Date: Wed, 26 Dec 2007 17:16:08 -0500
From: Bill Moran <wmoran@collaborativefusion.com>
To: Mark Mielke <mark@mark.mielke.cc>
Cc: Fernando Hevia <fhevia@ip-tel.com.ar>,
	'pgsql-performance' <pgsql-performance@postgresql.org>
Subject: Re: With 4 disks should I go for RAID 5 or RAID 10
Message-Id: <20071226171608.7fd6e625.wmoran@collaborativefusion.com>
In-Reply-To: <4772D042.8090402@mark.mielke.cc>
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	<0f9701c84801$399dcff0$8f01010a@iptel.com.ar>
	<20071226161430.72ef5d31.wmoran@collaborativefusion.com>
	<4772D042.8090402@mark.mielke.cc>
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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