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From: Edward Shishkin <edward.shishkin@gmail.com>
To: Chris Mason <chris.mason@oracle.com>,
	Edward Shishkin <edward.shishkin@gmail.com>,
	Mat <jackdachef@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, Ric
Subject: Re: Btrfs: broken file system design (was Unbound(?) internal fragmentation in Btrfs)
Date: Fri, 18 Jun 2010 18:22:39 +0200
Message-ID: <4C1B9D4F.6010008__1901.41732241518$1276878248$gmane$org@gmail.com> (raw)
In-Reply-To: <20100618151017.GN27466@think>

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@redhat.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..

  reply index

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-06-03 14:58 Unbound(?) internal fragmentation in Btrfs Edward Shishkin
     [not found] ` <AANLkTilKw2onQkdNlZjg7WVnPu2dsNpDSvoxrO_FA2z_@mail.gmail.com>
2010-06-18  8:03   ` Christian Stroetmann
2010-06-18 13:32   ` Btrfs: broken file system design (was Unbound(?) internal fragmentation in Btrfs) Edward Shishkin
2010-06-18 13:45     ` Daniel J Blueman
2010-06-18 16:50       ` Edward Shishkin
2010-06-23 23:40         ` Jamie Lokier
2010-06-24  3:43           ` Daniel Taylor
2010-06-24  4:51             ` Mike Fedyk
2010-06-24 22:06               ` Daniel Taylor
2010-06-25  9:15                 ` Btrfs: broken file system design Andi Kleen
2010-06-25 18:58                 ` Btrfs: broken file system design (was Unbound(?) internal fragmentation in Btrfs) Ric Wheeler
2010-06-26  5:18                   ` Michael Tokarev
2010-06-26 11:55                     ` Ric Wheeler
     [not found]                     ` <57784.2001:5c0:82dc::2.1277555665.squirrel@www.tofubar.com>
2010-06-26 13:47                       ` Ric Wheeler
2010-06-24  9:50             ` David Woodhouse
2010-06-18 18:15       ` Christian Stroetmann
2010-06-18 13:47     ` Chris Mason
2010-06-18 15:05       ` Edward Shishkin
     [not found]       ` <4C1B8B4A.9060308@gmail.com>
2010-06-18 15:10         ` Chris Mason
2010-06-18 16:22           ` Edward Shishkin [this message]
     [not found]           ` <4C1B9D4F.6010008@gmail.com>
2010-06-18 18:10             ` Chris Mason
2010-06-18 15:21       ` Christian Stroetmann
2010-06-18 15:22         ` Chris Mason
2010-06-18 15:56     ` Jamie Lokier
2010-06-18 19:25       ` Christian Stroetmann
2010-06-18 19:29       ` Edward Shishkin
2010-06-18 19:35         ` Chris Mason
2010-06-18 22:04           ` Balancing leaves when walking from top to down (was Btrfs:...) Edward Shishkin
     [not found]           ` <4C1BED56.9010300@redhat.com>
2010-06-18 22:16             ` Ric Wheeler
2010-06-19  0:03               ` Edward Shishkin
2010-06-21 13:15             ` Chris Mason
     [not found]               ` <20100621180013.GD17979@think>
2010-06-22 14:12                 ` Edward Shishkin
2010-06-22 14:20                   ` Chris Mason
2010-06-23 13:46                     ` Edward Shishkin
     [not found]                     ` <4C221049.501@gmail.com>
2010-06-23 23:37                       ` Jamie Lokier
2010-06-24 13:06                         ` Chris Mason
2010-06-30 20:05                           ` Edward Shishkin
     [not found]                           ` <4C2BA381.7040808@redhat.com>
2010-06-30 21:12                             ` Chris Mason
2010-07-09  4:16                 ` Chris Samuel
2010-07-09 20:30                   ` Chris Mason
2010-06-23 23:57         ` Btrfs: broken file system design (was Unbound(?) internal fragmentation in Btrfs) Jamie Lokier

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