From: Greg Smith <gsmith@gregsmith.com>
To: Peter T. Breuer <ptb@inv.it.uc3m.es>
Cc: Matthew Wakeling <matthew@flymine.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: Hardware vs Software RAID
Date: Wed, 25 Jun 2008 15:59:59 -0400 (EDT)
Message-ID: <Pine.GSO.4.64.0806251222450.24017@westnet.com> (raw)
In-Reply-To: <200806251605.m5PG5Il07546@inv.it.uc3m.es>
References: <200806251605.m5PG5Il07546@inv.it.uc3m.es>
On Wed, 25 Jun 2008, Peter T. Breuer wrote:
> I refrained from saying in my reply that I would set up a firewire-based
> link to ram in a spare old portable (which comes with a battery) if I
> wanted to do this cheaply.
Maybe, but this is kind of a weird setup. Not many people are going to
run a production database that way and us wandering into the details too
much risks confusing everybody else.
> The log is sync. Therefore it doesn't matter what the guarantees are, or
> at least I assume you are worrying about acks coming back before the
> write has been sent, etc. Only an actual net write will be acked by the
> firewire transport as far as I know.
That's exactly the issue; it's critical for database use that a disk not
lie to you about writes being done if they're actually sitting in a cache
somewhere. (S)ATA disks do that, so you have to turn that off for them to
be safe to use. Since the firewire enclosure is a black box, it's
difficult to know exactly what it's doing here, and history here says that
every type (S)ATA disk does the wrong in the default case. I expect that
for any Firewire/USB device, if I write to the disk, then issue a fsync,
it will return success from that once the data has been written to the
disk's cache--which is crippling behavior from the database's perspective
one day when you get a crash.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
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To: pgsql-performance@postgresql.org
Cc: gsmith@gregsmith.com, ptb@inv.it.uc3m.es, matthew@flymine.org
Subject: Re: Hardware vs Software RAID
In-Reply-To: <Pine.GSO.4.64.0806251222450.24017@westnet.com>
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