CentOS virt - Mar 2015 - Can't block-attach a file on a read only volume?

Hi All,

 

One more weird issue, this works on old xen but fails on 4.4:

 

xendev01 ~ # mkdir /mnt/test

xendev01 ~ # mount -t tmpfs - /mnt/test

xendev01 ~ # dd if=/dev/null of=/mnt/test/disk seek=100M bs=1

0+0 records in

0+0 records out

0 bytes (0 B) copied, 0.000201809 s, 0.0 kB/s

 

xendev01 ~ # /usr/sbin/xl block-attach nathannx "file:/mnt/test/disk"
"xvdd4"

DEBUG libxl__blktap_devpath 37 aio:/mnt/test/disk

DEBUG libxl__blktap_devpath 40 /dev/xen/blktap-2/tapdev20

 

xendev01 ~ # xl block-detach nathannx 51764

DEBUG libxl__device_destroy_tapdisk 66 type=aio:/mnt/test/disk
disk=:/mnt/test/disk

 

xendev01 ~ # mount -o remount,ro /mnt/test   

 

xendev01 ~ # /usr/sbin/xl block-attach nathannx "file:/mnt/test/disk"
"xvdd4"

DEBUG libxl__blktap_devpath 37 aio:/mnt/test/disk

libxl: error: libxl.c:2149:device_disk_add: failed to get blktap devpath for
0xd3abd0

libxl: error: libxl.c:1727:device_addrm_aocomplete: unable to (null) device

libxl_device_disk_add failed.

 

I'm not sure why xen would care if the disk is writable? Would be nice to be
able to mount these since many NFS storage arrays provide read only access to
snapshots.

 

- Nathan

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On Thu, Mar 12, 2015 at 8:05 PM, Nathan March <nathan at gt.net>
wrote:
> xendev01 ~ # mount -o remount,ro /mnt/test
>
>
>
> xendev01 ~ # /usr/sbin/xl block-attach nathannx
"file:/mnt/test/disk"
> "xvdd4"
>
> DEBUG libxl__blktap_devpath 37 aio:/mnt/test/disk
>
> libxl: error: libxl.c:2149:device_disk_add: failed to get blktap devpath
for
> 0xd3abd0
>
> libxl: error: libxl.c:1727:device_addrm_aocomplete: unable to (null) device
>
> libxl_device_disk_add failed.
>
>
>
> I'm not sure why xen would care if the disk is writable? Would be nice
to be
> able to mount these since many NFS storage arrays provide read only access
> to snapshots.
So by default, block-attach will set the access to a disk as
read-write.  If disk access is marked as read-write, the backend (in
this case tapdisk) will attempt to open the file read-write; if this
fails (as it will on read-only storage), the whole disk creation will
fail.  (This is true either of block-attach or domain creation.)

Normally you could fix this by adding ",ro" to your disk spec.  But in
the case of blktap, this option is ignored at the moment. :-/

I have, however, just taught xl to pay attention to this for blktap.
I've got fixes to both this issue and the extra-tapdisk-process issue
from the other thread in this local koji build:

http://cbs.centos.org/kojifiles/work/tasks/8801/8801/

If you could test those and let me know if it fixes your problem, I'd
appreciate it. :-)

As an aside, you're using the old disk format; the new disk format
looks more like this:

"vdev=hdc,format=raw,access=rw,target=/mnt/test/disk"

More info here:

/usr/share/doc/xen-doc-4.4.1/misc/xl-disk-configuration.txt

Thanks,
 -George

> http://cbs.centos.org/kojifiles/work/tasks/8801/8801/
> 
> If you could test those and let me know if it fixes your problem, I'd
> appreciate it. :-)
Confirmed, both issues are fixed. Thanks! Any plans to push those packages to
main mirrors?

- Nathan