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Date: Fri, 18 Jun 2010 20:15:34 +0200 From: Christian Stroetmann <stroetmann@...olab.com> To: Daniel J Blueman <daniel.blueman@...il.com> CC: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org, linux-btrfs@...r.kernel.org Subject: Re: Btrfs: broken file system design (was Unbound(?) internal fragmentation in Btrfs) Daniel J Blueman wrote: > On Fri, Jun 18, 2010 at 1:32 PM, Edward Shishkin > <edward.shishkin@...il.com> 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". >>>> >>>> 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. > I find this is somehow off-topic, but: For sure, it isn't impossible. History showed and present shows that there are exceptions. > 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 > Have fun Christian Stroetmann -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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