-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 David Rees wrote: > On Mon, Mar 3, 2008 at 12:08 PM, Tomasz Chmielewski <[EMAIL PROTECTED]> wrote: >> RAID5/6 have a performance penalty when compared to other RAID level >> because every single write (or, write IO operation) requires four disk >> IOs on two drives (two reads, and two writes), possibly harming other IO >> operations. > > Correction: > > Small writes (writes smaller than stripe size) require reads from > disk. Writes that are the size of a stripe or larger do not incur the > additional read penalty. > > For example if you have a 3-disk RAID 5 and a 256 KB stripe size, two > disks hold 128 KB of data and the third holds 128 KB of parity data. > > If you write less than 256 KB to a stripe, you first have to read the > data from the two data disks, calculate the parity with the new data > and write to all 3 disks. > > But if you are writing 256 KB or more, you can skip the read and > simply calculate the parity and write all 3 chunks to disk.
So would it then make sense for a backuppc data partition to use a smaller stripe size since most writes will be very small? > Having battery backed RAM on the RAID controller can help, because the > controller can lie to the OS and say the data is written to disk > immediately instead of waiting for an read-calculate-write cycle, > since it's sure that if it does lose power, it can store the data that > should be written to disk later when power is restored in addition to > buffering the reads/writes so that it can reorder them to reduce the > amount of seeking required. Is it possible to instruct linux to use it's memory to do this? If you have a UPS and feel that it is pretty unlikely to crash, you might be happy to get this kind of speed improvement. Regards, Adam -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFHzLhPGyoxogrTyiURArcmAJ9akT9v/+ru/ogOqRFecAEz8IKQFgCZAT8+ 2zjLqBFmjExZWT0adTTuWCc= =ycqA -----END PGP SIGNATURE----- ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ BackupPC-users mailing list BackupPC-users@lists.sourceforge.net List: https://lists.sourceforge.net/lists/listinfo/backuppc-users Wiki: http://backuppc.wiki.sourceforge.net Project: http://backuppc.sourceforge.net/
