On Fri, 12 Sep 2008, James Mansion wrote:
Scott Carey wrote:
Consumer drives will often read-ahead much more than server drives
optimized for i/o per second.
...
The Linux readahead setting is _definitely_ in the kernel, definitely uses
and fills the page cache, and from what I can gather, simply issues extra
I/O's to the hardware beyond the last one requested by an app in certain
situations. It does not make your I/O request larger, it just queues an
extra I/O following your request.
So ... fiddling with settings in Linux is going to force read-ahead, but the
read-ahead data will hit the controller cache and the system buffers.
And the drives use their caches for cyclinder caching implicitly (maybe the
SATA drives appear to preread more because the storage density per cylinder
is higher?)..
But is there any way for an OS or application to (portably) ask SATA, SAS or
SCSI drives to read ahead more (or less) than their default and NOT return
the data to the controller?
I've never heard of such a thing, but I'm no expert in the command sets for
any of this stuff.
I'm pretty sure that's not possible. the OS isn't supposed to even know
the internals of the drive.
David Lang
James
On Thu, Sep 11, 2008 at 12:54 PM, James Mansion
<james@xxxxxxxxxxxxxxxxxxxxxx <mailto:james@xxxxxxxxxxxxxxxxxxxxxx>> wrote:
Greg Smith wrote:
The point I was trying to make there is that even under
impossibly optimal circumstances, you'd be hard pressed to
blow out the disk's read cache with seek-dominated data even
if you read a lot at each seek point. That idea didn't make
it from my head into writing very well though.
Isn't there a bigger danger in blowing out the cache on the
controller and causing premature pageout of its dirty pages?
If you could get the readahead to work on the drive and not return
data to the controller, that might be dandy, but I'm sceptical.
James
-- Sent via pgsql-performance mailing list
(pgsql-performance@xxxxxxxxxxxxxx
<mailto:pgsql-performance@xxxxxxxxxxxxxx>)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
