xyzzy at speakeasy.org
Thu Feb 28 18:52:42 PST 2002
On Thu, 28 Feb 2002, Mark Hahn wrote:
> > Raid 5 is costly. As I understand it (not very well) there's 3 writes for
> > RAID 5. Reading can be slow, but not nearly as bad. Obviously you dont want
> I've never quite understood why people think R5 is expensive.
> yes, you have to do the parity calculation, and a 1 byte write
> does require an update of the parity block for that stripe,
> but it's not egregious.
In order to update a single RAID5 block you need to do two reads and two
writes. The two reads are the block to be written and the parity block. You
need to XOR the old data block ^ old parity block ^ new data block, to get the
new parity block. Then write the new parity and the new data. Much slower!
For instance, the 3ware 7410 card we get something like 75Mb/sec writes to 4
drives in raid 0 and 5.6MB/sec in raid 5.
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