performance fsarchive vs partimage

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dgerman
Posts: 6
Joined: Thu Jan 14, 2010 10:46 pm

performance fsarchive vs partimage

Post by dgerman » Mon Jan 18, 2010 2:22 am

Traditionally, in my experience, with one exception ( FSAVE [no you won't find it anymore])
programs that understand the file system are slower than those that backup on a raw sector basis.

This is especially true for filesystems that support extending files multiple times, where the extensions are separated from the previous portion of the file because of free space fragmentation. (Mac OS X solves this by rewriting any file it read as fragmented unfragmented basically defraging all the time on the fly.)

The slowness is because reading file BigExtendedManyTimes requires the disk to move the actuator at least "ManyTimes" and the fragments are not necessarily in order resulting in seek 1483, 1324, 1982, 241,4324,322. Now that all that is done, on to "BiggerFileNotExtendedButFragmentedDueToFreeSpaceFragmentation" . To collect all his fragments many of the same cylinders passed previously will be passed over again.

Another problem is a result of the fact that the files in a fileSystem backup are saved in alphabetical order not the order which the files were created (modified) in. They were allocated AT the order they were created. Very bad in the case of FILE1-100117, FILE2-100117, FILE1-100118, FILE2-100118,... new files suffixed with a date or increment.
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"Physical" sector based backups have the problem of understanding the allocation table of the volume and without knowledge of the file structure will back up allocationUnits which may be less (perhaps much less ) than full.
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These statements only applicable where the volume being backed up is the bottleneck and not the CPU(s) doing compression or the path to the destination or the destination.

admin
Site Admin
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Joined: Sat Feb 21, 2004 12:12 pm

Re: performance fsarchive vs partimage

Post by admin » Mon Jan 18, 2010 7:25 am

This is true. This is why partimage has been developed for a static usage (it works on blocks). But I really think today having more flexibility is really more important. The ability to restore a filesystem on a partition which is smaller is really useful. And it does not really matter if the speed is not optimal. BTW, a very good compression (lzma) helps to reduce the size of the archive, and then the I/O.

dgerman
Posts: 6
Joined: Thu Jan 14, 2010 10:46 pm

Re: performance fsarchive vs partimage

Post by dgerman » Mon Jan 18, 2010 12:54 pm

Do you have any statistics comparing fsarchive vs partimage on a "standard" test volume?
Like maybe a /usr volume immediately after an install ?

A really important speed consideration is that a "physical" backup is done with the system off line.

admin
Site Admin
Posts: 550
Joined: Sat Feb 21, 2004 12:12 pm

Re: performance fsarchive vs partimage

Post by admin » Mon Jan 18, 2010 8:45 pm

No I have never done benchmarks like that.If could be interesting to see what happens. We can make tests based on the same compression algorithms (gzip and bzip2 are both available in both partimage and fsarchiver). But I am also quite sure there is some space for optimizations in fsarchiver.

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