From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH 0/5 v2] add extent status tree caching
Date: Thu, 18 Jul 2013 20:07:38 -0400
Message-ID: <20130719000738.GD17938@thunk.org>
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 <51E8356C.9030603@redhat.com>
 <20130718235451.GA29997@gmail.com>
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On Fri, Jul 19, 2013 at 07:54:51AM +0800, Zheng Liu wrote:
> 
> I have talked with my colleague who is a MySQL contributor about whether
> MySQL tries to preallocate some files or not.  As far as I know, at
> least MySQL doesn't try to do it until now.  I don't have the source
> code of Oracle or DB2, these giant databases might use preallocation I
> guess.

Oracle and DB2 don't use preallocate, because they don't want the
metadata update overhead.  So for software packages that are really
critically worried about 99percentile latency, they will generally
either pre-zero the file ahead of time, so all of the extents are
written.  Or, they will use the out-of-tree nohidestale patch, and
mark all of the extents as written.  (If you are doing A/B benchmark
comparisons, using nohidestale means the setup overhead for each
benchmark run can be measured in minutes instead of hours...)

On at least one of the enterprise databases which I'm familiar with,
they don't pre-zero the entire database file, but they'll do it in
chunks of N megabytes.  That means they don't have the huge time lag
when the database is initially created, but then every so often, when
the database will suddenly use most of the disk bandwidth to zero the
next chunk of 16 or 32 or 64 megabytes.  (This tends to do a real
number on your 99.9 percentile latency numbers, if you care about such
things....)

						- Ted