Raid 5 performance

Ralph Doncaster ralph-Zd07PnzKK1IAvxtiuMwx3w at public.gmane.org
Wed Sep 22 12:59:19 UTC 2004


On Wed, 22 Sep 2004, Lennart Sorensen wrote:

> On Wed, Sep 22, 2004 at 07:58:01AM -0400, Ralph Doncaster wrote:
> > 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.
>
> Well in that case I guess you would be right.  That would apply if you
> actually ran the chunk size that small [...]

No, a larger chunk size doesn't eliminate the read for checksum
calculation when you have an odd number of data drives.

> I well agree that if you are writing a multiple of the size of data that
> fits acress the number of disks, you gain a bit from having 2&n data
> disks, although if I had 16k chunks and 3 data drives and wrote 48k, I
> would get the same benefit.

No.  You're wrong again.  But you'll have to figure it out on your own;
I'm not wasting any more of my time...

-Ralph

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