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Re: compact flash disks?

From: "James Mansion" <james(at)mansionfamily(dot)plus(dot)com>
To: "Csaba Nagy" <nagy(at)ecircle-ag(dot)com>,"Carlos Moreno" <moreno_pg(at)mochima(dot)com>,<rjpeace(at)earthlink(dot)net>
Cc: "postgres performance list" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: compact flash disks?
Date: 2007-04-04 05:18:52
Message-ID: HCEPKPMCAJLDGJIBCLGHOEKOHGAA.james@mansionfamily.plus.com (view raw or flat)
Thread:
Lists: pgsql-performance
>On sequential read speed HDs outperform flash disks... only on random
>access the flash disks are better. So if your application is a DW one,
>you're very likely better off using HDs.

This looks likely to be a non-issue shortly, see here:

http://www.reghardware.co.uk/2007/03/27/sams_doubles_ssd_capacity/

I still think this sort of devices will become the OLTP device
of choice before too long - even if we do have to watch the wear rate.

>WARNING:  modern TOtL flash RAMs are only good for ~1.2M writes per
>memory cell.  and that's the =good= ones.

Well, my original question was whether the physical update pattern
of the server does have hotspots that will tend to cause a problem
in normal usage if the wear levelling (such as it is) doesn't entirely
spread the load.  The sorts of application I'm interested in will not
update individual data elements very often.  There's a danger that
index nodes might be rewritted frequently, but one might want to allow
that indexes persist lazily and should be recovered from a scan after
a crash that leaves them dirty, so that they can be cached and avoid
such an access pattern.


Out of interest with respect to WAL - has anyone tested to see whether
one could tune the group commit to pick up slightly bigger blocks and
write the WAL using compression, to up the *effective* write speed of
media? Once again, most data I'm interested in is far from a random
pattern and tends to compress quite well.

If the WAL write is committed to the disk platter, is it OK for
arbitrary data blocks to have failed to commit to disk so we can
recover the updates for committed transactions?

Is theer any documentation on the write barrier usage?

James

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Subject: Re: Scaling SELECT:s with the number of disks on a stripe
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