[Qemu-devel] Global -snapshot not working where -drive ..., snapshot=on is fine

[Qemu-devel] Global -snapshot not working where -drive ..., snapshot=on is fine

[Charles Duffy]

Howdy.

Running qemu-kvm-0.10.4, I'm observing -snapshot to fail in conjunction 
with -drive in cases where either creating a new qcow2 backing store 
with qemu-img or using -drive ...,snapshot=on succeeds.

I'm trying to do a read-only mount of a disk image in need of ext3 
journal recovery.

Running without any snapshot support (with filesystem-level permissions 
allowing only read-only access to the disk), an (expected) failure 
occurs when trying to write to the disk:

> EXT3-fs: INFO: recovery required on readonly filesystem.
> EXT3-fs: write access will be enabled during recovery.
> kjournald starting.  Commit interval 5 seconds
> EXT3-fs: recovery complete.
> hda: dma_timer_expiry: dma status == 0x21
> hda: DMA timeout error

Running with snapshot support enabled globally via -snapshot also fails 
with a timeout, this one happening earlier during drive detection:

> ide1 at 0x170-0x177,0x376 on irq 15
> hda: max request size: 128KiB
> hda: 2016 sectors (1 MB) w/256KiB Cache, CHS=2/16/63, (U)DMA
> hda: cache flushes supported
>  hda:<4>hda: dma_timer_expiry: dma status == 0x61
> hda: DMA timeout error

Running against a writable qcow2 image backed by da.qcow2 works, as does 
setting snapshot=on as part of the -drive specifier for da.qcow2.

Any ideas?


The full command line in use looks like the following:

/usr/bin/qemu-kvm \
   -drive file=da.qcow2 \
   -m 384 \
   -no-reboot \
   -kernel /usr/lib64/guestfs/vmlinuz.centos-5.x86_64 \
   -initrd /usr/lib64/guestfs/initramfs.centos-5.x86_64.img \
   -append 'panic=1 console=ttyS0 guestfs=10.0.2.4:6666' \
   -nographic \
   -serial stdio \
   -net channel,6666:unix:/tmp/libguestfsCgbezn/sock,server,nowait \
   -net user,vlan=0 -net nic,model=virtio,vlan=0

[Qemu-devel] Re: Global -snapshot not working where -drive ..., snapshot=on is fine

[Charles Duffy]

Much to my relief (after looking at the source and wondering in 
bewilderment how the two cases could differ), this is actually a false 
alarm caused by an empty string passed by libguestfs on argv and 
interpreted by qemu as a filename.