linux-ext4 - [RFC PATCH 0/1] ext4: Try to better reuse recently freed space

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Date:	Thu,  4 Jul 2013 11:11:53 +0200
From:	Lukas Czerner <lczerner@...hat.com>
To:	linux-ext4@...r.kernel.org
Cc:	enwlinux@...il.com, Jose_Mario_Gallegos@...l.com,
	jordan_hargrave@...l.com, rwheeler@...hat.com
Subject: [RFC PATCH 0/1] ext4: Try to better reuse recently freed space

Hi all,

as I just recently discovered here http://www.ogris.de/blkalloc/ ext4 have
some unexpected allocation strategies which can cause some problems in
certain scenarios (see the 1/1 patch).

This is my attempt to fix this, however I think that this will need some
discussion because it changes some of the block allocator heuristic
quite significantly and I would like to make sure that it will not hurt
our performance in some widely used workloads. Eric, could you run your
performance testing with your test suite to see how it performs ?

Also, I think that the old behaviour might be quite helpful for SSD because
we're not overwriting existing blocks but rather trying to use new one, so
the firmware should have easier job. However I do not know how significant
impact if at all it might have. If we find out that this is helpful for
SSD we might make this conditional depending on the type of the device.

Which brings me to the other problem. Since the original behaviour is really
bad for thinly provisioned devices we would like to avoid using it even if
the underlying storage is SSD, however IIRC there is no way to distinguish
this from within the kernel.

See the comparison between the old and new allocator behaviour
http://people.redhat.com/lczerner/allocator/

Comments and testing is welcomed.

Thanks!
-Lukas
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