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

Deyan Chepishev - SuperHosting.BG dchepishev at superhosting.bg
Wed Aug 17 23:55:22 UTC 2011


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.

===> Hi,

I just realize that I actually did not realized Amar's answer. Thank you for 
bringing this to my attention. It really look like working solution I will give 
it a try. I tried to lower the percentage of min free disk space which looks 
like working so far, but hardcoding the min free space looks much better.

Thank you once again.


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