Date: Thu, 22 Sep 2005 23:05:08 -0400 From: Chuck Swiger <cswiger@mac.com> To: Francisco <francisco@natserv.net> Cc: Jeff Tchang <jeff.tchang@gmail.com>, freebsd-performance@freebsd.org Subject: Re: 3Ware 7500-4 Slow Message-ID: <433370E4.8060708@mac.com> In-Reply-To: <20050922215326.B50836@zoraida.natserv.net> References: <63f9d26505090417183dff415e@mail.gmail.com> <431C683B.1080803@mac.com> <20050922215326.B50836@zoraida.natserv.net>
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Francisco wrote:
> On Mon, 5 Sep 2005, Chuck Swiger wrote:
>> Small writes are pretty much the worst-case scenario for RAID-5,
>
> Such as mail servers?
So-so. RAID-5 is okay on a IMAP reader box, it's not so good for a pure SMTP
relay, especially one that does virus scanning.
> How about for a DB server which is mostly read only?
If your DB claims to support a RAID-5 configuration-- some DBs will change
their caching behavior to avoid thrashing a RAID-5 volume as much-- it might be
OK. If you're going to run a big DB, you really ought to be designing the disk
layout according to what the DB vendor recommends.
>> normal to see a very significant performance drop-- by up to an order
>> of magnitude-- from the performance of a bare drive.
>
> At which point Raid 5 starts to perform better?
> 6,8,10 drives?
Better for small writes? Never.
Although good hardware and lots of RAM to cache with can help a lot.
RAID systems have bus limitations on how wide they can go in terms of # of
drives, also in how much real bus bandwidth is available for very wide configs.
8 drives is a common maximum width.
> How about RAID 10 for a DB server?
This is a much better choice, close to ideal.
> I have been trying to convince the "powers that be" that SCSI would be
> much better.. but the price difference is just too astronomical for the
> capacities we need (500GB to 2 TB)
>
> Even 10K RPM IDE drives seem like would be a problem since they are
> mostly small in size.
Ten 72's would be in the right ballpark, that's about $2000. Ten of the
cheapest reasonable 80GB ATA drives would be about $800.
You could always ask:
"How much is your data worth to your company, again?"
You can get 146's for about $500 and even 300GB SCSI-3 drives exist.
--
-Chuck
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