Re: Bad iostat numbers

On Dec 5, 2006, at 8:54 PM, Greg Smith wrote:

> On Tue, 5 Dec 2006, Craig A. James wrote:
>
>> I'm not familiar with the inner details of software RAID, but the
>> only circumstance I can see where things would get corrupted is if
>> the RAID driver writes a LOT of blocks to one disk of the array
>> before synchronizing the others...
>
> You're talking about whether the discs in the RAID are kept
> consistant. While it's helpful with that, too, that's not the main
> reason a the battery-backed cache is so helpful. When PostgreSQL
> writes to the WAL, it waits until that data has really been placed
> on the drive before it enters that update into the database. In a
> normal situation, that means that you have to pause until the disk
> has physically written the blocks out, and that puts a fairly low
> upper limit on write performance that's based on how fast your
> drives rotate. RAID 0, RAID 1, none of that will speed up the time
> it takes to complete a single synchronized WAL write.
>
> When your controller has a battery-backed cache, it can immediately
> tell Postgres that the WAL write completed succesfully, while
> actually putting it on the disk later. On my systems, this results
> in simple writes going 2-4X as fast as they do without a cache.
> Should there be a PC failure, as long as power is restored before
> the battery runs out that transaction will be preserved.
>
> What Alex is rightly pointing out is that a software RAID approach
> doesn't have this feature. In fact, in this area performance can
> be even worse under SW RAID than what you get from a single disk,
> because you may have to wait for multiple discs to spin to the
> correct position and write data out before you can consider the
> transaction complete.

So... the ideal might be a RAID1 controller with BBU for the WAL and
something else, such as software RAID, for the main data array?

Cheers,
Steve