From: Marty Scholes <marty@outputservices.com>
To: John A Meinel <john@arbash-meinel.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Postgresql and Software RAID/LVM
Date: Tue, 07 Jun 2005 09:30:13 -0600
Message-ID: <42A5BD85.3060309@outputservices.com> (raw)
References: <42A4A06F.9040008@outputservices.com>
<42A52465.2000302@arbash-meinel.com>
John A Meinel wrote:
>
> Isn't this actually more of a problem for the meta-data to give out in a
> hardware situation? I mean, if the card you are using dies, you can't
> just get another one.
> With software raid, because the meta-data is on the drives, you can pull
> it out of that machine, and put it into any machine that has a
> controller which can read the drives, and a similar kernel, and you are
> back up and running.
Probably true. If you have a similar kernel and hardware and if you can
recover the state information, knowing where the state information is
stored. Those are some very big "ifs" during a hectic disaster.
> No, it hedges against *more* than one failure. But you can also do a
> RAID1 over a RAID5 in software. But if you are honestly willing to
> create a full RAID1, just create a RAID1 over RAID0. The performance is
> much better. And since you have a full RAID1, as long as both drives of
> a pairing don't give out, you can lose half of your drives.
True as well. The problem with RAID1 over RAID0 is that, during a drive
failure, you are one bad sector from disaster. Further, RAID5 does
automatic rebuild, whereas most RAID1 setups do not. RAID5 reduces the
amount of time that things are degraded, reducing the time that your
data is in danger.
> If you want the space, but you feel that RAID5 isn't redundant enough,
> go to RAID6, which uses 2 parity locations, each with a different method
> of storing parity, so not only is it more redundant, you have a better
> chance of finding problems.
Agreed, RAID6 is the future, but still won't keep the server running
when the RAID controller dies, or the SCSI/FC host adapter goes, or you
want to upgrade controller firmware, or you want to replace the media, or...
> So you are saying that you were able to replace the RAID controller
> without turning off the machine? I realize there does exist
> hot-swappable PCI cards, but I think you are overstating what you mean
> by "fully operational". For instance, it's not like you can access your
> data while it is being physically moved.
Detach mirror 1, uncable and move, recable and resync. Detach mirror 2,
uncable and move, recable and resync.
>
> I do think you had some nice hardware. But I know you can do all of this
> in software as well. It is usually a price/performance tradeoff. You
> spend quite a bit to get a hardware RAID card that can keep up with a
> modern CPU. I know we have an FC raid box at work which has a full 512MB
> of cache on it, but it wasn't that much cheaper than buying a dedicated
> server.
We run two Nexsan ATABoy2 arrays. These can be found in 1 TB
configurations for about $3,000 each, putting mirrored RAID5 storage at
$6 per GB. Is that a lot of money for storage? Maybe. In our case,
that's dirt cheap protection against storage-related downtime.
Marty
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To: pgsql-performance@postgresql.org
Cc: marty@outputservices.com, john@arbash-meinel.com
Subject: Re: Postgresql and Software RAID/LVM
In-Reply-To: <42A5BD85.3060309@outputservices.com>
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