When I've been converting from RAID1 to RAID5 I've been getting
stripes that only contain 1G regardless of how wide the stripe is. So
when I've done a large convert I've had to limit the blocks and then
do a balance of the target profile and repeat till finished.

Has anyone else seen similar?

On Wed, Mar 2, 2016 at 1:13 AM, Dan Blazejewski
[off-list ref] wrote:

Hey all,

Just wanted to follow up with this for anyone experiencing the same issue.

First, I tried Qu's suggestion, of re-balancing to single, then
re-balancing to RAID 6. I noticed when I completed the conversion to
single, that a few drives didn't receive an identical amount of data.
Balancing back to RAID 6 didn't totally work either. It definitely
made it better, but I still had multiple stripes of varying widths.
IIRC, I had one ~1.7TB stripe that went across all 7 drives, and then
a conglomerate of stripes ranging from 2-5 drives wide, and sizes 30GB
- 1TB. The majority of data was striped across all 7, but I was
concerned that as I added data, I'd run into the same situation as
before.

This process took quite a long time, as you guys expected. About 11
days for RAID 6 -> Single -> Raid 6. Patience is a virtue with large
arrays.



Henk, for some reason I didn't receive the email suggesting using the
-dstripes= filter until I was well into the conversion to single. Once
I finished the RAID 6 -> Single -> RAID 6, I attempted your method.
I'm happy to say that it worked, using -dstripes="1..6". This only
took about 30 hours, as most of the data was striped correctly. When
it finished, I was left with one RAID 6 profile, about ~2.50 TB
striped across all 7 drives. As I understand, running a balance with
the -dstripes="1..$drivecount-1" filter will force BTRFS to balance
chunks that are not evenly striped across all drives. I will
definitely have to keep this trick in mind in the future.


A side note, I'm happy with how robust BTRFS is becoming. I had a
sustained power outage while I wasn't home that resulted in an unclean
shutdown in the middle of the balance. (I had preciously disconnected
my UPS' USB connector to move the server to a different room and
forgot to reconnect it. Doh!). When power was returned, it started
right back up where it left off with no corruption or data loss. I
have backups, but I wasn't looking forward to the idea of restoring 11
TB of data.

Than you everyone for your help, and thank you for putting all this
work into BTRFS. Your efforts are truly appreciated.

Regards,
Dan

On Thu, Feb 18, 2016 at 8:36 PM, Qu Wenruo [off-list ref] wrote:

quoted

Henk Slager wrote on 2016/02/19 00:27 +0100:

quoted

On Thu, Feb 18, 2016 at 3:03 AM, Qu Wenruo [off-list ref]
wrote:

quoted

Dan Blazejewski wrote on 2016/02/17 18:04 -0500:

quoted

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

quoted

quoted

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.

quoted

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

quoted

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