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

[Lin Ma]

On 2020-07-08 15:11, Paolo Bonzini wrote:
> 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.

Yeah, the rescan-scsi-bus.sh does result in a READ CAPACITY command.

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

Got it, The 'block_resize' is the recommended or necessary step, Even 
for
passthrough disk online resizing.

Thanks for your information,
Lin