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  • From: Ismael Luceno <ismael.luceno AT gmail.com>
  • To: sm-discuss AT lists.ibiblio.org <sm-discuss AT lists.ibiblio.org>
  • Subject: [SM-Discuss] BTRFS: Migration Report
  • Date: Sun, 27 May 2012 08:11:29 -0300

Hi, fellow mages. I would like to share my experience with btrfs.

To save some time, I've used a Debian Sid LiveCD. The whole process
took about two hours. I'm not exactly sure how much, should have timed
it.

The disk is a Hitachi HTS72757 750GB 7500RPM (real size 698GB)
and the original filesystem was an ext4 of 690GB, of which 400GB were
free space.

btrfs-convert itself took about an hour to convert the ext4 volume, and
then resizing it to 590GB took about 30 minutes.

BTW, keep in mind that the brtfs tool doesn't report the limits of the
filesystem, so it makes you guess. I think newer version of Gparted
support brtfs (the annoying part of it is that always forces
filesystem checks, and it's not a good idea in this case...).

I recovered about 20GB by deleting the ext2_saved volume. But it might
come handy, so unless you have a full backup at hand, I would not
recommend removing it.

Also, remember there's no fsck.brtfs (btrfsck is not production-ready),
so you need to disable checking or provide a replacement for
fsck.brtfs.

I've moved all the grub stuff and kernels to another partition... I
didn't feel like patching grub2. Backporting the module might be a good
idea (or we could just borrow some other distro's work :P).

Also, before someone asks:

ismael@pirotess:~$ df -h
/dev/sda1 592G 180G 270G 41% /

ismael@pirotess:~$ btrfs fi df /
Data: total=395.00GB, used=125.92GB
System: total=32.00MB, used=36.00KB
Metadata: total=196.00GB, used=53.83GB

It might seem confusing, but what's happening is that btrfs is putting
quite a lot of data along with the metadata, and it's not reporting the
space reserved for metadata as free. The data-to-metadata ratio of 2:1
makes it quite dramatic.

Looks like it behaves pretty well with small files. I'm not sure
about big ones. Also I have concerns about filesystem images,
databases, etc.

I noticed booting is slightly faster now, but I will not be sure
how well it performs until defragmenting (since the migration doesn't
leave it on an ideal shape).

That's all I guess :).

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