On 07/29/2013 10:01 AM, Chris Murphy wrote:
On Jul 29, 2013, at 6:30 AM, "Daniel P. Berrange" <berrange@xxxxxxxxxx> wrote:
Yep, we need to be able to report free space on filesystems, so that
apps provisioning virtual machines can get an idea of how much storage
they can provide to VMs without risk of over comitting.
I agree that we really want the kernel, or at least a reusable shared
library, to provide some kind of interface to determine this, rather
than requiring every userspace app which cares to re-invent the wheel.
What does it mean for an app to use stat to get free space, and then proceeds to create too big a VM image in a directory that has a quota set? I still think apps are asking an inappropriate/unqualified question by asking for volume free space, instead of what's available to them for a specified path.
Chris Murphy
When you use thinly provisioned storage, the file system itself does not know
how much physical storage is really backing it so stat, df and friends *really*
have no way to tell.
Think of it as the equivalent of virtual memory backed by physical DRAM - the
virtual storage is backed by physical disk.
It is up to the admin/installation tools to provision enough real storage to
make this work. If you provision 10 file systems with a virtual 1TB each and
only back it with 2TB of real disk, you will need to monitor the space (via
device mapper tools) and dynamically throw in more disk when the physical pool
runs low.
Regards,
Ric
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