From: Greg Smith <gsmith@gregsmith.com>
To: PFC <lists@peufeu.com>
Cc: James Mansion <james@mansionfamily.plus.com>
Cc: Laszlo Nagy <gandalf@shopzeus.com>
Cc: pgsql-performance@postgresql.org, Tony Nagy <tony@shopzeus.com>
Subject: Re: Planning a new server - help needed
Date: Sun, 30 Mar 2008 00:45:36 -0400 (EDT)
Message-ID: <Pine.GSO.4.64.0803300031060.5780@westnet.com> (raw)
In-Reply-To: <op.t8r6d212cigqcu@apollo13.peufeu.com>
References: <47ECB4F6.1040901@shopzeus.com>
<Pine.GSO.4.64.0803281300240.2521@westnet.com>
<47EE1B1F.20506@mansionfamily.plus.com>
<op.t8r6d212cigqcu@apollo13.peufeu.com>
On Sat, 29 Mar 2008, PFC wrote:
>> Why do you claim that 'More platters also means slower seeks
>> and generally slower performance.'?
> More platters -> more heads -> heavier head assembly -> slower seek
> time
I recall seeing many designs with more platters that have slower seek
times in benchmarks, and I always presumed this was the reason. That's
the basis for that comment. I'll disclaim that section a bit.
> Actually, now that 8.3 can sync to disk every second instead of at every
> commit, I wonder, did someone do some enlightening benchmarks ?
I've seen some really heavy workloads where using async commit helped
group commits in a larger batches usefully, but I personally haven't found
it to be all that useful if you're already got a caching controller to
accelerate writes on the kinds of hardware most people have. It's a great
solution for situations without a usable write cache though.
> Also, there is a thing called write barriers, which supposedly could be
> used to implement fsync-like behaviour without the penalty, if the disk,
> the OS, the controller, and the filesystem support it (that's a lot of
> ifs)...
The database can't use fsync-like behavior for the things it calls fsync
for; it needs the full semantics. You're either doing the full operation,
or you're cheating and it doesn't do what it's supposed to. Write
barriers aren't any improvement over a good direct I/O sync write setup
for the WAL. There may be some limited value to that approach for the
database writes at checkpoint time, but again there's a real fsync coming
at the end of that and it's not satisfied until everything is on disk (or
in a good disk controller cache).
> Gigabyte should revamp their i-RAM to use ECC RAM of a larger
> capacity... and longer lasting battery backup...
I saw a rumor somewhere that they were close to having a new version of
that using DDR2 ready, which would make it pretty easy to have 8GB on
there.
> I wonder, how many write cycles those Flash drives can take before
> reliability becomes a problem...
The newer SSD drives with good write leveling should last at least as long
as you'd expect a mechanical drive to, even in a WAL application. Lesser
grades of flash used as disk could be a problem though.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
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To: pgsql-performance@postgresql.org
Cc: gsmith@gregsmith.com, lists@peufeu.com, james@mansionfamily.plus.com, gandalf@shopzeus.com, tony@shopzeus.com
Subject: Re: Planning a new server - help needed
In-Reply-To: <Pine.GSO.4.64.0803300031060.5780@westnet.com>
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