Re: How to improve db performance with $7K? - Mailing list pgsql-performance

I've been doing some reading up on this, trying to keep up here,
and have found out that (experts, just yawn and cover your ears)

1) some SATA drives (just type II, I think?) have a "Phase Zero"
    implementation of Tagged Command Queueing (the special sauce
    for SCSI).
2) This SATA "TCQ" is called NCQ and I believe it basically
    allows the disk software itself to do the reordering
    (this is called "simple" in TCQ terminology) It does not
    yet allow the TCQ "head of queue" command, allowing the
    current tagged request to go to head of queue, which is
    a simple way of manifesting a "high priority" request.

3) SATA drives are not yet multi-initiator?

Largely b/c of 2 and 3, multi-initiator SCSI RAID'ed drives
are likely to whomp SATA II drives for a while yet (read: a
year or two) in multiuser PostGres applications.



-----Original Message-----
From: pgsql-performance-owner@postgresql.org [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of Greg Stark
Sent: Thursday, April 14, 2005 2:04 PM
To: Kevin Brown
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] How to improve db performance with $7K?


Kevin Brown <kevin@sysexperts.com> writes:

> Greg Stark wrote:
>
>
> > I think you're being misled by analyzing the write case.
> >
> > Consider the read case. When a user process requests a block and
> > that read makes its way down to the driver level, the driver can't
> > just put it aside and wait until it's convenient. It has to go ahead
> > and issue the read right away.
>
> Well, strictly speaking it doesn't *have* to.  It could delay for a
> couple of milliseconds to see if other requests come in, and then
> issue the read if none do.  If there are already other requests being
> fulfilled, then it'll schedule the request in question just like the
> rest.

But then the cure is worse than the disease. You're basically describing exactly what does happen anyways, only you're
delayingmore requests than necessary. That intervening time isn't really idle, it's filled with all the requests that
weredelayed during the previous large seek... 

> Once the first request has been fulfilled, the driver can now schedule
> the rest of the queued-up requests in disk-layout order.
>
> I really don't see how this is any different between a system that has
> tagged queueing to the disks and one that doesn't.  The only
> difference is where the queueing happens.

And *when* it happens. Instead of being able to issue requests while a large seek is happening and having some of them
satisfiedthey have to wait until that seek is finished and get acted on during the next large seek. 

If my theory is correct then I would expect bandwidth to be essentially equivalent but the latency on SATA drives to be
increasedby about 50% of the average seek time. Ie, while a busy SCSI drive can satisfy most requests in about 10ms a
busySATA drive would satisfy most requests in 15ms. (add to that that 10k RPM and 15kRPM SCSI drives have even lower
seektimes and no such IDE/SATA drives exist...) 

In reality higher latency feeds into a system feedback loop causing your application to run slower causing bandwidth
demandsto be lower as well. It's often hard to distinguish root causes from symptoms when optimizing complex systems. 

--
greg


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