linux-kernel - Re: Btrfs: broken file system design (was Unbound(?) internal fragmentation in Btrfs)

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Date:	Fri, 18 Jun 2010 18:22:39 +0200
From:	Edward Shishkin <edward.shishkin@...il.com>
To:	Chris Mason <chris.mason@...cle.com>,
	Edward Shishkin <edward.shishkin@...il.com>,
	Mat <jackdachef@...il.com>, LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, Ric Wheeler <rwheeler@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	The development of BTRFS <linux-btrfs@...r.kernel.org>
Subject: Re: Btrfs: broken file system design (was Unbound(?) internal fragmentation
 in Btrfs)

Chris Mason wrote:
> On Fri, Jun 18, 2010 at 05:05:46PM +0200, Edward Shishkin wrote:
>   
>> Chris Mason wrote:
>>     
>>> On Fri, Jun 18, 2010 at 03:32:16PM +0200, Edward Shishkin wrote:
>>>       
>>>> Mat wrote:
>>>>         
>>>>> On Thu, Jun 3, 2010 at 4:58 PM, Edward Shishkin <edward@...hat.com> wrote:
>>>>>           
>>>>>> Hello everyone.
>>>>>>
>>>>>> I was asked to review/evaluate Btrfs for using in enterprise
>>>>>> systems and the below are my first impressions (linux-2.6.33).
>>>>>>
>>>>>> The first test I have made was filling an empty 659M (/dev/sdb2)
>>>>>> btrfs partition (mounted to /mnt) with 2K files:
>>>>>>
>>>>>> # for i in $(seq 1000000); \
>>>>>> do dd if=/dev/zero of=/mnt/file_$i bs=2048 count=1; done
>>>>>> (terminated after getting "No space left on device" reports).
>>>>>>
>>>>>> # ls /mnt | wc -l
>>>>>> 59480
>>>>>>
>>>>>> So, I got the "dirty" utilization 59480*2048 / (659*1024*1024) = 0.17,
>>>>>> and the first obvious question is "hey, where are other 83% of my
>>>>>> disk space???" I looked at the btrfs storage tree (fs_tree) and was
>>>>>> shocked with the situation on the leaf level. The Appendix B shows
>>>>>> 5 adjacent btrfs leafs, which have the same parent.
>>>>>>
>>>>>> For example, look at the leaf 29425664: "items 1 free space 3892"
>>>>>> (of 4096!!). Note, that this "free" space (3892) is _dead_: any
>>>>>> attempts to write to the file system will result in "No space left
>>>>>> on device".
>>>>>>             
>>> There are two easy ways to fix this problem.  Turn off the inline
>>> extents (max_inline=0) or allow splitting of the inline extents.  I
>>> didn't put in the splitting simply because the complexity was high while
>>> the benefits were low (in comparison with just turning off the inline
>>> extents).
>>>       
>> Hello, Chris. Thanks for response!
>> I afraid that both ways won't fix the problem. Look at this leaf:
>>
>> [...]
>> leaf 29425664 items 1 free space 3892 generation 8 owner 5
>> fs uuid 50268d9d-2a53-4f4d-b3a3-4fbff74dd956
>> chunk uuid 963ba49a-bb2b-48a3-9b35-520d857aade6
>>        item 0 key (320 XATTR_ITEM 3817753667) itemoff 3917 itemsize 78
>>                location key (0 UNKNOWN 0) type 8
>>                namelen 16 datalen 32 name: security.selinux
>> [...]
>>
>> There is no inline extents, and what are you going to split here?
>> All leafs must be at least a half filled, otherwise we loose all
>> boundaries, which provides non-zero utilization..
>>     
>
> Right, there is no inline extent because we require them to fit entirely
> in the leaf.  So we end up with mostly empty leaves because the inline
> item is large enough to make it difficult to push around but not large
> enough to fill the leaf.
>   

How about left and right neighbors? They contain a lot of
free space (1572 and 1901 respectively).
I am not happy with the very fact of such shallow leafs which
contain only one small (xattr) item..
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