Re: Ideas for btrfs-convert fix(or rework)

[Roman Mamedov <rm@romanrm.net>]

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On Tue, 10 Nov 2015 16:16:26 +0800
Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:

> So if things are correct, the btrfs you converted should still be in 
> mixed-bg mode.
> A recent 'btrfs fi df' command should show things like:
> Data+Metadata, DUP: total=512.00MiB, used=68.23MiB

Actually not quite. After all the rebalancing, I currently have:

Data, single: total=8.37TiB, used=7.50TiB
System, DUP: total=32.00MiB, used=592.00KiB
Metadata, DUP: total=8.00GiB, used=6.42GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

> > But as we know recently there were reports that it now causes corruption or
> > does not work. So I wonder what happened that broke it, and is there really no
> > simpler fix? Even if requiring the user to balance to get rid of the bloated
> > Metadata like I had to.
> >
> 
> Yes, current btrfs-convert has problem on marking chunk meta or data.
> Resulting some data extents may lay in metadata chunk, and vice verse.
> At least it won't pass latest btrfsck.
> 
> In that incorrect chunk layout, I'm not surprised any thing go wrong.

Made an experiment:

# dd if=/dev/zero bs=1M count=1 seek=100000 of=100G
1+0 records in
1+0 records out
1048576 bytes (1.0 MB) copied, 0.0023896 s, 439 MB/s

# losetup /dev/loop7 100G

# mkfs.ext4 /dev/loop7 

// Success

# mount /dev/loop7 /mnt/tmp1/

// Copy some files there

# df -h /mnt/tmp1/
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop7       96G  2.6G   89G   3% /mnt/tmp1

# umount /mnt/tmp1/

# btrfs-convert -d /dev/loop7 
creating btrfs metadata.
creating ext2fs image file.
cleaning up system chunk.
conversion complete.

# mount /dev/loop7 /mnt/tmp1/

# btrfs fi df /mnt/tmp1/
Data, single: total=65.00GiB, used=1.32GiB
System, single: total=32.00MiB, used=12.00KiB
Metadata, single: total=32.63GiB, used=2.91GiB
GlobalReserve, single: total=92.00MiB, used=0.00B

# btrfs sub del /mnt/tmp1/ext2_saved
Transaction commit: none (default)
Delete subvolume '/mnt/tmp1/ext2_saved'

# btrfs fi df /mnt/tmp1/
Data, single: total=10.00GiB, used=120.12MiB
System, single: total=32.00MiB, used=4.00KiB
Metadata, single: total=4.00GiB, used=2.37GiB
GlobalReserve, single: total=52.00MiB, used=0.00B

// It has 2.6 GB of files, and most of them are now accounted as "Metadata".

# btrfs fi balance start -m /mnt/tmp1/
Done, had to relocate 5 out of 7 chunks

// They are now reclassified as data:

# btrfs fi df /mnt/tmp1/
Data, single: total=10.00GiB, used=2.49GiB
System, single: total=32.00MiB, used=4.00KiB
Metadata, single: total=1.00GiB, used=468.00KiB
GlobalReserve, single: total=4.00MiB, used=0.00B

// Unmount, mount, checked all files integrity with 'cfv', all files are
present and there is no corruption.

So this behaves just like I remember it to, after conversion at first a large
part of the data will show up as Metadata, but a balance fixes that. And most
importantly no corruption issues. Maybe it's just because this is only a small
test FS, but my production one earlier went through the same process without
problems as well.

Using btrfs-tools v3.17 and kernel 3.18.21

-- 
With respect,
Roman

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