Raid 5 performance

[Ralph Doncaster]

On Tue, 21 Sep 2004, Lennart Sorensen wrote:

> On Tue, Sep 21, 2004 at 05:42:16PM -0400, Ralph Doncaster wrote:
> > To avoid having to read from the odd data disk in order to calculate the
> > checksum when doing a write.
>
> You have to know the value of n-1 disks in order to calculate the xor
> value for the last disk, so you either have to read before write on one
> disk, or read from all disks and do a write.  I don't see how either is
> affected by number of disks.

OK, I'll make one last try to explain this.  The smallest RAID chunk size
supported by Linux is 4KB.  The sector size on an IDE drive is 512-bytes.
When writing a 4KB chunk to 2, 4, or 8 data drives, no read is necessary
for checksum calculation.

-Ralph

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