tahoe-lafs/docs/performance.txt

= Performance costs for some common operations =

=== Publishing an A-byte immutable file ===

cost:  O(A)

notes: An immutable file upload requires an additional I/O pass over the entire
       source file before the upload process can start, since convergent
       encryption derives the encryption key in part from the contents of the
       source file.

=== Publishing an A-byte mutable file ===

cost:  O(A) + a large constant for RSA + memory usage.

notes: Tahoe-LAFS generates a new RSA keypair for each mutable file that
       it publishes to a grid. This takes up to 1 or 2 seconds on a
       typical desktop PC.

       Part of the process of encrypting, encoding, and uploading a
       mutable file to a Tahoe-LAFS grid requires that the entire file
       be in memory at once. For larger files, this may cause
       Tahoe-LAFS to have an unacceptably large memory footprint (at
       least when uploading a mutable file).

=== Downloading B bytes of an A-byte immutable file ===

time/cost until the read is satisfied: variable; up to O(A).
cost of the entire operation:          O(A) if the file isn't cached.

notes: When asked to read an arbitrary range of an immutable file,
       Tahoe-LAFS will download from the beginning of the file up until
       it has enough of the file to satisfy the requested read.
       Depending on where in the file the requested range is, this can
       mean that the entire file is downloaded before the request is
       satisfied. Tahoe-LAFS will continue to download the rest of the
       file even after the request is satisfied, so in any case where the
       file actually has to downloaded from the grid, reading part of an
       immutable file will result in downloading all of the immutable
       file. Ticket #798 is a proposal to change this behavior.

       Tahoe-LAFS will cache files that are read in this manner for a
       short while, so subsequent reads of the same file may be served
       entirely from cache, depending on what part of the file they need
       to read, what part of the file was read by previous reads, and
       how much time has elapsed since the last read.

=== Downloading B bytes of an A-byte mutable file ===

cost:  O(A)

notes: As currently implemented, mutable files must be downloaded in
       their entirety before any part of them can be read. We are
       exploring fixes for this; see ticket #393 for more information.

=== Modifying B bytes of an A-byte mutable file ===

cost:  O(A)

notes: If you upload a changed version of a mutable file that you
       earlier put onto your grid with, say, 'tahoe put --mutable',
       Tahoe-LAFS will replace the old file with the new file on the
       grid, rather than attempting to modify only those portions of the
       file that have changed. Modifying a file in this manner is
       essentially uploading the file over again, except that it re-uses
       the existing RSA keypair instead of generating a new one.

=== Adding/Removing B bytes in an A-byte mutable file ===

cost:  O(A)

notes: Modifying any part of a mutable file in Tahoe-LAFS requires that
       the entire file be downloaded, modified, held in memory while it
       is encrypted and encoded, and then re-uploaded. Note that this
       sort of modification is mostly used internally for directories,
       and isn't something that the WUI, CLI, or other interfaces will
       do -- instead, they will simply overwrite the file to be
       modified, as described in "Modifying B bytes of an A-byte mutable
       file".

=== Adding an entry to an A-entry directory ===

cost:  O(A) (roughly)
notes: In Tahoe-LAFS, directories are implemented as specialized mutable
       files. So adding an entry to a directory is essentially adding B
       (actually, 300-330) bytes somewhere in an existing mutable file.

=== Listing an A entry directory ===

cost:  O(A)

notes: Listing a directory requires that the mutable file storing the
       directory be downloaded from the grid. So listing an A entry
       directory requires downloading a (roughly) 330 * A byte mutable
       file, since each directory entry is about 300-330 bytes in size.

=== Checking an A-byte file ===

cost:  variable; between O(N) and O(S), where N is the number of shares
       generated when the file was initially uploaded, and S is the
       number of servers on your grid.

notes: To check a file, Tahoe-LAFS queries the servers that it knows
       about until it either runs out of servers, or finds all of the
       shares that were originally uploaded. Note that neither of these
       values directly depend on the size of the file. This is
       relatively inexpensive, compared to the verify and repair
       operations.

=== Verifying an A-byte file ===

cost: O(A)

notes: To verify a file, Tahoe-LAFS downloads all of the ciphertext
       shares that were originally uploaded to the grid and integrity
       checks them. This is, for well-behaved grids, likely to be more
       expensive than downloading an A-byte file, since only a fraction
       of these shares are necessary to recover the file.

=== Repairing an A-byte file (mutable or immutable) ===

cost:  variable; up to around O(A)

notes: To repair a file, Tahoe-LAFS generates and uploads missing shares
       in the same way as when it initially uploads the file. So,
       depending on how many shares are missing, this can be about as
       expensive as initially uploading the file in the first place.