Messages in this thread | | | Date | Mon, 1 Nov 2010 17:58:52 +0200 | Subject | Re: Horrible btrfs performance due to fragmentation | From | Felipe Contreras <> |
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On Mon, Nov 1, 2010 at 12:47 AM, Hugo Mills <hugo-lkml@carfax.org.uk> wrote: > On Mon, Nov 01, 2010 at 12:36:58AM +0200, Felipe Contreras wrote: >> On Mon, Nov 1, 2010 at 12:25 AM, cwillu <cwillu@cwillu.com> wrote: >> > btrfs fi defrag isn't recursive. "btrfs filesystem defrag /home" will >> > defragment the space used to store the folder, without touching the >> > space used to store files in that folder. >> >> Yes, that came up on the IRC, but: >> >> 1) It doesn't make sense: "btrfs filesystem" doesn't allow a fileystem >> as argument? Why would anyone want it to be _non_ recursive? > > You missed the subsequent discussion on IRC about the interaction > of COW with defrag. Essentially, if you've got two files that are COW > copies of each other, and one has had something written to it since, > it's *impossible* for both files to be defragmented, without making a > full copy of both: > > Start with a file (A, etc are data blocks on the disk): > > file1 = ABCDEF > > Cow copy it: > > file1 = ABCDEF > file2 = ABCDEF > > Now write to one of them: > > file1 = ABCDEF > file2 = ABCDxF > > So, either file1 is contiguous, and file2 is fragmented (with the > block x somewhere else on disk), or file2 is contiguous, and file1 is > fragmented (with E somewhere else on disk). In fact, we've determined > by experiment that when you defrag a file that's sharing blocks with > another one, the file gets copied in its entirety, thus separating the > blocks of the file and its COW duplicate.
Ok, but the fragmentation would not be an issue in this case.
>> 2) The filesystem should not degrade performance so horribly no matter >> how long the it has been used. Even git has automatic garbage >> collection. > > Since, I believe, btrfs uses COW very heavily internally for > ensuring consistency, you can end up with fragmenting files and > directories very easily. You probably need some kind of scrubber that > goes looking for non-COW files that are fragmented, and defrags them > in the background.
Or when going through all the fragments of a file, have a counter, and if it exceeds certain limit mark it somehow, so that it gets defragmented at least to a certain extent.
-- Felipe Contreras -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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