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

[Lin Ma <lma@suse.de>]

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