[Gluster-users] How to correctly distribute OpenStack VM files...

[Ben Turner]

I ran into this in my testing as well.  I think that this is happening because Cinder names its image files in the format image-$UUID.  My guess is that the similar filenames are hashing to the same brick as I have seen similar behavior naming files like test1, test2, ect.  Looking at this problem the first/simplest thing I could think of was for cinder to use a more unique filename and hope that they don't hash to the same brick.  Maybe even just use $UUID, but I don't know enough about the EHA to make very educated suggestions.

Something completely experimental I have been toying with is the idea of running both gluster(with the NUFA xlator) and cinder on all compute nodes.  Gluster would group all of the local storage across compute nodes the into a single name space and replicate it.  NUFA would prefer the local node for creating the image and hopefully reduce some overhead of going over the wire(although data would still need to be replicated).  I doubt the folks in either the glusterfs or OS camps would suggest this but, on paper at least, I was thinking this could help several things that I have ran into.

As far as striping goes I haven't looked into that as an option, and tbh I prolly won't until it becomes supported on the Red Hat downstream bits.  But it sounds like a possibility.  The ALU scheduler isn't something I am familiar with so I don't have a comment there.

I'll keep an eye on this thread, let me know what ya come up with!

-b

----- Original Message -----
> From: "Xavier Trilla" <xavier.trilla at silicontower.net>
> To: gluster-users at gluster.org
> Sent: Thursday, August 1, 2013 8:52:36 PM
> Subject: [Gluster-users] How to correctly distribute OpenStack VM files...
> 
> 
> 
> Hi,
> 
> 
> 
> We have been playing for a while with GlusterFS (Now with ver 3.4). We are
> running tests and playing with it to check if GlusterFS can be really used
> as the distributed storage for OpenStack block storage (Cinder) as new
> features in KVM, GlusterFS and OpenStack are pointing to GlusterFS as the
> future of OpenStack open source block and object storage.
> 
> 
> 
> But we’ve found a problem just when we started playing with GlusterFS… The
> way distribute translator (DHT) balances the load. I mean, we understand and
> see the benefits of metadata less setup. Using hashes based on filenames and
> assigning a hash range to each brick is clever, reliable and fast, but from
> our understanding there is a big problem when it comes to storing VM images
> of a OpenStack deployment.
> 
> 
> 
> I mean, OpenStack Block Storage (Cinder) assigns a name to each volume it
> creates (GUID), so GlusterFS does a hash of the filename and decides in
> which brick it should be stored. But as in this scenario we don’t have many
> files (I mean, we would just have one big file per VM) we may end with a
> really unbalanced storage.
> 
> 
> 
> Let’s say we have a 4 bricks setup with DHT distribute, and we want to store
> 100 VMs there, so the ideal scenario would be:
> 
> 
> 
> Brick1: 25 VMs
> 
> Brick2: 25 VMs
> 
> Brick3: 25 VMs
> 
> Brick4: 25 VMs
> 
> 
> 
> As VMs are IO intensive it’s really important to correctly balance the load,
> as each brick has a limited amount of IOPS, but as DHT is just based on a
> filename HASH, we could end with something like the following scenario (Or
> even worse):
> 
> 
> 
> Brick1: 50 VMs
> 
> Brick2: 10 VMs
> 
> Brick3: 35 VMs
> 
> Brick4: 5 VMs
> 
> 
> 
> And if we scale this out, things may get even worse. I mean, we may end with
> almost all VM file in one or two bricks and all the other bricks almost
> empty. And if we use growing VM disk image files like qcow2 the option
> “min-free-disk” will not prevent all VMs disk image files being stored in
> the same brick. So, I understand DHT works well for large amount of small
> files, but for few big IO intensive files doesn’t seem to be a really good
> solution… (I mean, we are looking for a solution able to handle around 32
> bricks and around 1500 VM for the initial deployment and able to scale up to
> 256 bricks and 12000 VMs :/ )
> 
> 
> 
> So, anybody has a suggestion about how to handle this? I mean so far we only
> see two options: Either using legacy unify translator with ALU scheduler or
> either use cluster/stripe translator with a big block-size so at least load
> gets balanced across all bricks in some way. But obviously we don’t like
> unify as it needs a namespace brick, and using stripping seems to have an
> impact on performance and really complicates backup/restore/recovery
> strategies.
> 
> 
> 
> So, suggestions? :)
> 
> 
> 
> Thanks!
> 
> 
> 
> Saludos cordiales,
> 
> Xavier Trilla P.
> 
> Silicon Hosting
> 
> 
> 
> ¿Sabías que ahora en SiliconHos ting
> 
> resolvemos tus dudas técnicas gratis?
> 
> 
> 
> Más información en: siliconhosting.com/qa/
> 
> 
> 
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