Re: [Qemu-devel] check trim/unmap

[Paolo Bonzini <pbonzini@redhat.com>]

Il 28/01/2014 11:31, ching ha scritto:
> My config is Gentoo x64 stable branch, kernel 3.10, libvirt 1.1.3, qemu 1.5, lvm2（non-thin） on ssd
>
> How can i check that if:
> 1. qemu receives trim/unmap from guest
> 2. qemu is punching hole/issue blkdiscards/writing zeros?

First of all, I suggest that you use current QEMU git.  The trim/unmap 
feature was completed after 1.7 was released.

To use trim/discard, you need to use the discard=on option for QEMU's 
-drive command-line option.  You also need to use cache=none (because of 
a Linux kernel bug, QEMU may disable thin provisioning in other cache 
modes).

In libvirt, this means adding cache='none and discard='on' like this:

     <driver name='qemu' type='qcow2' cache='none' discard='on'/>

You can check if QEMU is punching a hole into a file using "qemu-img 
map" on the file.  You must not run "qemu-img map" while the VM is 
running though; that can give incorrect results.  There is no equivalent 
for block devices yet.

Paolo