "Don't Do That".
|
|
|
|
The second line of defense is to keep "S" (the number of competing versions)
|
|
lower than N/k. If this holds true, at least one competing version will have
|
|
k shares and thus be recoverable. Note that server unavailability counts
|
|
against us here: the old version stored on the unavailable server must be
|
|
included in the value of S.
|
|
|
|
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.
|