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docs/performance.txt: split out CPU from network, expand on mutable costs

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Brian Warner 2010-02-23 23:38:13 -05:00
parent cc2fcd700c
commit 8ba5363196
1 changed files with 41 additions and 32 deletions
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docs/performance.txt
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@ -2,7 +2,8 @@
=== Publishing an A-byte immutable file ===
cost: O(A)
network: A
memory footprint: N/k*128KiB
notes: An immutable file upload requires an additional I/O pass over the entire
source file before the upload process can start, since convergent
@ -11,7 +12,9 @@ notes: An immutable file upload requires an additional I/O pass over the entire
=== Publishing an A-byte mutable file ===
cost: O(A) + a large constant for RSA + memory usage.
network: A
memory footprint: N/k*A
cpu: O(A) + a large constant for RSA keypair generation
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
@ -25,8 +28,8 @@ notes: Tahoe-LAFS generates a new RSA keypair for each mutable file that
=== 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.
network: A
memory footprint: 128KiB
notes: When asked to read an arbitrary range of an immutable file,
Tahoe-LAFS will download from the beginning of the file up until
@ -47,7 +50,8 @@ notes: When asked to read an arbitrary range of an immutable file,
=== Downloading B bytes of an A-byte mutable file ===
cost: O(A)
network: A
memory footprint: N/k*A
notes: As currently implemented, mutable files must be downloaded in
their entirety before any part of them can be read. We are
@ -55,7 +59,8 @@ notes: As currently implemented, mutable files must be downloaded in
=== Modifying B bytes of an A-byte mutable file ===
cost: O(A)
network: A
memory footprint: N/k*A
notes: If you upload a changed version of a mutable file that you
earlier put onto your grid with, say, 'tahoe put --mutable',
@ -65,51 +70,54 @@ notes: If you upload a changed version of a mutable file that you
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 ===
=== Inserting/Removing B bytes in an A-byte mutable file ===
cost: O(A)
network: A
memory footprint: N/k*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
the entire file be downloaded, modified, held in memory while it is
encrypted and encoded, and then re-uploaded. A future version of the
mutable file layout ("LDMF") may provide efficient inserts and
deletes. 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)
network: O(A)
memory footprint: N/k*A
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)
network: O(A)
memory footprint: N/k*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 ===
=== Performing a file-check on 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.
network: O(S), where S is the number of servers on your grid
memory footprint: negligible
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.
notes: To check a file, Tahoe-LAFS queries all the servers that it knows
about. 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 ===
=== Performing a file-verify on an A-byte file ===
cost: O(A)
network: N/k*A
memory footprint: N/k*128KiB
notes: To verify a file, Tahoe-LAFS downloads all of the ciphertext
shares that were originally uploaded to the grid and integrity
@ -119,9 +127,10 @@ notes: To verify a file, Tahoe-LAFS downloads all of the ciphertext
=== Repairing an A-byte file (mutable or immutable) ===
cost: variable; up to around O(A)
network: variable; up to around O(A)
memory footprint: from 128KiB to (1+N/k)*128KiB
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
notes: To repair a file, Tahoe-LAFS downloads the file, and generates/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.