Does your application depend on, or report, free disk space? Re: F20 Self Contained Change: OS Installer Support for LVM Thin Provisioning

[Ric Wheeler]

On 07/29/2013 10:01 AM, Chris Murphy wrote:
> On Jul 29, 2013, at 6:30 AM, "Daniel P. Berrange" <berrange at redhat.com> 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