Raid 5 performance

[Lennart Sorensen]

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 (and I thought the chunk size was
how much to write to each disk at a time, not how big a chunk across
all disks were).  I think from benchmarks I have seen most people find
64k chunks per disk a good balance.  Of course on updating data in a
block, you might have to read the block, modify and write back, but that
is always true so no real loss there.

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.

Lennart Sorensen
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