Re: Questions about online resizing a lun passthrough disk with virtio-scsi

[Paolo Bonzini]

On 08/07/20 16:44, lma wrote:
> 
> Is the 'block_resize' mandatory to notify guest os after online resizing
> a lun passed through disk? I'm curious it because I found there're couple
> of ways can make guest os realize the disk capacity change.
> e.g:
> * run 'block_resize' via qmp to let virtio-scsi notify the frontend about
>   capacity change.
> * run 'rescan-scsi-bus.sh -s' inside guest.
> * run 'sg_readcap --16 /dev/sda' inside guest.
> 
> I knew that the purpose of 'block_resize' is not only to notify guest os,
> but also to update some internal structure's member, say bs->total_sectors.
> What if I forgot to run 'block_resize', but run 'rescan-scsi-bus.sh -s'
> in guest?

Request start and length are checked even for passthrough disks (see
scsi_disk_dma_command in hw/scsi/scsi-disk.c, called by
scsi_block_dma_command), but the maximum LBA is snooped from READ
CAPACITY commands (see scsi_read_complete in hw/scsi/scsi-generic.c).
So as long as rescan-scsi-bus.sh results in a READ CAPACITY command, it
should work.

It's not recommended however, because block_resize will report the
change to the guest directly with a CAPACITY HAS CHANGED unit attention
condition.

Thanks,

Paolo