[Qemu-devel] fstrim and cache=none

[Qemu-devel] fstrim and cache=none

[Dusty Mabe]

Hi,

Is it valid to have cache=none and discard=unmap together? I notice
that when I have cache=none the fstrim operations inside of my guests
gives me an error and i get this from the sys filesystem:

[root@guest ~]# cat /sys/block/sdb/queue/discard*
4096
0
0


This wasn't very obvious to me to begin with and just wanted to see if
this is a bug or as designed.


libvirt 1.0.6
qemu 1.5.0

Thanks,
Dusty

Re: [Qemu-devel] fstrim and cache=none

[Paolo Bonzini]

Il 02/06/2013 22:21, Dusty Mabe ha scritto:
> Hi,
> 
> Is it valid to have cache=none and discard=unmap together? I notice
> that when I have cache=none the fstrim operations inside of my guests
> gives me an error

What error?  Can you add an extra disk and try

dd if=/dev/urandom of=/dev/sdc bs=1M count=32
sg_unmap -l0 -n64 /dev/sdc

from within the guest?

> and i get this from the sys filesystem:
> 
> [root@guest ~]# cat /sys/block/sdb/queue/discard*
> 4096
> 0
> 0

This is expected.  It won't change between discard=unmap and
discard=ignore, too.  (Tip: Use "grep . /sys/block/sdb/queue/discard*"
so that it shows the filename).

> 
> 
> This wasn't very obvious to me to begin with and just wanted to see if
> this is a bug or as designed.

It should be.  It may be a Linux bug, too.

Paolo

Re: [Qemu-devel] fstrim and cache=none

[Dusty Mabe]

Paolo,

It is possible that I was mistaken. Now when I switch back to
cache=none I am still able to execute trim operations. I'll
investigate a little more into my exact steps that I took and see if I
can come up with a reproducer for the issue I observed.

Thanks,
Dusty

Re: [Qemu-devel] fstrim and cache=none

[Dusty Mabe]

On Mon, Jun 3, 2013 at 5:39 PM, Dusty Mabe <dustymabe@gmail.com> wrote:
>
> It is possible that I was mistaken. Now when I switch back to
> cache=none I am still able to execute trim operations. I'll
> investigate a little more into my exact steps that I took and see if I
> can come up with a reproducer for the issue I observed.
>

Paolo,

I was mistaken. I had added a disk to the guest and then created an
ext4 FS on the disk. I then noticed that I couldn't perform fstrim
operations. I then examined the xml and the only difference between
this disk and the other disk in the system was the cache=none. So I
removed cache=none and then shutdown/booted the guest. After the
reboot all worked fine so I assumed it was the cache=none line. This
was wrong.

Actually it is the formatting of the filesystem. When i create an FS
in the guest it disables discard. There are also some errors printed
in /var/log/messages.

Please see the output in file at the following location:

http://dustymabe.com/content/qemu-devel-mail.txt


Looks like it could be a bug (linux bug?) but I'm not sure.

Thoughts?
Dusty

Re: [Qemu-devel] fstrim and cache=none

[Paolo Bonzini]

Il 04/06/2013 03:02, Dusty Mabe ha scritto:
> On Mon, Jun 3, 2013 at 5:39 PM, Dusty Mabe <dustymabe@gmail.com> wrote:
>>
>> It is possible that I was mistaken. Now when I switch back to
>> cache=none I am still able to execute trim operations. I'll
>> investigate a little more into my exact steps that I took and see if I
>> can come up with a reproducer for the issue I observed.
>>
> 
> Paolo,
> 
> I was mistaken. I had added a disk to the guest and then created an
> ext4 FS on the disk. I then noticed that I couldn't perform fstrim
> operations. I then examined the xml and the only difference between
> this disk and the other disk in the system was the cache=none. So I
> removed cache=none and then shutdown/booted the guest. After the
> reboot all worked fine so I assumed it was the cache=none line. This
> was wrong.
> 
> Actually it is the formatting of the filesystem. When i create an FS
> in the guest it disables discard. There are also some errors printed
> in /var/log/messages.
> 
> Please see the output in file at the following location:
> 
> http://dustymabe.com/content/qemu-devel-mail.txt

Please attach the QEMU command line too.  I'll try to reproduce.

Paolo

Re: [Qemu-devel] fstrim and cache=none

[Dusty Mabe]

On Tue, Jun 4, 2013 at 2:53 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> Please attach the QEMU command line too.  I'll try to reproduce.
>

The command line is below (sorry it's so large). A little context: sda
is a raw disk image backed by a file, while sdb is backed by a thin
logical volume in the host.

/usr/bin/qemu-system-x86_64 -machine accel=kvm -name Fed19 -S -machine
pc-i440fx-1.5,accel=kvm,usb=off -m 2048 -realtime mlock=off -smp
1,sockets=1,cores=1,threads=1 -uuid
f8c227a7-432e-377b-275e-7efbae3c41f9 -no-user-config -nodefaults
-chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/Fed19.monitor,server,nowait
-mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc
-no-shutdown -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2
-device virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x4 -drive
file=/guests/Fed19.img,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=1
-drive file=/dev/vgthin/lv3,if=none,id=drive-scsi0-0-0-1,format=raw,cache=none,discard=unmap
-device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=1,drive=drive-scsi0-0-0-1,id=scsi0-0-0-1
-drive if=none,id=drive-ide0-1-0,readonly=on,format=raw -device
ide-cd,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 -netdev
tap,fd=26,id=hostnet0,vhost=on,vhostfd=27 -device
virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:89:0f:df,bus=pci.0,addr=0x3
-chardev pty,id=charserial0 -device
isa-serial,chardev=charserial0,id=serial0 -vnc 127.0.0.1:0 -vga cirrus
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5


Dusty

Re: [Qemu-devel] fstrim and cache=none

[Dusty Mabe]

On Tue, Jun 4, 2013 at 8:01 AM, Dusty Mabe <dustymabe@gmail.com> wrote:
> On Tue, Jun 4, 2013 at 2:53 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> Please attach the QEMU command line too.  I'll try to reproduce.
>>
>
> The command line is below (sorry it's so large). A little context: sda
> is a raw disk image backed by a file, while sdb is backed by a thin
> logical volume in the host.

Did you manage to find anything?

Dusty

Re: [Qemu-devel] fstrim and cache=none

[Paolo Bonzini]

Il 11/06/2013 19:18, Dusty Mabe ha scritto:
> On Tue, Jun 4, 2013 at 8:01 AM, Dusty Mabe <dustymabe@gmail.com> wrote:
>> On Tue, Jun 4, 2013 at 2:53 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>>
>>> Please attach the QEMU command line too.  I'll try to reproduce.
>>>
>>
>> The command line is below (sorry it's so large). A little context: sda
>> is a raw disk image backed by a file, while sdb is backed by a thin
>> logical volume in the host.
> 
> Did you manage to find anything?

No, I was (and am :)) on vacation.

Paolo

Re: [Qemu-devel] fstrim and cache=none

[Dusty Mabe]

On Wed, Jun 12, 2013 at 8:49 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> Il 11/06/2013 19:18, Dusty Mabe ha scritto:
>> On Tue, Jun 4, 2013 at 8:01 AM, Dusty Mabe <dustymabe@gmail.com> wrote:

>> Did you manage to find anything?
>
> No, I was (and am :)) on vacation.
>

Paolo,

Hope you are enjoying your vacation. I have since deleted the VM I was
doing this on but it won't be too hard to rebuild if you need me to as
I have documented my steps and guest xml as part of a series of blog
posts I did on fstrim within VMs. If you are interested you can find
them all at the URLs below. I decided to write the posts because I
could not find much step-by-step documentation on how to do this on
the web. Please feel free to distribute to any who may find them
useful.

http://dustymabe.com/2013/06/11/recover-space-from-vm-disk-images-by-using-discardfstrim/
http://dustymabe.com/2013/06/21/guest-discardfstrim-on-thin-lvs/
http://dustymabe.com/2013/06/26/enabling-qemu-guest-agent-anddddd-fstrim-again/

Thanks,
Dusty