Re: WAL+Os on a single disk

On Thu, Jun 24, 2010 at 5:14 AM, Matthew Wakeling <matthew@flymine.org> wrote:
> On Wed, 23 Jun 2010, Scott Marlowe wrote:
>>>
>>> We have a 12 x 600G hot swappable disk system (raid 10)
>>> and 2 internal disk  ( 2x 146G)
>>>
>>> Does it make sense to put the WAL and OS on the internal disks
>>
>> So for us, the WAL and OS and logging on the same data set works well.
>
> Generally, it is recommended that you put the WAL onto a separate disc to
> the data. However, in this case, I would be careful. It may be that the 12
> disc array is more capable. Specifically, it is likely that the 12-disc
> array has a battery backed cache, but the two internal drives (RAID 1
> presumably) do not. If this is the case, then putting the WAL on the
> internal drives will reduce performance, as you will only be able to commit
> a transaction once per revolution of the internal discs. In contrast, if the
> WAL is on a battery backed cache array, then you can commit much more
> frequently.

This is not strictly true of the WAL, which writes sequentially and
more than one transaction at a time.  As you said though, test it to
be sure.