[ovirt-users] Reclaim disk space of thin disks

[Yaniv Kaul]

On Thu, Mar 31, 2016 at 3:44 PM, Grundmann, Christian <
Christian.Grundmann at fabasoft.com> wrote:

> Hi,
>
> @ I'm unsure if that size should change - we've allocated blocks, they
> are still allocated from our perspective. From the storage point of view,
> there's a hole that can be used for other purposes.
>
> I'd look on the storage side.
>
>
>
> That’s the problem, if the space is still allocated it can’t be used for
> other VMs so it won’t help. Is there a possibility to deallocate the free
> Blocks of the Disk? I think virt-sparsify would do exactly that but can’t
> be used with ovirt?
>

Well, it's tricky ;-)  - the space is allocated, in the sense that the
storage 'promised' to give you X GBs of storage. It just didn't say where
does blocks are going to be. So once you unmap a block, it may take it and
give it to someone else. This is what thin provisioning is all about. You
get logical blocks which map to physical blocks. Once those physical blocks
have been deallocated, they may be used by something else, yet the logical
block is still yours - and if you'll write to it, you'll get a different
physical block.
Y.


>
>
> Thx
>
> Christian
>
>
>
>
>
>
>
>
>
>
>
> *Von:* Yaniv Kaul [mailto:ykaul at redhat.com]
> *Gesendet:* Donnerstag, 31. März 2016 14:33
>
> *An:* Grundmann, Christian <Christian.Grundmann at fabasoft.com>
> *Cc:* users at ovirt.org
> *Betreff:* Re: [ovirt-users] Reclaim disk space of thin disks
>
>
>
> On Thu, Mar 31, 2016 at 1:16 PM, Grundmann, Christian <
> Christian.Grundmann at fabasoft.com> wrote:
>
> Hi,
>
> @1. Did you use the hook?
>
> yes, without the hook fstrim would not start,
>
> @2. Where did you see it's the same size? Should be looked at from the
> storage side (assuming, btw, it supports discard!)
>
> I looked at the “actual Size” under Storage
>
>
>
> I'm unsure if that size should change - we've allocated blocks, they are
> still allocated from our perspective. From the storage point of view,
> there's a hole that can be used for other purposes.
>
> I'd look on the storage side.
>
>
>
> I don’t know how ovirt handles FC storage and if this will allow discard.
>
>
>
> It doesn't really care - thee discard should be passed to the underlying
> storage. Most should support UNMAP these days. But you can easily check, I
> assume with a small test (use sg3_utils for example).
>
> Y.
>
>
>
>
>
> Thx
>
> Christian
>
>
>
>
>
> *Von:* Yaniv Kaul [mailto:ykaul at redhat.com]
> *Gesendet:* Donnerstag, 31. März 2016 12:13
>
>
> *An:* Grundmann, Christian <Christian.Grundmann at fabasoft.com>
> *Cc:* users at ovirt.org
> *Betreff:* Re: [ovirt-users] Reclaim disk space of thin disks
>
>
>
>
>
>
>
> On Thu, Mar 31, 2016 at 1:05 PM, Grundmann, Christian <
> Christian.Grundmann at fabasoft.com> wrote:
>
> Hi,
>
> seems not to work with FC Storage,
>
>
>
> I Zero filled the Disk with dd, removed the file and did a fstrim which
> told me it trimmed the right amount of space,
>
> But the disk Image stays full size.
>
>
>
> 1. Did you use the hook?
>
> 2. Where did you see it's the same size? Should be looked at from the
> storage side (assuming, btw, it supports discard!)
>
> Y.
>
>
>
>
>
> Thx
>
> Christian
>
>
>
> *Von:* Yaniv Kaul [mailto:ykaul at redhat.com]
> *Gesendet:* Mittwoch, 30. März 2016 22:29
> *An:* Grundmann, Christian <Christian.Grundmann at fabasoft.com>
> *Cc:* users at ovirt.org
> *Betreff:* Re: [ovirt-users] Reclaim disk space of thin disks
>
>
>
>
>
>
>
> On Wed, Mar 30, 2016 at 10:04 AM, Grundmann, Christian <
> Christian.Grundmann at fabasoft.com> wrote:
>
> Hi,
>
> is there a way to „shrink“ a thin disk image to reclaim deleted space like
> using virt-sparsify?
>
>
>
> Yes, if the underlying file system supports it (NFS, for example, might
> need to be 4.2 and above).
>
>
>
> Or can it be done by making a snapshot and clone the VM to a new one?
>
>
>
> Another suggestion: use the VDSM hooks for discard ('diskunmap') and run
> 'fstrim' from within the guest.
>
> Y.
>
>
>
>
>
> Thanks a lot
>
> Christian
>
>
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