Fresel Michal - hi competence e.U. wrote at about 18:59:12 +0100 on Wednesday,
March 23, 2011:
> hi
>
> i was thinking about the current state of integrity-tests
> ("RsyncCsumCacheVerifyProb")
>
> on compressed files this means
> # decompress
> # create checksum of decompressed
> # compare cached checksums
>
If checksum caching is on then the checksum is stored at the end of
the compressed file. So the file does *not* need to be decompressed.
Technically, the checksum is only added to the file the 2nd time the
file is encountered which I imagine is probably due to the fact that
the native rsync algorithm only transmits the block and full file
checksums when the file already exists (otherwise perhaps only the
full file checksum is transmitted)
> what about:
> on first run
> # create checksum of compressed file = checksum_compressed_1
> # decompress
> # create checksum of decompressed
> # compare checksums (of cache)
> # if checksums (of cache) are the same - recreate checksum of compressed
> (just in case we had some disk issue meanwile) = checksum_compressed_2
> # compare checksum_compressed_1 to checksum_compressed_2
> # if the same - save checksum of compressed file for future use
>
> the next run we just need to checksum the compressed file as (the internal
> cached checksums) were already verified above
>
> results in faster rechecks afterwards:
> -) no need to decompress
> -) checksum-calculations on compressed files is faster (less MB's to
> calculate)
Seems like a lot of extra work since Craig's clever implementation
leverages existing rsync checksum calculations and gives the block and
full file md4 checksums for free.
Also, while it might be nice to have checksums of both the compressed
and decompressed files to speed up an independent file integrity test,
that is a whole other question.
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