Henk Slager wrote on 2016/02/19 00:27 +0100:
On Thu, Feb 18, 2016 at 3:03 AM, Qu Wenruo <quwenruo@xxxxxxxxxxxxxx> wrote:
Dan Blazejewski wrote on 2016/02/17 18:04 -0500:
Hello,
I upgraded my kernel to 4.4.2, and btrfs-progs to 4.4. I also added
another 4TB disk and kicked off a full balance (currently 7x4TB
RAID6). I'm interested to see what an additional drive will do to
this. I'll also have to wait and see if a full system balance on a
newer version of BTRFS tools does the trick or not.
I also noticed that "btrfs device usage" shows multiple entries for
Data, RAID 6 on some drives. Is this normal? Please note that /dev/sdh
is the new disk, and I only just started the balance.
# btrfs dev usage /mnt/data
/dev/sda, ID: 5
Device size: 3.64TiB
Data,RAID6: 1.43TiB
Data,RAID6: 1.48TiB
Data,RAID6: 320.00KiB
Metadata,RAID6: 2.55GiB
Metadata,RAID6: 1.50GiB
System,RAID6: 16.00MiB
Unallocated: 733.67GiB
/dev/sdb, ID: 6
Device size: 3.64TiB
Data,RAID6: 1.48TiB
Data,RAID6: 320.00KiB
Metadata,RAID6: 1.50GiB
System,RAID6: 16.00MiB
Unallocated: 2.15TiB
/dev/sdc, ID: 7
Device size: 3.64TiB
Data,RAID6: 1.43TiB
Data,RAID6: 732.69GiB
Data,RAID6: 1.48TiB
Data,RAID6: 320.00KiB
Metadata,RAID6: 2.55GiB
Metadata,RAID6: 982.00MiB
Metadata,RAID6: 1.50GiB
System,RAID6: 16.00MiB
Unallocated: 25.21MiB
/dev/sdd, ID: 1
Device size: 3.64TiB
Data,RAID6: 1.43TiB
Data,RAID6: 732.69GiB
Data,RAID6: 1.48TiB
Data,RAID6: 320.00KiB
Metadata,RAID6: 2.55GiB
Metadata,RAID6: 982.00MiB
Metadata,RAID6: 1.50GiB
System,RAID6: 16.00MiB
Unallocated: 25.21MiB
/dev/sdf, ID: 3
Device size: 3.64TiB
Data,RAID6: 1.43TiB
Data,RAID6: 732.69GiB
Data,RAID6: 1.48TiB
Data,RAID6: 320.00KiB
Metadata,RAID6: 2.55GiB
Metadata,RAID6: 982.00MiB
Metadata,RAID6: 1.50GiB
System,RAID6: 16.00MiB
Unallocated: 25.21MiB
/dev/sdg, ID: 2
Device size: 3.64TiB
Data,RAID6: 1.43TiB
Data,RAID6: 732.69GiB
Data,RAID6: 1.48TiB
Data,RAID6: 320.00KiB
Metadata,RAID6: 2.55GiB
Metadata,RAID6: 982.00MiB
Metadata,RAID6: 1.50GiB
System,RAID6: 16.00MiB
Unallocated: 25.21MiB
/dev/sdh, ID: 8
Device size: 3.64TiB
Data,RAID6: 320.00KiB
Unallocated: 3.64TiB
Not sure how that multiple chunk type shows up.
Maybe all these shown RAID6 has different number of stripes?
Indeed, its 4 different sets of stripe-widths, i.e. how many drives is
striped accross. Someone has suggested to indicate this in the output
of btrfs de us comand some time ago.
The fs has only RAID6 profile and I am not fully sure if the
'Unallocated' numbers are correct (on RAID10 they are 2x too high
with unpatched v4.4 progs), but anyhow the lower devid's are way too
full.
From the size, one can derive how many devices (or stipe-width):
732.69GiB 4, 1.43TiB 5, 1.48TiB 6, 320.00KiB 7
Qu, in regards to your question, I ran RAID 1 on multiple disks of
different sizes. I believe I had a mix of 2x4TB, 1x2TB, and 1x3TB
drive. I replaced the 2TB drive first with a 4TB, and balanced it.
Later on, I replaced the 3TB drive with another 4TB, and balanced,
yielding an array of 4x4TB RAID1. A little while later, I wound up
sticking a fifth 4TB drive in, and converting to RAID6. The sixth 4TB
drive was added some time after that. The seventh was added just a few
minutes ago.
Personally speaking, I just came up to one method to balance all these
disks, and in fact you don't need to add a disk.
1) Balance all data chunk to single profile
2) Balance all metadata chunk to single or RAID1 profile
3) Balance all data chunk back to RAID6 profile
4) Balance all metadata chunk back to RAID6 profile
System chunk is so small that normally you don't need to bother.
The trick is, as single is the most flex chunk type, only needs one disk
with unallocated space.
And btrfs chunk allocater will allocate chunk to device with most
unallocated space.
So after 1) and 2) you should found that chunk allocation is almost
perfectly balanced across all devices, as long as they are in same size.
Now you have a balance base layout for RAID6 allocation. Should make things
go quite smooth and result a balanced RAID6 chunk layout.
This is a good trick to get out of 'the RAID6 full' situation. I have
done some RAID5 tests on 100G VM disks with kernel/tools 4.5-rcX/v4.4,
and various balancing starts, cancels, profile converts etc, worked
surprisingly well, compared to my experience a year back with RAID5
(hitting bugs, crashes).
A RAID6 full balance with this setup might be very slow, even if the
fs would be not so full. The VMs I use are on a mixed SSD/HDD
(bcache'd) array so balancing within the last GB(s), so almost no
workspace, still makes progress. But on HDD only, things can take very
long. The 'Unallocated' space on devid 1 should be at least a few GiB,
otherwise rebalancing will be very slow or just not work.
That's true the rebalance of all chunks will be quite slow.
I just hope OP won't encounter super slow
BTW, the 'unallocated' space can on any device, as btrfs will choose
devices by the order of unallocated space, to alloc new chunk.
In the case of OP, balance itself should continue without much porblem
as several devices have a lot of unallocated space.
The way from RAID6 -> single/RAID1 -> RAID6 might also be more
acceptable w.r.t. speed in total. Just watch progress I would say.
Maybe its not needed to do a full convert, just make sure you will
have enough workspace before starting a convert from single/RAID1 to
RAID6 again.
With v4.4 tools, you can do filtered balance based on stripe-width, so
it avoids complete balance again of block groups that are already
allocated across the right amount of devices.
In this case, avoiding the re-balance of the '320.00KiB group' (in the
means time could be much larger) you could do this:
btrfs balance start -v -dstripes=1..6 /mnt/data
Super brilliant idea!!!
I didn't realize that's the silver bullet for such use case.
BTW, can stripes option be used with convert?
IMHO we still need to use single as a temporary state for those not
fully allocated RAID6 chunks.
Or we won't be able to alloc new RAID6 chunk with full stripes.
Thanks,
Qu
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