Re: SCSI vs SATA - Mailing list pgsql-performance

On 4-Apr-07, at 8:46 AM, Andreas Kostyrka wrote:

> * Peter Kovacs <maxottovonstirlitz@gmail.com> [070404 14:40]:
>> This may be a silly question but: will not 3 times as many disk
>> drives
>> mean 3 times higher probability for disk failure? Also rumor has it
>> that SATA drives are more prone to fail than SCSI drivers. More
>> failures will result, in turn, in more administration costs.
> Actually, the newest research papers show that all discs (be it
> desktops, or highend SCSI) have basically the same failure statistics.
>
> But yes, having 3 times the discs will increase the fault probability.

I highly recommend RAID6 to anyone with more than 6 standard SATA
drives in a single array. It's actually fairly probable that you will
lose 2 drives in a 72 hour window (say over a long weekend) at some
point.

> Andreas
>>
>> Thanks
>> Peter
>>
>> On 4/4/07, david@lang.hm <david@lang.hm> wrote:
>>> 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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