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

[Daniel J Blueman <daniel.blueman@gmail.com>]

On Fri, Jun 18, 2010 at 1:32 PM, Edward Shishkin
<edward.shishkin@gmail.com> 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".
>>>
>>> Internal fragmentation (see Appendix A) of those 5 leafs is
>>> (1572+3892+1901+3666+1675)/4096*5 = 0.62. This is even worse then
>>> ext4 and xfs: The last ones in this example will show fragmentation
>>> near zero with blocksize <= 2K. Even with 4K blocksize they will
>>> show better utilization 0.50 (against 0.38 in btrfs)!
>>>
>>> I have a small question for btrfs developers: Why do you folks put
>>> "inline extents", xattr, etc items of variable size to the B-tree
>>> in spite of the fact that B-tree is a data structure NOT for variable
>>> sized records? This disadvantage of B-trees was widely discussed.
>>> For example, maestro D. Knuth warned about this issue long time
>>> ago (see Appendix C).
>>>
>>> It is a well known fact that internal fragmentation of classic Bayer's
>>> B-trees is restricted by the value 0.50 (see Appendix C). However it
>>> takes place only if your tree contains records of the _same_ length
>>> (for example, extent pointers). Once you put to your B-tree records
>>> of variable length (restricted only by leaf size, like btrfs "inline
>>> extents"), your tree LOSES this boundary. Moreover, even worse:
>>> it is clear, that in this case utilization of B-tree scales as zero(!).
>>> That said, for every small E and for every amount of data N we
>>> can construct a consistent B-tree, which contains data N and has
>>> utilization worse then E. I.e. from the standpoint of utilization
>>> such trees can be completely degenerated.
>>>
>>> That said, the very important property of B-trees, which guarantees
>>> non-zero utilization, has been lost, and I don't see in Btrfs code any
>>> substitution for this property. In other words, where is a formal
>>> guarantee that all disk space of our users won't be eaten by internal
>>> fragmentation? I consider such guarantee as a *necessary* condition
>>> for putting a file system to production.

Wow...a small part of me says 'well said', on the basis that your
assertions are true, but I do think there needs to be more
constructivity in such critique; it is almost impossible to be a great
engineer and a great academic at once in a time-pressured environment.

If you can produce some specific and suggestions with code references,
I'm sure we'll get some good discussion with potential to improve from
where we are.

Thanks,
  Daniel
-- 
Daniel J Blueman