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Re: Effects of setting linux block device readahead size

From: "Scott Carey" <scott(at)richrelevance(dot)com>
To: "Mark Wong" <markwkm(at)gmail(dot)com>
Cc: "Greg Smith" <gsmith(at)gregsmith(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>, "Gabrielle Roth" <gorthx(at)gmail(dot)com>, "Selena Deckelmann" <selenamarie(at)gmail(dot)com>
Subject: Re: Effects of setting linux block device readahead size
Date: 2008-09-10 18:09:32
Message-ID: a1ec7d000809101109g7b4cad14j9891a5de8c0574ba@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
I am planning my own I/O tuning exercise for a new DB and am setting up some
fio profiles.  I appreciate the work and will use some of yours as a
baseline to move forward.  I will be making some mixed mode fio profiles and
running our own application and database as a test as well.  I'll focus on
ext3 versus xfs (Linux) and zfs (Solaris) however, and expect to be working
with sequential transfer rates many times larger than your test and am
interested in performance under heavy concurrency -- so the results may
differ quite a bit.

I'll share the info I can.


On Wed, Sep 10, 2008 at 10:38 AM, Mark Wong <markwkm(at)gmail(dot)com> wrote:

> On Wed, Sep 10, 2008 at 9:26 AM, Scott Carey <scott(at)richrelevance(dot)com>
> wrote:
> > How does that readahead tunable affect random reads or mixed random /
> > sequential situations?  In many databases, the worst case scenarios
> aren't
> > when you have a bunch of concurrent sequential scans but when there is
> > enough random read/write concurrently to slow the whole thing down to a
> > crawl.   How the file system behaves under this sort of concurrency
> >
> > I would be very interested in a mixed fio profile with a "background
> writer"
> > doing moderate, paced random and sequential writes combined with
> concurrent
> > sequential reads and random reads.
>
> The data for the other fio profiles we've been using are on the wiki,
> if your eyes can take the strain.  We are working on presenting the
> data in a more easily digestible manner.  I don't think we'll add any
> more fio profiles in the interest of moving on to doing some sizing
> exercises with the dbt2 oltp workload.  We're just going to wrap up a
> couple more scenarios first and get through a couple of conference
> presentations.  The two conferences in particular are the Linux
> Plumbers Conference, and the PostgreSQL Conference: West 08, which are
> both in Portland, Oregon.
>
> Regards,
> Mark
>

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