Why "discard":"unmap" is the default option for disks

[Han Han]

On Thu, Oct 22, 2020 at 2:40 PM Peter Krempa <pkrempa at redhat.com> wrote:

> On Thu, Oct 22, 2020 at 10:57:05 +0800, Han Han wrote:
> > Hello,
> > I find "discard":"unmap" is defaultly enabled in  qemu cmdline(libvirt
> > v6.6, qemu v5.1):
> > XML:
> > <disk type="file" device="disk">
> >       <driver name="qemu" type="qcow2"/>
> >       <source file="/var/lib/libvirt/images/new.qcow2" index="2"/>
> >       <backingStore/>
> >       <target dev="sda" bus="scsi"/>
> >       <alias name="scsi0-0-0-0"/>
> >       <address type="drive" controller="0" bus="0" target="0" unit="0"/>
>
> [...]
>
> > QEMU cmdline:
>
> The -blockdev definition of storage splits up a storage into a protocol
> node, which handles storage access:
>
> In this case it's a 'file':
>
> > ... -blockdev
> >
> {"driver":"file","filename":"/var/lib/libvirt/images/new.qcow2","node-name":"libvirt-2-storage","auto-read-only":true,
> > *"discard":"unmap"*}
>
> You can see, that 'discard: unmap' is enabled only here.
>
>
> The second layer is a 'format' node which handles the actual qcow2
> format:
>
> >-blockdev
> >
> {"node-name":"libvirt-2-format","read-only":false,"driver":"qcow2","file":"libvirt-2-storage","backing":null}
>
> Here you can see that there's no 'discard' setting, since you didn't
> configure any setting in the disk.
>
> Now the above configuration is used as we want to allow the QCOW2 driver
> to discard it's unused internal sectors/structures. This can happen for
> example when you delete an internal snapshot. All the unused sectors get
> unref'd and discarded. This wouldn't be possible without enabling unmap
> for the protocol node and the qcow2 image would not decrease size
> otherwise.
>
> The user setting is configured only for the 'format' node since it
> handles whether the guest discard gets honoured or not.
>
> The 'format' node is then installed as the disk backend:
>
> Thx

> > -device
> >
> scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,device_id=drive-scsi0-0-0-0,drive=libvirt-2-format,id=scsi0-0-0-0,bootindex=1
>
> [...]
>
>
> > I think it's from
> >
> https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_block.c#L1211
> > But I cannot find the reason from commit msgs or documents.
> > Could you please explain it?
>
> See above. I'm curious though why are you asking? You didn't mention
> that there's any problem with that.
>
> Yeah. No problems here. I was just a little surprised to find discard in
qemu cmdline when no such configurations in libvirt domain XML.
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