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

Raghavendra Bhat raghavendrabhat at gluster.com
Thu Sep 8 05:35:05 UTC 2011

This is how it is supposed to work.

Suppose a distribute volume is created with 2 bricks. 1st brick is having
25GB of free space, 2nd disk has 35 GB of free space. If one sets a 30GB of
minimum-free-disk through volume set (gluster volume set <volname>
min-free-disk 30GB), then whenever files are created, if the file is hashed
to the 1st brick (which has 25GB of free space), then actual file will be
created in the 2nd brick to which a linkfile will be created in the 1st
brick. So the linkfile points to the actual file. A warning message
indicating minimum free disk limit has been crosses and adding more nodes
will be printed in the glusterfs log file. So any file which is hashed to
the 1st brick will be created in the 2nd brick.

Once the free space of 2nd brick also comes below 30 GB, then the files will
be created in the respective hashed bricks only. There will be a warning
message in the log file about the 2nd brick also crossing the minimum free
disk limit.

Raghavendra Bhat
On Wed, Sep 7, 2011 at 4:27 PM, Dan Bretherton <d.a.bretherton at reading.ac.uk
> wrote:

> On 17/08/11 16:19, Dan Bretherton wrote:
>>> Dan Bretherton wrote:
>>>> On 15/08/11 20:00, gluster-users-request at gluster.**org<gluster-users-request at gluster.org>wrote:
>>>>> Message: 1
>>>>> Date: Sun, 14 Aug 2011 23:24:46 +0300
>>>>> From: "Deyan Chepishev - SuperHosting.BG"<dchepishev@**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@**superhosting.bg<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://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<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<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<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.
> -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
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