Gluster users - Jun 2012 - shrinking volume

Hi all - is there a safe way to shrink an active gluster volume without 
losing files?

I've used remove-brick before, but this causes the files on that brick 
to be removed from the volume. Which is fine for some situations.

But I'm trying to remove a brick without losing files. This is because 
our file usage can grow dramatically over short periods. During those 
times we add a lot of buffer to our gluster volume, to keep it at about 
50% usage. After things settle down and file usage isn't changing as 
much, we'd like to remove some bricks in order to keep usage at about 
80%. (These bricks are AWS EBS volumes - we want to remove the bricks to 
save a little $ when things are slow.)

So what I'd like to do is the following. This is a simple distributed 
volume, no replication.

* Let gluster know I want to remove a brick
* No new files will go to that brick
* Gluster starts copying files from that brick to other bricks, 
essentially rebalancing the data
* Once all files have been duplicated onto other bricks, the brick is 
marked as "removed" and I can do a normal remove-brick
* Over the course of this procedure the files are always available 
because there's always at least one active copy of every file

This procedure seems very similar to replace-brick, except the goal 
would be to evenly distribute to all other active bricks (without 
interfering with pre-exiting files), not one new brick.

Is there any way to do this?

I *could* just do my remove-brick, then manually distribute the files 
from that old brick back onto the volume, but that would cause those 
files to become unavailable for some amount of time.

Many thanks for all your help,

- brian

In 3.3, this is exactly what 'remove-brick' does.  It migrates the data
off an active volume, and when it's done, allows the removed brick to be
upgraded, shut down, killed off etc.

gluster volume  remove-brick <vol> server:/brick  start

(takes a while to start up, but then goes fairly rapidly.)

following is the result of a recent remove-brick I did with 3.3

% gluster volume  remove-brick gl bs1:/raid1  status
     Node Rebalanced-files          size       scanned      failures
status
---------      -----------   -----------   -----------   -----------
------------
localhost                2  10488397779           12            0    in
progress
      bs2                0            0            0            0    not
started
      bs3                0            0            0            0    not
started
      bs4                0            0            0            0    not
started

(time passes )

$ gluster volume  remove-brick gl bs1:/raid2  status
                                    Node Rebalanced-files          size
scanned      failures         status
                               ---------      -----------   -----------
-----------   -----------   ------------
                               localhost              952  26889337908
8306            0      completed



Note that once the 'status' says completed, you need to issue the
remove-brick command again to actually finalize the operation.

And that 'remove-brick' command will not clear the dir structure on the
removed brick.


On Thu, 2012-06-21 at 12:29 -0400, Brian Cipriano wrote:
> Hi all - is there a safe way to shrink an active gluster volume without 
> losing files?
> 
> I've used remove-brick before, but this causes the files on that brick 
> to be removed from the volume. Which is fine for some situations.
> 
> But I'm trying to remove a brick without losing files. This is because 
> our file usage can grow dramatically over short periods. During those 
> times we add a lot of buffer to our gluster volume, to keep it at about 
> 50% usage. After things settle down and file usage isn't changing as 
> much, we'd like to remove some bricks in order to keep usage at about 
> 80%. (These bricks are AWS EBS volumes - we want to remove the bricks to 
> save a little $ when things are slow.)
> 
> So what I'd like to do is the following. This is a simple distributed 
> volume, no replication.
> 
> * Let gluster know I want to remove a brick
> * No new files will go to that brick
> * Gluster starts copying files from that brick to other bricks, 
> essentially rebalancing the data
> * Once all files have been duplicated onto other bricks, the brick is 
> marked as "removed" and I can do a normal remove-brick
> * Over the course of this procedure the files are always available 
> because there's always at least one active copy of every file
> 
> This procedure seems very similar to replace-brick, except the goal 
> would be to evenly distribute to all other active bricks (without 
> interfering with pre-exiting files), not one new brick.
> 
> Is there any way to do this?
> 
> I *could* just do my remove-brick, then manually distribute the files 
> from that old brick back onto the volume, but that would cause those 
> files to become unavailable for some amount of time.
> 
> Many thanks for all your help,
> 
> - brian
> 
> 
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