[Gluster-users] Advice on rebuilding underlying filesystem

[Machiel Groeneveld]

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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