Date: Sat, 11 Feb 2006 15:21:29 -0800 From: soralx@cydem.org To: bakul@BitBlocks.com Cc: freebsd-hackers@freebsd.org Subject: Re: RAID5 on athlon64 machines Message-ID: <200602111521.29263.soralx@cydem.org> In-Reply-To: <200602112044.k1BKi3u8083329@gate.bitblocks.com> References: <200602112044.k1BKi3u8083329@gate.bitblocks.com>
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> Theoretically the sequential write rate should be same or > higher than the sequential read rate. Given an N+1 disk Seq write rate for the whole RAID5 array will always be lower than the write rate for it's single disk. See 'http://en.wikipedia.org/wiki/RAID#RAID_5' " Traditional RAID5 A1 A2 A3 Ap B1 B2 Bp B3 C1 Cp C2 C3 Dp D1 D2 D3 [...] If another block, or some portion of a block, is written on that same stripe the parity block (or some portion of the parity block) is recalculated and rewritten. For small writes, this requires reading the old parity, reading the old data, writing the new parity, and writing the new data. [...] The parity blocks are not read on data reads, since this would be unnecessary overhead and would diminish performance. The parity blocks are read, however, when a read of a data sector results in a cyclic redundancy check (CRC) error." Timestamp: 0x43EE7046 [SorAlx] http://cydem.org.ua/ ridin' VN1500-B2
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