Message-ID: <4613053F.5070401@polimetrix.com>
Date: Tue, 03 Apr 2007 18:54:07 -0700
From: Geoff Tolley <geoff@polimetrix.com>
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To: Ron <rjpeace@earthlink.net>
Cc: "jason@ohloh.net" <jason@ohloh.net>,
	pgsql-performance@postgresql.org
Subject: Re: SCSI vs SATA
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Ron wrote:
> At 07:07 PM 4/3/2007, Ron wrote:
>> For random IO, the 3ware cards are better than PERC
>>
>> > Question: will 8*15k 73GB SCSI drives outperform 24*7K 320GB SATA II 
>> drives?
>>
>> Nope.  Not even if the 15K 73GB HDs were the brand new Savvio 15K 
>> screamers.
>>
>> Example assuming 3.5" HDs and RAID 10 => 4 15K 73GB vs 12 7.2K 320GB
>> The 15K's are 2x faster rpm, but they are only ~23% the density => 
>> advantage per HD to SATAs.
>> Then there's the fact that there are 1.5x as many 7.2K spindles as 15K 
>> spindles...
> Oops make that =3x= as many 7.2K spindles as 15K spindles...

I don't think the density difference will be quite as high as you seem to 
think: most 320GB SATA drives are going to be 3-4 platters, the most that a 
73GB SCSI is going to have is 2, and more likely 1, which would make the 
SCSIs more like 50% the density of the SATAs. Note that this only really 
makes a difference to theoretical sequential speeds; if the seeks are 
random the SCSI drives could easily get there 50% faster (lower rotational 
latency and they certainly will have better actuators for the heads). 
Individual 15K SCSIs will trounce 7.2K SATAs in terms of i/os per second.

What I always do when examining hard drive options is to see if they've 
been tested (or a similar model has) at http://www.storagereview.com/ - 
they have a great database there with lots of low-level information 
(although it seems to be down at the time of writing).

But what's likely to make the largest difference in the OP's case (many 
inserts) is write caching, and a battery-backed cache would be needed for 
this. This will help mask write latency differences between the two 
options, and so benefit SATA more. Some 3ware cards offer it, some don't, 
so check the model.

How the drives are arranged is going to be important too - one big RAID 10 
is going to be rather worse than having arrays dedicated to each of 
pg_xlog, indices and tables, and on that front the SATA option is going to 
grant more flexibility.

If you care about how often you'll have to replace a failed drive, then the 
SCSI option no question, although check the cases for hot-swapability.

HTH,
Geoff