Okay. So here is my understanding/summary based on the discussion.

Can you please confirm?

Thanks,

Shubha

Am 11.08.2020 um 19:22 hat Nir Soffer geschrieben:

On Tue, Aug 11, 2020 at 7:21 PM Kevin Wolf <kwolf@redhat.com> wrote:

Am 11.08.2020 um 17:44 hat Nir Soffer geschrieben:

On Mon, Aug 10, 2020 at 11:53 AM Kevin Wolf <kwolf@redhat.com> wrote:

Am 09.08.2020 um 23:50 hat Nir Soffer geschrieben:

On Wed, Jul 29, 2020 at 2:30 PM Shubha Kulkarni
<shubha.kulkarni@oracle.com> wrote:

Thanks for the feedback Nir.

I agree in general that having an additional engine config for disk
level error handling default would be the right way. It would be

good

to

decide the granularity. Would it make sense to have this for a

specific

disk type like lun or would you prefer to make it generic for all

types?

This must be for a specific disk type, since for thin images on block
storage we cannot support propagating errors to the guest. This will
break thin provisioning.

Is werror=enospc not enough for thin provisioning to work? This will
still stop the guest for any other kinds of I/O errors.

Right, this should work, and what we actually use now for propagating
errors for anything but cdrom.

Hm, wait, the options you quote below are all either 'stop' or 'report',
but never 'enospc'. Is 'enospc' used for yet another kind of disk?

Currently as a user there is no good way to get enospc, this is what Shubha
is trying to fix.

For LUN using werror=enospc,rerror=enospc seems wrong, but we do this for

many years.

This is how we handle cdrom:

-device

ide-cd,bus=ide.2,id=ua-346e176c-f983-4510-af4b-786b368efdd6,bootindex=2,werror=report,rerror=report

Makes sense to me. This is read-only and removable media. Stopping the
guest usually makes sense so that it won't assume the disk is broken,
but if it happens with removable media, you can just eject and re-insert
the same image and it's fixed.

BTW this was changed since users typically leave cdrom attached
with otherwise unused ISO storage domain (NFS). When the NFS server breaks
the VM was stopped.

Image:

-device

virtio-blk-pci,iothread=iothread1,scsi=off,bus=pci.5,addr=0x0,drive=libvirt-2-format,id=ua-1d93fa9e-1665-40d7-9ffc-770513242795,bootindex=1,write-cache=on,serial=1d93fa9e-1665-40d7-9ffc-770513242795,werror=stop,rerror=stop

I assume this is the one that could use 'enospc'?

Yes, if we propagate errors, this will become werror=enospc,rerror=enospc

LUN:

-device

virtio-blk-pci,iothread=iothread2,scsi=off,bus=pci.7,addr=0x0,drive=libvirt-1-format,id=ua-19b06845-2c54-422d-921b-6ec0ee2e935b,write-cache=on,werror=stop,rerror=stop

\

Kevin, any reason not to use werror=report,rerror=report for LUN when
we want to propagate errors to the guest?

If you want to propagate errors, then 'report' is the right setting.

What does "LUN" mean exactly?

When we attach a multipath device, this is called "Direct LUN" in oVirt.
The underlying device can iSCSI or FC, managed by the user, or managed by
Cindelib.

We have 3 options:

1. As virtio or virtio-scsi

    <disk type='block' device='disk' snapshot='no'>
      <driver name='qemu' type='raw' cache='none' error_policy='stop'
io='native' iothread='1'/>
      <source dev='/dev/mapper/360014058657c2a1941841348f19c1a50' index='1'>
        <seclabel model='dac' relabel='no'/>
      </source>
      <backingStore/>
      <target dev='vdb' bus='virtio'/>
      <alias name='ua-c1bf9168-00f0-422f-a190-9ddf6bcd449b'/>
      <address type='pci' domain='0x0000' bus='0x06' slot='0x00'
function='0x0'/>
    </disk>

-blockdev
'{"driver":"host_device","filename":"/dev/mapper/360014058657c2a1941841348f19c1a50","aio":"native","node-name":"libvirt-1-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"}'
\
-blockdev
'{"node-name":"libvirt-1-format","read-only":false,"cache":{"direct":true,"no-flush":false},"driver":"raw","file":"libvirt-1-storage"}'
\
-device
virtio-blk-pci,iothread=iothread1,scsi=off,bus=pci.6,addr=0x0,drive=libvirt-1-format,id=ua-c1bf9168-00f0-422f-a190-9ddf6bcd449b,write-cache=on,werror=stop,rerror=stop
\

2. same + passthrough

    <disk type='block' device='lun' sgio='filtered' snapshot='no'>
      <driver name='qemu' type='raw' cache='none' error_policy='stop'
io='native'/>
      <source dev='/dev/mapper/360014058657c2a1941841348f19c1a50' index='1'>
        <seclabel model='dac' relabel='no'/>
      </source>
      <backingStore/>
      <target dev='sda' bus='scsi'/>
      <alias name='ua-c1bf9168-00f0-422f-a190-9ddf6bcd449b'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

-blockdev
'{"driver":"host_device","filename":"/dev/mapper/360014058657c2a1941841348f19c1a50","aio":"native","node-name":"libvirt-1-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"}'
\
-blockdev
'{"node-name":"libvirt-1-format","read-only":false,"cache":{"direct":true,"no-flush":false},"driver":"raw","file":"libvirt-1-storage"}'
\
-device
scsi-block,bus=ua-50240806-3d5a-4e5b-a220-bc394698a641.0,channel=0,scsi-id=0,lun=0,drive=libvirt-1-format,id=ua-c1bf9168-00f0-422f-a190-9ddf6bcd449b,werror=stop,rerror=stop
\

3. same + privileged I/O

    <disk type='block' device='lun' sgio='unfiltered' snapshot='no'>
      <driver name='qemu' type='raw' cache='none' error_policy='stop'
io='native'/>
      <source dev='/dev/mapper/360014058657c2a1941841348f19c1a50' index='1'>
        <seclabel model='dac' relabel='no'/>
      </source>
      <backingStore/>
      <target dev='sda' bus='scsi'/>
      <alias name='ua-c1bf9168-00f0-422f-a190-9ddf6bcd449b'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

-blockdev
'{"driver":"host_device","filename":"/dev/mapper/360014058657c2a1941841348f19c1a50","aio":"native","node-name":"libvirt-1-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"}'
\
-blockdev
'{"node-name":"libvirt-1-format","read-only":false,"cache":{"direct":true,"no-flush":false},"driver":"raw","file":"libvirt-1-storage"}'
\
-device
scsi-block,bus=ua-9c2c7e43-d32d-4ea4-9cfd-e2bb36d26fdb.0,channel=0,scsi-id=0,lun=0,drive=libvirt-1-format,id=ua-c1bf9168-00f0-422f-a190-9ddf6bcd449b,werror=stop,rerror=stop
\

It doesn't seem to be passthrough, so is

it just that you have some restriction like that it's always raw?

Yes, we don't support qcow2 for these disks (yet). Theoretically we
can support qcow2 to enable incremental backup. But the qcow2 image
will never be larger than the block device, actually smaller to leave
room for metadata.  Thin provisioning is done on the storage side.

Maybe
I would use 'enospc' for consistency even though you never expect this
error to happen. But 'report' is fine, too.

enospc looks wrong since this error should not be possible, and if it
happens we cannot handle it. Sounds like good way to confuse futures
maintainers of this code.

If it's a different code path anyway, then I agree. If you would have to
make it a special case just for this, I'd rather add a comment.

Maybe libvirt or qemu did not support "report" when this code was added in
2010?

At least as far as QEMU is concerned, 'report' has existed as long as
rerror/werror in general. If you're interested, commit 428c570512c added
it in 2009.

Kevin