Re: [GENERAL] Slow PITR restore

On Thu, 13 Dec 2007, Gregory Stark wrote:

> Note that even though the processor is 99% in wait state the drive is 
> only handling about 3 MB/s. That translates into a seek time of 2.2ms 
> which is actually pretty fast...But note that if this were a raid array 
> Postgres's wouldn't be getting any better results. A Raid array wouldn't 
> improve i/o latency at all and since it's already 99% waiting for i/o 
> Postgres is not going to be able to issue any more.

If it's a straight stupid RAID array, sure.  But when you introduce a good 
write caching controller into the mix, that can batch multiple writes, 
take advantage of more elevator sorting, and get more writes/seek 
accomplished.  Combine that improvement with having multiple drives as 
well and the PITR performance situation becomes very different; you really 
can get more than one drive in the array busy at a time.  It's also true 
that you won't see everything that's happening with vmstat because the 
controller is doing the low-level dispatching.

I'll try to find time to replicate the test Tom suggested, as I think my 
system is about middle ground between his and Joshua's.  In general I've 
never been able to get any interesting write throughput testing at all 
without at least a modest caching controller in there.  Just like Tom's 
results, with a regular 'ole drive everything gets seek bottlenecked, WIO 
goes high, and it looks like I've got all the CPU in the world.  I run a 
small Areca controller with 3 drives on it (OS+DB+WAL) at home to at least 
get close to a real server.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD