linux-ext4 - Re: [PATCH 2/2] Add batched discard support for ext4

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Message-ID: <alpine.LFD.2.00.1004262038310.29999@localhost>
Date:	Mon, 26 Apr 2010 20:42:58 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
cc:	Jeff Moyer <jmoyer@...hat.com>, Ric Wheeler <rwheeler@...hat.com>,
	Jan Kara <jack@...e.cz>,
	Greg Freemyer <greg.freemyer@...il.com>,
	Eric Sandeen <sandeen@...hat.com>,
	Mark Lord <kernel@...savvy.com>, linux-ext4@...r.kernel.org,
	Edward Shishkin <eshishki@...hat.com>,
	Eric Sandeen <esandeen@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 2/2] Add batched discard support for ext4 - using
 rbtree

On Mon, 26 Apr 2010, Lukas Czerner wrote:

> Create an ioctl which walks through all the free extents in each
> allocating group and discard those extents. As an addition to improve
> its performance one can specify minimum free extent length, so ioctl
> will not bother with shorter extents.
> 
> This of course means, that with each invocation the ioctl must walk
> through whole file system, checking and discarding free extents, which
> is not very efficient. The best way to avoid this is to keep track of
> deleted (freed) blocks. Then the ioctl have to trim just those free
> extents which were recently freed.
> 
> In order to implement this I have created new structure
> ext4_deleted_data which represents deleted extent in per-group rbtree.
> When blocks are freed, extents are stored in the tree (possibly merged
> with other extents). The ioctl then can walk through the tree, take out
> extents from the tree, compare them with the free extents and possibly
> trim them.
> 
> Note that there is support for setting minimum extent length in ioctl
> call, so it will not bother with shorter extents. Also, when the
> previously deleted range is taken from the tree and it is not entirely
> free, the free fragments are discarded and extents shorter than minlen
> are NOT returned back to the tree to avoid fragmentation of the tree
> which could lead to the big memory consumption.
> 
> But you may notice, that there is one problem. bb_bitmap_deleted does
> not survive umount. To bypass the problem the first ioctl call have to
> walk through whole file system trimming all free extents.
> 
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>

For now it just ignores the small extents to avoid fragmentation. As I
said before, I agree that they should not be ignored, I just need to
figure out the way to do this efficiently. 

Also it was not properly tested yet.


-Lukas.
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