On 2016-11-29 12:20, Florian Lindner wrote:
Hello,
I have 4 harddisks with 3TB capacity each. They are all used in a btrfs RAID 5.
It has come to my attention, that there
seem to be major flaws in btrfs' raid 5 implementation. Because of that, I want
to convert the the raid 5 to a raid 10
and I have several questions.
* Is that possible as an online conversion?
Yes, as long as you have a complete array to begin with (converting from
a degraded raid5/6 array has the same issues as rebuilding a degraded
raid5/6 array).
* Since my effective capacity will shrink during conversions, does btrfs check
if there is enough free capacity to
convert? As you see below, right now it's probably too full, but I'm going to
delete some stuff.
No, you'll have to do the math yourself. This would be a great project
idea to place on the wiki though.
* I understand the command to convert is
btrfs balance start -dconvert=raid10 -mconvert=raid10 /mnt
Correct?
Yes, but I would personally convert first metadata then data. The
raid10 profile gets better performance than raid5, so converting the
metadata first (by issuing a balance just covering the metadata) should
speed up the data conversion a bit).
* What disks are allowed to fail? My understanding of a raid 10 is like that
disks = {a, b, c, d}
raid0( raid1(a, b), raid1(c, d) )
This way (a XOR b) AND (c XOR d) are allowed to fail without the raid to fail
(either a or b and c or d are allowed to fail)
How is that with a btrfs raid 10?
A BTRFS raid10 can only sustain one disk failure. Ideally, it would
work like you show, but in practice it doesn't.
* Any other advice? ;-)
You'll actually get significantly better performance with no loss of
data safety by running BTRFS in raid1 mode on top of two RAID0 volumes
(LVM/MD/hardware doesn't matter much). I do this myself and see roughly
10-20% improved performance on average with my workloads.
If you do decide to do this, it's theoretically possible to do so
online, but it's kind of tricky, so I won't post any instructions for
that here unless someone asks for them.
Thanks a lot,
Florian
Some information of my filesystem:
# btrfs filesystem show /
Label: 'data' uuid: 57e5b9e9-01ae-4f9e-8a3d-9f42204d7005
Total devices 4 FS bytes used 7.57TiB
devid 1 size 2.72TiB used 2.72TiB path /dev/sda4
devid 2 size 2.72TiB used 2.72TiB path /dev/sdb4
devid 3 size 2.72TiB used 2.72TiB path /dev/sdc4
devid 4 size 2.72TiB used 2.72TiB path /dev/sdd4
# btrfs filesystem df /
Data, RAID5: total=8.14TiB, used=7.56TiB
System, RAID5: total=96.00MiB, used=592.00KiB
Metadata, RAID5: total=12.84GiB, used=11.06GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
Based on this output, you will need to delete some data before you can
convert to raid10. With 4 2.72TiB drives, you're looking at roughly
5.44TiB of usable space, so you're probably going to have to delete at
least 2-3TiB of data from this filesystem before converting.
If you're not already using transparent compression, it could probably
help some with this, but it likely won't save you more than a few
hundred GB unless you are storing lots of data that compresses very well.
# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda4 11T 7.6T 597G 93% /
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