Re: raid10 hard disk choice

On Thu, May 21, 2009 at 5:41 PM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> wrote:
> On Thu, May 21, 2009 at 2:29 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Thu, May 21, 2009 at 8:59 AM, Matthew Wakeling <matthew(at)flymine(dot)org> wrote:
>>> On Thu, 21 May 2009, Linos wrote:
>>>>
>>>>        i have to buy a new server and in the budget i have (small) i have
>>>> to select one of this two options:
>>>>
>>>> -4 sas 146gb 15k rpm raid10.
>>>> -8 sas 146gb 10k rpm raid10.
>>>
>>> It depends what you are doing. I think in most situations, the second option
>>> is better, but there may be a few situations where the reverse is true.
>>
>> One possible case of this - I believe that 15K drives will allow you
>> to commit ~250 times per second (15K/60) vs. ~166 times per second
>> (10K/60).  If you have a lot of small write transactions, this might
>> be an issue.
>
> But in a RAID-10 you aggreate pairs like RAID-0, so you could write
> 250(n/2) times per second on 15k where n=4 and 166(n/2) for 10k drives
> where n=8.  So 500 versus 664... ?  Or am I getting it wrong.

Well, that would be true if every write used a different disk, but I
don't think that will be the case in practice. The WAL writes are
very small, so often you'll have multiple writes even to the same
block. But even if they're to different blocks they're likely to be
in the same RAID stripe.

...Robert