The rebalance walks the directory tree and re-allocates the layout map
to match the number of bricks you have. Then it walks the tree and moves
files that don't match that map.
On 04/11/2014 03:01 PM, Andrew Smith wrote:
Since I am concerned about how the rebalance will work, in particular the
sequence of how files are moved. can someone set me straight?
Does the system sort through the files one by one and figure
out their correct location, or does it sort out the bricks one by one,
identifying the files on each brick that need to be relocated and shipping
them off?
On Apr 11, 2014, at 5:49 PM, Joe Julian <joe@xxxxxxxxxxxxxxxx> wrote:
I knew that deprecating something that's that useful was a bad idea.
Remove brick, iirc, marks the brick for removal and does a rebalance, so all it's files (and I would expect any others that need be) are redistributed.
Add brick does nothing but add it to the volume, after which a rebalance (at least the rebalance fix-layout) needs to be performed to allow it to accept files.
Removing another brick is going to try to use the same rebalance algorithm to distribute the files that were on the brick being removed. They won't go to the "emptiest" brick but rather will be distributed based on the hash mapping.
I'm afraid the most efficient online method is going to be: remove-brick, add-brick, rebalance, repeat as necessary. Luckily with each brick you convert to xfs, the process should go faster and have less of an impact on your performance.
If you can afford downtime, you could always just copy from a BTRFS brick to an XFS one while the brick is offline, being sure to include the extended attributes.
On 04/11/2014 02:38 PM, Andrew Smith wrote:
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@xxxxxxxxx> wrote:
Isn't that what replace-brick is for?
On 11 Apr 2014, at 23:32, Andrew Smith <smith.andrew.james@xxxxxxxxx> 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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