On Sun, May 06, 2012 at 01:07:06PM +1000, Mike Sampson wrote:
> On Sat, May 5, 2012 at 10:52 PM, Hugo Mills <h...@carfax.org.uk> wrote:
> > On Sat, May 05, 2012 at 10:37:01PM +1000, Mike Sampson wrote:
> >> Hello list,
> >>
> >> recently reformatted my home partition from XFS to RAID1 btrfs. I used
> >> the default options to mkfs.btrfs except for enabling raid1 for data
> >> as well as metadata. Filesystem is made up of two 1TB drives.
> >>
> >> mike@mercury (0) pts/3 ~ $ sudo btrfs filesystem show
> >> Label: none uuid: f08a8896-e03e-4064-9b94-9342fb547e47
> >> Total devices 2 FS bytes used 888.06GB
> >> devid 1 size 931.51GB used 931.51GB path /dev/sdb1
> >> devid 2 size 931.51GB used 931.49GB path /dev/sdc1
> >>
> >> Btrfs Btrfs v0.19
> >>
> >> mike@mercury (0) pts/3 ~ $ btrfs filesystem df /home
> >> Data, RAID1: total=893.48GB, used=886.60GB
> >> Data: total=8.00MB, used=0.00
> >> System, RAID1: total=8.00MB, used=136.00KB
> >> System: total=4.00MB, used=0.00
> >> Metadata, RAID1: total=38.00GB, used=2.54GB
> >> Metadata: total=8.00MB, used=0.00
> >>
> >> As can be seen I don't have a lot of free space left and while I am
> >> planning on adding more storage soon I would like to gain a little
> >> breathing room until I can do this. While I don't have a lot of space
> >> remaining in Data, RAID1 I do have a good chunk in Metadata, RAID1.
> >> 2.5GB used out of 38GB. Does this excess become available
> >> automatically to the file system when the block groups in Data, RAID1
> >> are exhausted or, if not, is there a way to manually reallocate them?
> >
> > Youre best bet at the moment is to try a partial balance of
> > metadata chunks:
> >
> > # btrfs balance start -m /home
> >
> > That will rewrite all of your metadata, putting it through the
> > allocator again, and removing the original allocated chunks. This
> > should have the effect of reducing the allocation of metadata chunks.
> >
> > You will need a 3.3 kernel, or later, and an up-to-date userspace
> > from cmason's git repository.
>
> Gave this a shot and it did help.
>
> mike@mercury (0) pts/3 ~/.../btrfs-progs git:master $ uname -r
> 3.3.4-2-ARCH
>
> mike@mercury (0) pts/3 ~/.../btrfs-progs git:master $ sudo ./btrfs
> balance start -m /home
>
> Done, had to relocate 40 out of 934 chunks
>
> mike@mercury (0) pts/3 ~/.../btrfs-progs git:master $ ./btrfs
> filesystem df /home
> Data, RAID1: total=900.97GB, used=880.06GB
> Data: total=8.00MB, used=0.00
> System, RAID1: total=32.00MB, used=136.00KB
> System: total=4.00MB, used=0.00
> Metadata, RAID1: total=30.50GB, used=2.54GB
>
> There is now 8GB less in Metadata and I was able to delete some files
> as well to free up space. There is still a lot of wasted space in the
> metadata block groups. It seems that it allocates more metadata block
> groups than required for my filesystem. This will do until I am able
> to add a couple of devices to the system. Is there anyway to adjust
> the block group allocation strategy at filesystem creation?
No. Chunk allocator currently allocates a lot more chunks than actually
needed, and it impacts both balancing and normal operation. Try this:
# btrfs balance start -musage=10 /home
This is suboptimal, but it should get rid of more chunks.
Thanks,
Ilya
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