linux-ext4 - Re: inconsistent file placement

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Message-ID: <4C337D16.9000200@redhat.com>
Date:	Tue, 06 Jul 2010 13:59:34 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	tytso@....edu
CC:	Daniel Taylor <Daniel.Taylor@....com>, linux-ext4@...r.kernel.org
Subject: Re: inconsistent file placement

tytso@....edu wrote:
> On Mon, Jul 05, 2010 at 06:49:34PM -0700, Daniel Taylor wrote:
>> I realize that it is enerally not a good idea to tune
>> an operating system, or subsystem, for benchmarking, but
>> there's something that I don't understand about ext[234]
>> that is badly affecting our product.  File placement on
>> newly-created file systems is inconsistent.  I can't,
>> yet, call it a bug, but I really need to understand what
>> is happening, and I cannot find, in the source code, the
>> source of the randomization (related to "goal"???).
> 
> In ext3, it really is random.  The randomness you're looking for can
> be found in fs/ext3/ialloc.c:find_group_orlov(), when it calls
> get_random_bytes().  This is responsible for "spreading" directories
> so they are spread across the block groups, to try to prevent
> fragmented files.  Yes, if all you care about is benchmarks which only
> use 10% of the entire file system, and for which the benchmarks don't
> adequately simulate file system aging, the algorithms in ext3 will
> cause a lot of variability.

However, from the test description it looks like it is writing
a file to the root dir, so there should be no parent-dir random spreading,
right?

-Eric

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