On 7/8/10 2:18 PM, Timothy.Noonan@xxxxxxx wrote:
How does the linux machine know that there is a BBU installed and to
change its behavior or change the behavior of Postgres? I am
experiencing performance issues, not with searching but more with IO.
It doesn't. It trusts the disk controller. Linux says, "Flush your cache" and the controller says, "OK, it's flushed." In the case of a BBU controller, the controller can say that almost instantly because it's got the data in a battery-backed memory that will survive even if the power goes out. In the case of a non-BBU controller (RAID or non-RAID), the controller has to actually wait for the head to move to the right spot, then wait for the disk to spin around to the right sector, then write the data. Only then can it say, "OK, it's flushed."
So to Linux, it just appears to be a disk that's exceptionally fast at flushing its buffers.
Craig
-----Original Message-----
From: pgsql-performance-owner@xxxxxxxxxxxxxx
[mailto:pgsql-performance-owner@xxxxxxxxxxxxxx] On Behalf Of Craig James
Sent: Thursday, July 08, 2010 4:02 PM
To: pgsql-performance@xxxxxxxxxxxxxx
Subject: Re: performance on new linux box
On 7/8/10 12:47 PM, Ryan Wexler wrote:
On Thu, Jul 8, 2010 at 12:46 PM, Kevin Grittner
<Kevin.Grittner@xxxxxxxxxxxx<mailto:Kevin.Grittner@xxxxxxxxxxxx>>
wrote:
Ryan Wexler<ryan@xxxxxxxxxxxxxxxx<mailto:ryan@xxxxxxxxxxxxxxxx>>
wrote:
> One thing I don't understand is why BBU will result in a huge
> performance gain. I thought BBU was all about power failures?
Well, it makes it safe for the controller to consider the write
complete as soon as it hits the RAM cache, rather than waiting for
persistence to the disk itself. It can then schedule the writes
in
a manner which is efficient based on the physical medium.
Something like this was probably happening on your non-server
machines, but without BBU it was not actually safe. Server class
machines tend to be more conservative about not losing your data,
but without a RAID controller with BBU cache, that slows writes
down
to the speed of the rotating disks.
-Kevin
Thanks for the explanations that makes things clearer. It still
amazes
me that it would account for a 5x change in IO.
It's not exactly a 5x change in I/O, rather it's a 5x change in
*transactions*. Without a BBU Postgres has to wait for each transaction
to by physically written to the disk, which at 7200 RPM (or 10K or 15K)
means a few hundred per second. Most of the time Postgres is just
sitting there waiting for the disk to say, "OK, I did it." With BBU,
once the RAID card has the data, it's virtually guaranteed it will get
to the disk even if the power fails, so the RAID controller says, "OK, I
did it" even though the data is still in the controller's cache and not
actually on the disk.
It means there's no tight relationship between the disk's rotational
speed and your transaction rate.
Craig
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