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Date: Tue, 28 Jan 2014 20:17:35 +0800
From: ching <lsching17@gmail.com>
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Subject: Re: [Qemu-devel] check trim/unmap
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To: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org

Thanks for the information. Hopefully, there will be better debug/tracing facility for this. This is useful for sysadmin to ensure the whole storage stack is functioning as expected.

ching


On 28/01/2014 06:46 PM, Paolo Bonzini wrote:
> 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