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. 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. -Dave ------------------------------------------------------------------------- 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 [email protected] List: https://lists.sourceforge.net/lists/listinfo/backuppc-users Wiki: http://backuppc.wiki.sourceforge.net Project: http://backuppc.sourceforge.net/
