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Re: SCSI vs SATA

From: david(at)lang(dot)hm
To: Geoff Tolley <geoff(at)polimetrix(dot)com>
Cc: Ron <rjpeace(at)earthlink(dot)net>, "jason(at)ohloh(dot)net" <jason(at)ohloh(dot)net>,pgsql-performance(at)postgresql(dot)org
Subject: Re: SCSI vs SATA
Date: 2007-04-04 04:15:03
Message-ID: Pine.LNX.4.64.0704032106210.30746@asgard.lang.hm (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, 3 Apr 2007, Geoff Tolley wrote:

> 
> 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.

true, but with 3x as many drives (and 4x the capacity per drive) the SATA 
system will have to do far less seeking

for that matter, with 20ish 320G drives, how large would a parition be 
that only used the outer pysical track of each drive? (almost certinly 
multiple logical tracks) if you took the time to set this up you could 
eliminate seeking entirely (at the cost of not useing your capacity, but 
since you are considering a 12x range in capacity, it's obviously not your 
primary concern)

> 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.

note that the CMU and Google studies both commented on being surprised at 
the lack of difference between the reliability of SCSI and SATA drives.

David Lang

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