Re: Fwd: Discard is not working

[Michael Tokarev <mjt@tls.msk.ru>]

07.02.2014 04:03, Paolo Bonzini wrote:
> Michael, can you look at this?
> 
> Paolo
> 
> -------- Messaggio originale --------
> From: chickenmarkus@freenet.de
> To: kvm@vger.kernel.org
> 
> Hello,
> 
> after reading the "invitation" for some one-off questions without
> subscribing excuse my disturbing, please.
> 
> At first my setup of host:
> 
>  * in general Debian Wheezy
>  * Kernel: 3.11-0.bpo.2-amd64
> 
> (http://packages.debian.org/wheezy-backports/linux-image-3.11-0.bpo.2-amd64)
>  * LVM 2.02.98 (http://packages.debian.org/jessie/lvm2)
>  * thin-provisioning-tools 0.2.8-1
>    (http://packages.debian.org/jessie/thin-provisioning-tools)
>  * Qemu-KVM: 1.7.0 (http://packages.debian.org/jessie/qemu-kvm)
> 
> The new kernel and dirty things from Jessie are for working thin volumes
> with discard on my ssd. Everything's fine up to this point.
> 
> Afterwards I start a guest (also Debian Wheezy) over the following
> command (over libvirt) as root (for test only):
> 
>    kvm [...] -drive
> 
> file=/dev/ssd0/sarabi,if=none,id=drive-scsi0-0-0-0,format=raw,discard=unmap -device
> scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0-0-0-0,id=scsi0-0-0-0,bootindex=2
>    [...]
> 
> Neither lsblk -D nor the try of fstrim ends positive. Both say that
> discard is not supported.
> I read that guests need at least kernel 3.4 to support discard (and
> virtio-scsi). But the same backports kernel 3.11 did not changed anything.

This is the first time I tried to deal with discard.  I've no discard-capable
hardware, so never bothered to even look how it works (or not).

I fired up my sample linux guest here, using 3.10 kernel on host and 3.2 kernel
on guest, using something similar to what your command looks like:

 -device megasas,id=scsi0 \
 -drive file=test.raw,if=none,id=sd0,discard=unmap \
 -device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,drive=sd0

Note: this is a plain file (test.raw) on an ext4 filesystem on a device
without discard support (regular rotational hard drive).

In the guest, I can see discard granularity (4K) for that drive in lsblk -D
output.

fstrim does not error out, but I don't know if it actually does something or
not.

The same result can be observed with 3.10 kernel in guest, and with other
scsi devices/drivers (eg lsi53c895a).

However, when I export the same test.raw file using virtio, like this:

 -drive file=test.raw,if=virtio,discard=unmap

it doesn't work: guest does not see the virtio drive as discardable.

> Did I understand something wrong? Is the command wrong? Are any
> requirements not met?

So I've no idea what's going on here... ;)

Thanks,

/mjt