[Gluster-users] Advice on rebuilding underlying filesystem

[Andrew Smith]

My understanding is that “replace-brick” is deprecated 

http://www.gluster.org/pipermail/gluster-users/2012-October/034473.html

And that the “add-brick” followed by “remove-brick” should behave
the same way.

It does not behave as predicted, I think, because my system is 
unbalanced. I have no idea whether or no the “replace-brick” 
command will behave differently. 

Andy


On Apr 11, 2014, at 5:34 PM, Machiel Groeneveld <machielg at gmail.com> wrote:

> Isn't that what replace-brick is for?
> 
> 
>> On 11 Apr 2014, at 23:32, Andrew Smith <smith.andrew.james at gmail.com> wrote:
>> 
>> 
>> Hi, I have a problem, which I hope for your sake, is uncommon.
>> 
>> I built a Gluster  volume with 8 bricks, 4 80TB and 4 68TB with 
>> a total capacity of about 600TB. The underlying filesystem 
>> is BTRFS. 
>> 
>> I found out after the system was half full that BTRFS was a 
>> bad idea. BTRFS doesn’t have inodes. It allocates some fraction
>> of the disk space to metadata and when it runs out, it allocates
>> more. This allocation process on large volumes is painfully slow
>> and brings effective write speeds down to only a few MB/s with long 
>> timeouts. The data can be read at high speeds, but writing to the
>> volume is a big fat mess. Reading is still fairly fast though, 
>> so access to the my data by users is acceptable. 
>> 
>> I need to keep this volume available and I don’t have a second 
>> copy of the hardware to rebuild the system on. So, I need to do 
>> an in-situ transition from BTRFS to XFS. 
>> 
>> To do this, I first cleared out some data to free up metadata space,
>> and then with much difficulty managed to do a 
>> 
>>  # gluster volume remove-brick 
>> 
>> I retired the removed brick and then reformatted it with XFS and added
>> it back to my Gluster volume. At this point, I thought I was nearly 
>> home. I thought I could retire a second brick and the data would 
>> be copied to the empty brick. However, this is not what happens.
>> Some data ends up on the newly added brick, but some of the data 
>> flows elsewhere, which due to the BTRFS problem is a nightmare.
>> 
>> I assume this is because when I took my volume from 8 bricks to 7, it 
>> became unbalanced. The data on the brick that I was retiring 
>> belongs on several different bricks and so I am not just doing a 
>> substitution.
>> 
>> I need to be able to tell my Gluster volume to include all the bricks, 
>> but do not write files to any of the BTRFS bricks so that it puts data
>> only on the XFS brick. If I could somehow tell Gluster that these bricks
>> were full, that would suffice. 
>> 
>> I could do a "rebalance migrate-data" to make make the data on the BTRFS
>> volumes more uniform, but I don’t know how this will work. Does reposition
>> the data brick by brick or file by file. Brick by brick would be bad, since
>> the last brick to rebalance would need to receive all the data that it requires
>> before it would get to write data out to free up metadata space. 
>> 
>> There is a “rebalance-brick” option in the man page, but I don’t see that 
>> documented. This may be useful, but I have no idea what it will do.
>> 
>> Is there a solution to my problem? Whip it and start over is not helpful. 
>> Any help on how I can predict where data will go will also help.
>> 
>> Andy
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