Re: Btrfs: broken file system design (was Unbound(?) internal fragmentationin Btrfs)

[Edward Shishkin]

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@xxxxxxxxxx> 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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