Discard is not working

Discard is not working

[chickenmarkus]

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.

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

Bye Markus

Fwd: Discard is not working

[chickenmarkus]

Hello,

I did not get response to my private mail address. So I'm subscribed now.

Bye Markus


-------- Original-Nachricht --------
Betreff: 	Discard is not working
Datum: 	Wed, 05 Feb 2014 23:53:56 +0100
Von: 	chickenmarkus@freenet.de
An: 	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.

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

Bye Markus

Re: Fwd: Discard is not working

[Michael Tokarev]

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

Re: Fwd: Discard is not working

[chickenmarkus]

Hello,

I played around with raw images now.

Megasas is not supported by libvirt. The default controller crashs 
during installation while accessing to disk.

    qemu-system-x86_64:
    /build/qemu-j0uSxv/qemu-1.7.0+dfsg/hw/scsi/lsi53c895a.c:541:
    lsi_do_dma: Assertion `s->current' failed.

So I took Debian Testing (Jessie with kernel 3.12) to use virtio-scsi 
over following options.

    -device virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x4 \
    -drive
    file=/home/vm/trim.raw,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

Result of this: fstrim works.
Take a look into lsblk -D: DISC-ZERO=0? I read DISC-ZERO has to be "1" 
to propagate discard.

Switch back to my thin volume vm. DISC-ZERO=0 and fstrim works! Booting 
from old kernel 3.2 - works too.
LVM recognize the increasing of thin volume after discard.

Conclusion of my issues:
> Did I understand something wrong?
Yes!
Discard granularity (DISC-GRAN) of lsblk -D is the important number. It 
must not be "0B".

I did so much the last weeks. Maybe the error of fstrim occurs before 
getting thin volumes work.
Thanks a lot for your help (and hint to try it with an image)!

Bye Markus



Am 07.02.2014 17:36, schrieb Michael Tokarev:
> 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
>
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: Fwd: Discard is not working

[Paolo Bonzini]

Il 07/02/2014 17:36, Michael Tokarev ha scritto:
> 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 Michael for the experiments!  I missed this email.

virtio-blk does not support discard, so this is expected.

Regarding lsblk -D, DISC-ZERO is never true in VMs (not yet at least!), 
but the important one is DISC-GRAN.

Paolo