Raid 5 performance

[John Macdonald]

On Wed, Sep 22, 2004 at 10:40:24AM -0400, Ralph Doncaster wrote:
> > If a write
> > spans some, but not all, of the data disks in a stripe, then
> > data must be read from the disk to compute the new parity info.
> This is a correct statement, and this is what happens when you write 16KB
> chunks to a stripe of 4 disks (3 data + 1 parity).  The Linux RAID code
> doesn't coalesce writes of multiple chunks (i.e. 3x 16KB chunks) to
> optimize away the theoretically unecessary read for parity computation.

So, the driver does not compute the parity directly from the
data for a 3+1 setup when you write exactly one entire data
stripe of 12k, but it does for a 4+1 setup when you write
exactly one data stripe of 16k?

That's insane.

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