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247 lines
7.7 KiB
ReStructuredText
247 lines
7.7 KiB
ReStructuredText
.. -*- coding: utf-8-with-signature -*-
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============================================
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Performance costs for some common operations
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============================================
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1. `Publishing an A-byte immutable file`_
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2. `Publishing an A-byte mutable file`_
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3. `Downloading B bytes of an A-byte immutable file`_
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4. `Downloading B bytes of an A-byte mutable file`_
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5. `Modifying B bytes of an A-byte mutable file`_
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6. `Inserting/Removing B bytes in an A-byte mutable file`_
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7. `Adding an entry to an A-entry directory`_
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8. `Listing an A entry directory`_
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9. `Checking an A-byte file`_
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10. `Verifying an A-byte file (immutable)`_
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11. `Repairing an A-byte file (mutable or immutable)`_
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``K`` indicates the number of shares required to reconstruct the file
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(default: 3)
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``N`` indicates the total number of shares produced (default: 10)
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``S`` indicates the segment size (default: 128 KiB)
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``A`` indicates the number of bytes in a file
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``B`` indicates the number of bytes of a file that are being read or
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written
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``G`` indicates the number of storage servers on your grid
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Most of these cost estimates may have a further constant multiplier: when a
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formula says ``N/K*S``, the cost may actually be ``2*N/K*S`` or ``3*N/K*S``.
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Also note that all references to mutable files are for SDMF-formatted files;
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this document has not yet been updated to describe the MDMF format.
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Publishing an ``A``-byte immutable file
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=======================================
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when the file is already uploaded
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---------------------------------
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If the file is already uploaded with the exact same contents, same
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erasure coding parameters (K, N), and same added convergence secret,
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then it reads the whole file from disk one time while hashing it to
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compute the storage index, then contacts about N servers to ask each
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one to store a share. All of the servers reply that they already have
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a copy of that share, and the upload is done.
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disk: A
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cpu: ~A
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network: ~N
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memory footprint: S
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when the file is not already uploaded
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-------------------------------------
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If the file is not already uploaded with the exact same contents, same
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erasure coding parameters (K, N), and same added convergence secret,
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then it reads the whole file from disk one time while hashing it to
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compute the storage index, then contacts about N servers to ask each
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one to store a share. Then it uploads each share to a storage server.
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disk: 2*A
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cpu: 2*~A
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network: N/K*A
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memory footprint: N/K*S
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Publishing an ``A``-byte mutable file
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=====================================
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cpu: ~A + a large constant for RSA keypair generation
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network: A
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memory footprint: N/K*A
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notes: Tahoe-LAFS generates a new RSA keypair for each mutable file that it
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publishes to a grid. This takes up to 1 or 2 seconds on a typical desktop PC.
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Part of the process of encrypting, encoding, and uploading a mutable file to a
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Tahoe-LAFS grid requires that the entire file be in memory at once. For larger
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files, this may cause Tahoe-LAFS to have an unacceptably large memory footprint
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(at least when uploading a mutable file).
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Downloading ``B`` bytes of an ``A``-byte immutable file
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=======================================================
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cpu: ~B
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network: B
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notes: When Tahoe-LAFS 1.8.0 or later is asked to read an arbitrary
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range of an immutable file, only the S-byte segments that overlap the
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requested range will be downloaded.
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(Earlier versions would download from the beginning of the file up
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until the end of the requested range, and then continue to download
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the rest of the file even after the request was satisfied.)
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Downloading ``B`` bytes of an ``A``-byte mutable file
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=====================================================
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cpu: ~A
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network: A
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memory footprint: A
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notes: As currently implemented, mutable files must be downloaded in
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their entirety before any part of them can be read. We are
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exploring fixes for this; see ticket #393 for more information.
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Modifying ``B`` bytes of an ``A``-byte mutable file
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===================================================
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cpu: ~A
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network: A
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memory footprint: N/K*A
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notes: If you upload a changed version of a mutable file that you
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earlier put onto your grid with, say, 'tahoe put --mutable',
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Tahoe-LAFS will replace the old file with the new file on the
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grid, rather than attempting to modify only those portions of the
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file that have changed. Modifying a file in this manner is
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essentially uploading the file over again, except that it re-uses
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the existing RSA keypair instead of generating a new one.
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Inserting/Removing ``B`` bytes in an ``A``-byte mutable file
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============================================================
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cpu: ~A
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network: A
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memory footprint: N/K*A
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notes: Modifying any part of a mutable file in Tahoe-LAFS requires that
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the entire file be downloaded, modified, held in memory while it is
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encrypted and encoded, and then re-uploaded. A future version of the
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mutable file layout ("LDMF") may provide efficient inserts and
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deletes. Note that this sort of modification is mostly used internally
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for directories, and isn't something that the WUI, CLI, or other
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interfaces will do -- instead, they will simply overwrite the file to
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be modified, as described in "Modifying B bytes of an A-byte mutable
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file".
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Adding an entry to an ``A``-entry directory
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===========================================
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cpu: ~A
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network: ~A
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memory footprint: N/K*~A
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notes: In Tahoe-LAFS, directories are implemented as specialized mutable
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files. So adding an entry to a directory is essentially adding B
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(actually, 300-330) bytes somewhere in an existing mutable file.
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Listing an ``A`` entry directory
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================================
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cpu: ~A
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network: ~A
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memory footprint: N/K*~A
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notes: Listing a directory requires that the mutable file storing the
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directory be downloaded from the grid. So listing an A entry
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directory requires downloading a (roughly) 330 * A byte mutable
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file, since each directory entry is about 300-330 bytes in size.
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Checking an ``A``-byte file
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===========================
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cpu: ~G
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network: ~G
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memory footprint: negligible
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notes: To check a file, Tahoe-LAFS queries all the servers that it knows
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about. Note that neither of these values directly depend on the size
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of the file. This is relatively inexpensive, compared to the verify
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and repair operations.
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Verifying an A-byte file (immutable)
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====================================
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cpu: ~N/K*A
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network: N/K*A
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memory footprint: N/K*S
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notes: To verify a file, Tahoe-LAFS downloads all of the ciphertext
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shares that were originally uploaded to the grid and integrity checks
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them. This is (for grids with good redundancy) more expensive than
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downloading an A-byte file, since only a fraction of these shares would
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be necessary to recover the file.
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Verifying an A-byte file (mutable)
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==================================
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cpu: ~N/K*A
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network: N/K*A
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memory footprint: N/K*A
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notes: To verify a file, Tahoe-LAFS downloads all of the ciphertext
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shares that were originally uploaded to the grid and integrity checks
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them. This is (for grids with good redundancy) more expensive than
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downloading an A-byte file, since only a fraction of these shares would
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be necessary to recover the file.
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Repairing an ``A``-byte file (mutable or immutable)
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===================================================
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cpu: variable, between ~A and ~N/K*A
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network: variable; between A and N/K*A
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memory footprint (immutable): (1+N/K)*S
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(SDMF mutable): (1+N/K)*A
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notes: To repair a file, Tahoe-LAFS downloads the file, and
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generates/uploads missing shares in the same way as when it initially
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uploads the file. So, depending on how many shares are missing, this
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can cost as little as a download or as much as a download followed by
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a full upload.
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Since SDMF files have only one segment, which must be processed in its
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entirety, repair requires a full-file download followed by a full-file
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upload.
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