From mboxrd@z Thu Jan 1 00:00:00 1970 From: Edward Shishkin Subject: Re: Btrfs: broken file system design (was Unbound(?) internal fragmentation in Btrfs) Date: Fri, 18 Jun 2010 18:22:39 +0200 Message-ID: <4C1B9D4F.6010008__1901.41732241518$1276878248$gmane$org@gmail.com> References: <4C07C321.8010000@redhat.com> <4C1B7560.1000806@gmail.com> <20100618134755.GG27466@think> <4C1B8B4A.9060308@gmail.com> <20100618151017.GN27466@think> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed To: Chris Mason , Edward Shishkin , Mat , LKML , linux-fsdevel@vger.kernel.org, Ric Return-path: In-Reply-To: <20100618151017.GN27466@think> List-ID: 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 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..