[Gluster-users] cluster.min-free-disk separate for each, brick

[Dan Bretherton]

On 15/08/11 20:00, gluster-users-request at gluster.org wrote:
> Message: 1
> Date: Sun, 14 Aug 2011 23:24:46 +0300
> From: "Deyan Chepishev - SuperHosting.BG"<dchepishev at superhosting.bg>
> Subject: [Gluster-users] cluster.min-free-disk  separate for each
> 	brick
> To: gluster-users at gluster.org
> Message-ID:<4E482F0E.3030604 at superhosting.bg>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> Hello,
>
> I have a gluster set up with very different brick sizes.
>
> brick1: 9T
> brick2: 9T
> brick3: 37T
>
> with this configuration if I set the parameter cluster.min-free-disk to 10% it
> applies to all bricks which is quite uncomfortable with these brick sizes,
> because 10% for the small bricks are ~ 1T but for the big brick it is ~3.7T and
> what happens at the end is that if all brick go to 90% usage and I continue
> writing, the small ones eventually fill up to 100% while the big one has enough
> free space.
>
> My question is, is there a way to set cluster.min-free-disk per brick instead
> setting it for the entire volume or any other way to work around this problem ?
>
> Thank you in advance
>
> Regards,
> Deyan
>
Hello Deyan,

I have exactly the same problem and I have asked about it before - see 
links below.

http://community.gluster.org/q/in-version-3-1-4-how-can-i-set-the-minimum-amount-of-free-disk-space-on-the-bricks/
http://gluster.org/pipermail/gluster-users/2011-May/007788.html

My understanding is that the patch referred to in Amar's reply in the 
May thread prevents a "migrate-data" rebalance operation failing by 
running out of space on smaller bricks, but that doesn't solve the 
problem we are having.  Being able to set min-free-disk for each brick 
separately would be useful, as would being able to set this value as a 
number of bytes rather than a percentage.  However, even if these 
features were present we would still have a problem when the amount of 
free space becomes less than min-free-disk, because this just results in 
a warning message in the logs and doesn't actually prevent more files 
from being written.  In other words, min-free-disk is a soft limit 
rather than a hard limit.  When a volume is more than 90% full there may 
still be hundreds of gigabytes of free space spread over the large 
bricks, but the small bricks may each only have a few gigabytes left of 
even less.  Users do "df" and see lots of free space in the volume so 
they continue writing files.  However, when GlusterFS chooses to write a 
file to a small brick, the write fails with "device full" errors if the 
file grows too large, which is often the case here with files typically 
several gigabytes in size for some applications.

I would really like to know if there is a way to make min-free-disk a 
hard limit.  Ideally, GlusterFS would chose a brick on which to write a 
file based on how much free space it has left rather than choosing a 
brick at random (or however it is done now).  That would solve the 
problem of non-uniform brick sizes without the need for a hard 
min-free-disk limit.

Amar's comment in the May thread about QA testing being done only on 
volumes with uniform brick sizes prompted me to start standardising on a 
uniform brick size for each volume in my cluster.  My impression is that 
implementing the features needed for users with non-uniform brick sizes 
is not a priority for Gluster, and that users are all expected to use 
uniform brick sizes.  I really think this fact should be stated clearly 
in the GlusterFS documentation, in the sections on creating volumes in 
the Administration Guide for example.  That would stop other users from 
going down the path that I did initially, which has given me a real 
headache because I am now having to move tens of terabytes of data off 
bricks that are larger than the new standard size.

Regards
Dan.

-- 
Mr. D.A. Bretherton
Computer System Manager
Environmental Systems Science Centre
Harry Pitt Building
3 Earley Gate
University of Reading
Reading, RG6 6AL
UK

Tel. +44 118 378 5205
Fax: +44 118 378 6413