From: Zooko O'Whielacronx Date: Tue, 4 Jan 2011 06:54:55 +0000 (-0800) Subject: docs: update performance.rst to describe the difference between already-uploaded... X-Git-Url: https://git.rkrishnan.org/about.html?a=commitdiff_plain;h=3523b50716e655757b10594c86fdcf7432e1cac8;p=tahoe-lafs%2Ftahoe-lafs.git docs: update performance.rst to describe the difference between already-uploaded and not-already-uploaded, to parameterize segment size, and to use "~A" to mean "approximately A" --- diff --git a/docs/performance.rst b/docs/performance.rst index 4165b776..95a05143 100644 --- a/docs/performance.rst +++ b/docs/performance.rst @@ -14,26 +14,66 @@ Performance costs for some common operations 10. `Performing a file-verify on an A-byte file`_ 11. `Repairing an A-byte file (mutable or immutable)`_ +``K`` indicates the number of shares required to reconstruct the file +(default: 3) + +``N`` indicates the total number of shares produced (default: 10) + +``S`` indicates the segment size (default: 128 KiB) + +``A`` indicates the number of bytes in a file + +``B`` indicates the number of bytes of a file which are being read or +written + +``G`` indicates the number of storage servers on your grid + Publishing an ``A``-byte immutable file ======================================= -network: A +when the file is already uploaded +--------------------------------- + +If the file is already uploaded with the exact same contents, same +erasure coding parameters (K, N), and same added convergence secret, +then it reads the whole file from disk one time while hashing it to +compute the storage index, then contacts about N servers to ask each +one to store a share. All of the servers reply that they already have +a copy of that share, and the upload is done. + +disk: A + +cpu: ~A + +network: ~N + +memory footprint: N/K*S + +when the file is not already uploaded +------------------------------------- + +If the file is not already uploaded with the exact same contents, same +erasure coding parameters (K, N), and same added convergence secret, +then it reads the whole file from disk one time while hashing it to +compute the storage index, then contacts about N servers to ask each +one to store a share. Then it uploads each share to a storage server. + +disk: 2*A + +cpu: 2*~A -memory footprint: N/k*128KiB +network: ~N + ~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. +memory footprint: N/K*S Publishing an ``A``-byte mutable file ===================================== network: A -memory footprint: N/k*A +memory footprint: N/K*A -cpu: O(A) + a large constant for RSA keypair generation +cpu: ~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 typical desktop PC. @@ -48,10 +88,10 @@ Downloading ``B`` bytes of an ``A``-byte immutable file network: B -memory footprint: 128KiB +cpu: ~A -notes: When Tahoe-LAFS 1.8.0 or later is asked to read an arbitrary range -of an immutable file, only the 128-KiB segments that overlap the +notes: When Tahoe-LAFS 1.8.0 or later is asked to read an arbitrary +range of an immutable file, only the S-byte segments that overlap the requested range will be downloaded. (Earlier versions would download from the beginning of the file up @@ -74,7 +114,7 @@ Modifying ``B`` bytes of an ``A``-byte mutable file network: A -memory footprint: N/k*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', @@ -89,7 +129,7 @@ Inserting/Removing ``B`` bytes in an ``A``-byte mutable file network: A -memory footprint: N/k*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 @@ -104,9 +144,9 @@ file". Adding an entry to an ``A``-entry directory =========================================== -network: O(A) +network: ~A -memory footprint: N/k*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 @@ -115,9 +155,9 @@ files. So adding an entry to a directory is essentially adding B Listing an ``A`` entry directory ================================ -network: O(A) +network: ~A -memory footprint: N/k*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 @@ -127,7 +167,7 @@ file, since each directory entry is about 300-330 bytes in size. Performing a file-check on an ``A``-byte file ============================================= -network: O(S), where S is the number of servers on your grid +network: ~G, where G is the number of servers on your grid memory footprint: negligible @@ -139,9 +179,9 @@ and repair operations. Performing a file-verify on an ``A``-byte file ============================================== -network: N/k*A +network: N/K*A -memory footprint: N/k*128KiB +memory footprint: N/K*S notes: To verify a file, Tahoe-LAFS downloads all of the ciphertext shares that were originally uploaded to the grid and integrity @@ -152,9 +192,9 @@ of these shares are necessary to recover the file. Repairing an ``A``-byte file (mutable or immutable) =================================================== -network: variable; up to around O(A) +network: variable; up to around ~A -memory footprint: from 128KiB to (1+N/k)*128KiB +memory footprint: from S to (1+N/K)*S 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.