On 07/29/2013 10:18 AM, Daniel P. Berrange wrote:
On Mon, Jul 29, 2013 at 08:01:23AM -0600, 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.
From an API POV, libvirt doesn't need/care about the free space on the
volume underlying the filesystem. We actually only care about the free
space in a given directory that we're using for disk images. It just
happens that we implement this using statvfs() currently. So when I
ask for an API above, don't take this to mean I want a statvfs() that
knows about sparse volumes. An API or syscall that provides free space
for individual directories is fine with me.
Daniel
Just another note, it is never safe to assume that storage under any file system
is yours for the taking.
If application A does a stat or statvfs() call, sees 1GB of space left and then
does a write, we could easily lose that race to any other application.
If you want to reserve space, you need to grab the space yourself (always works
with a large "write()" but preallocation can also help without dm-thin).
The difference with dm-thin is that all applications on all file systems backed
by the same block pool compete for that space.
Another worry here is that preallocation is a file system concept, thinly
provisioned storage (dm-thin or array backed), only sees proper writes so you
need to write to space to really claim it.
Ric
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