Il 02/03/2017 14:11, Jason Dillaman ha scritto:
On Thu, Mar 2, 2017 at 8:09 AM, Massimiliano Cuttini <max@xxxxxxxxxxxxx> wrote:
Ok,
then, if the command comes from the hypervisor that hold the image is it
safe?
No, it needs to be issued from the guest VM -- not the hypervisor that
is running the guest VM. The reason is that it's a black box to the
hypervisor and it won't know what sectors can be safely discarded.
This is true if you talk about the filesystem.
So the command
fstrim
would be the case for sure.
But if we talk about the block device.
The command
blkdiscard
could not run on a VM which see images as localdisks without any thin
provisioning.
This command should be casted by the Hypervisor not the guest.
... or not?
But if the guest VM on the same Hypervisor try to using the image, what
happen?
If you trim from outside the guest, I would expect you to potentially
corrupt the image (if the fstrim tool doesn't stop you first since the
filesystem isn't mounted).
Ok make it sense on fstrim, but with blkdiscard?
Are these safe tools? (aka: safely exit with error instead of try the
command and ruin the image?).
Should I consider a snapshot before go?
As I mentioned, the only safe way to proceed would be to run the trim
from within the guest VM or wait until Ceph adds the rbd CLI tooling
to safely sparsify an image.
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