On Fri, Jun 18, 2010 at 1:32 PM, Edward Shishkin
<edward.shishkin@xxxxxxxxx> 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".
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