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

Dan Bretherton d.a.bretherton at reading.ac.uk
Wed Sep 7 10:57:13 UTC 2011

On 17/08/11 16:19, Dan Bretherton wrote:
>> Dan Bretherton wrote:
>>> 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.
>> Hello,
>> This is really bad news, because I already migrated my data and I 
>> just realized that I am screwed because Gluster just does not care 
>> about the brick sizes.
>> It is impossible to move to uniform brick sizes.
>> Currently we use 2TB  HDDs, but the disks are growing and soon we 
>> will probably use 3TB hdds or whatever other larges sizes appear on 
>> the market. So if we choose to use raid5 and some level of redundancy 
>> (for example 6hdds in raid5, no matter what their size is) this 
>> sooner or later will lead us to non uniform bricks which is a problem 
>> and it is not correct to expect that we always can or want to provide 
>> uniform size bricks.
>> With this way of thinking if we currently have 10T from 6x2T in hdd5, 
>> at some point when there is a 10T on a single disk we will have to 
>> use no raid just because gluster can not handle non uniform bricks.
>> Regards,
>> Deyan
> I think Amar might have provided the answer in his posting to the 
> thread yesterday, which has just appeared in my autospam folder.
> http://gluster.org/pipermail/gluster-users/2011-August/008579.html
>> With size option, you can have a hardbound on min-free-disk
> This means that you can set a hard limit on min-free-disk, and set a 
> value in GB that is bigger than the biggest file that is ever likely 
> to be written.  This looks likely to solve our problem and make 
> non-uniform brick sizes a practical proposition.  I wish I had known 
> about this back in May when I embarked on my cluster restructuring 
> exercise; the issue was discussed in this thread in May as well:  
> http://gluster.org/pipermail/gluster-users/2011-May/007794.html
> Once I have moved all the data off the large bricks and standardised 
> on a uniform brick size, it will be relatively easy to stick to this 
> because I use LVM.  I create logical volumes for new bricks when a 
> volume needs extending.  The only problem with this approach is what 
> happens when the amount of free space left on a server is less than 
> the size of the brick you want to create.  The only option then would 
> be to use new servers, potentially wasting several TB of free space on 
> existing servers.  The standard brick size for most of my volumes is 
> 3TB, which allows me to use a mixture of small servers and large 
> servers in a volume and limits the amount of free space that would be 
> wasted if there wasn't quite enough free space on a server to create 
> another brick.  Another consequence of having 3TB bricks is that a 
> single server typically has two more more bricks belonging to a the 
> same volume, although I do my best to distribute the volumes across 
> different servers in order to spread the load.  I am not aware of any 
> problems associated with exporting multiple bricks from a single 
> server and it has not caused me any problems so far that I am aware of.
> -Dan.
Hello Deyan,

Have you tried giving min-free-disk a value in gigabytes, and if so does 
it prevent new files being written to your bricks when they are nearly 
full?  I recently tried it myself and found that min-free-disk had no 
effect all.  I deliberately filled my test/backup volume and most of the 
bricks became 100 full.  I set min-free-disk to "20GB", as reported in 
"gluster volume ... info" below.

cluster.min-free-disk: 20GB

Unless I am doing something wrong it seems as though we can not "have a 
hardbound on min-free-disk" after all, and uniform brick size is 
therefore an essential requirement.  It still doesn't say that in the 
documentation, at least not in the volume creation sections.


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

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

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