linux-kernel - Re: Horrible btrfs performance due to fragmentation

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Date:	Sun, 31 Oct 2010 16:25:41 -0600
From:	cwillu <cwillu@...llu.com>
To:	Felipe Contreras <felipe.contreras@...il.com>
Cc:	Calvin Walton <calvin.walton@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-btrfs@...r.kernel.org, Chris Mason <chris.mason@...cle.com>
Subject: Re: Horrible btrfs performance due to fragmentation

On Sun, Oct 31, 2010 at 1:58 PM, Felipe Contreras
<felipe.contreras@...il.com> wrote:
> On Mon, Oct 11, 2010 at 6:46 PM, Calvin Walton <calvin.walton@...il.com> wrote:
>> On Mon, 2010-10-11 at 03:30 +0300, Felipe Contreras wrote:
>>> I use btrfs on most of my volumes on my laptop, and I've always felt
>>> booting was very slow, but definitely sure is slow, is starting up
>>> Google Chrome:
>>>
>>> encrypted ext4: ~20s
>>> btrfs: ~2:11s
>>>
>>> I have tried different things to find out exactly what is the issue,
>>> but haven't quite found it yet.
>>
>> If you've been using this volume for a while, it could just have become
>> badly fragmented. You could try btrfs's fancy online defragmentation
>> abilities to see if that'll give you an improvement:
>>
>> # btrfs filesystem defragment /mountpoint/of/volume
>>
>> Let us know if that helps, of course :)
>
> I finally managed to track down this issue. Indeed the fragmentation
> is horrible, and 'btrfs filesystem defragment' doesn't help:
>
> % cat History-old > History
> % btrfs filesystem defragment /home
> % echo 3 > /proc/sys/vm/drop_caches
>
> % time dd if=History of=/dev/null && time dd if=History-old of=/dev/null
> 109664+0 records in
> 109664+0 records out
> 56147968 bytes (56 MB) copied, 1.90015 s, 29.5 MB/s
> dd if=History of=/dev/null  0.08s user 0.29s system 15% cpu 2.458 total
> 109664+0 records in
> 109664+0 records out
> 56147968 bytes (56 MB) copied, 97.772 s, 574 kB/s
> dd if=History-old of=/dev/null  0.07s user 0.80s system 0% cpu 1:37.79 total
>
> I think this is a serious issue that *must* be fixed for 1.0. I filed
> a bug for this:
> https://bugzilla.kernel.org/show_bug.cgi?id=21562

btrfs fi defrag isn't recursive.  "btrfs filesystem defrag /home" will
defragment the space used to store the folder, without touching the
space used to store files in that folder.
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