Re: Hardware vs Software RAID

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(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD