In response to Mark Mielke <mark@mark.mielke.cc>:
> Bill Moran wrote:
> > In order to recalculate the parity, it has to have data from all disks. Thus,
> > if you have 4 disks, it has to read 2 (the unknown data blocks included in
> > the parity calculation) then write 2 (the new data block and the new
> > parity data) Caching can help some, but if your data ends up being any
> > size at all, the cache misses become more frequent than the hits. Even
> > when caching helps, you max speed is still only the speed of a single
> > disk.
> >
> If you have 4 disks, it can do either:
>
> 1) Read the old block, read the parity block, XOR the old block with
> the parity block and the new block resulting in the new parity block,
> write both the new parity block and the new block.
> 2) Read the two unknown blocks, XOR with the new block resulting in
> the new parity block, write both the new parity block and the new block.
>
> You are emphasizing 2 - but the scenario is also overly simplistic.
> Imagine you had 10 drives on RAID 5. Would it make more sense to read 8
> blocks and then write two (option 2, and the one you describe), or read
> two blocks and then write two (option 1). Obviously, if option 1 or
> option 2 can be satisfied from cache, it is better to not read at all.
Good point that I wasn't aware of.
> I note that you also disagree with Dave, in that you are not claiming it
> performs consistency checks on read. No system does this as performance
> would go to the crapper.
I call straw man :)
I don't disagree. I simply don't know. There's no reason why it _couldn't_
do consistency checking as it ran ... of course, performance would suck.
Generally what you expect out of RAID 5|6 is that it can rebuild a drive
in the event of a failure, so I doubt if anyone does consistency checking
by default, and I wouldn't be surprised if a lot of systems don't have
the option to do it at all.
--
Bill Moran
Collaborative Fusion Inc.
http://people.collaborativefusion.com/~wmoran/
wmoran@collaborativefusion.com
Phone: 412-422-3463x4023
