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

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Date:	Mon, 1 Nov 2010 00:36:58 +0200
From:	Felipe Contreras <felipe.contreras@...il.com>
To:	cwillu <cwillu@...llu.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 Mon, Nov 1, 2010 at 12:25 AM, cwillu <cwillu@...llu.com> wrote:
> 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.

Yes, that came up on the IRC, but:

1) It doesn't make sense: "btrfs filesystem" doesn't allow a fileystem
as argument? Why would anyone want it to be _non_ recursive?

2) The filesystem should not degrade performance so horribly no matter
how long the it has been used. Even git has automatic garbage
collection.

-- 
Felipe Contreras
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