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

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Date:	Fri, 18 Jun 2010 14:10:11 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Edward Shishkin <edward.shishkin@...il.com>
Cc:	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)

On Fri, Jun 18, 2010 at 06:22:39PM +0200, Edward Shishkin wrote:
> 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..

Sure, the balancing can also be made more aggressive.  This should be
very easy to fix.

-chris

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