Re: [PATCH] nbd: server: Report holes for raw images

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

19.02.2021 19:58, Eric Blake wrote:
> On 2/19/21 10:42 AM, Eric Blake wrote:
> 
>>> To me, data=false looks compatible with NBD_STATE_HOLE. From user point
>>> of view, getting same results from qemu-nbd and qemu-img is more
>>> important than being more correct about allocation status.
>>
>> More to the point, here is our inconsistency:
>>
>> In nbd/server.c, we turn !BDRV_BLOCK_ALLOCATED into NBD_STATE_HOLE
>>
>> In block/nbd.c, we turn !NBD_STATE_HOLE into BDRV_BLOCK_DATA
>>
>> The fact that we are not doing a round-trip conversion means that one of
>> the two places is wrong.  And your argument that the server side is
>> wrong makes sense to me.
> 
> In fact, when I went back and researched when this was introduced (see
> commit e7b1948d51 in 2018), we may have been aware of the inconsistency
> between client and server, but didn't make up our minds at the time:
> https://lists.gnu.org/archive/html/qemu-devel/2018-03/msg03465.html
> "? Hm, don't remember, what we decided about DATA/HOLE flags mapping.."
> 
>>
>> I'll wait a few days for any other reviewer commentary before taking
>> this through my NBD tree.
>>
> 


I can add the following.

First, link to my research of block_status in Qemu: https://lists.gnu.org/archive/html/qemu-devel/2020-04/msg05136.html

And about HOLE and ZERO..

As I've noted in the research above, SCSI may return HOLE & !ZERO:

from SCSI:
Logical Block Provisioning Read Zeros (LBPRZ) bit
1     If the logical block provisioning read zeros (LBPRZ) bit is set to one, then, for an unmapped LBA specified by a read operation, the deviceserver shall send user data with all bits set to zero to the data-in buffer.
0     If the TPRZ bit is set to zero, then, for an unmapped LBA specified by a read operation, the device server may send user data with all bitsset to any value to the data-in buffer.

So we can have an unmapped area that can be read as any random data. Same thing can be said about null-co driver with read-zeroes=false

Also, qcow2 support ALLOCATED ZERO clusters which reads as zero but data is allocated - they are reasonable to report as ZERO & !HOLE

And of-course UNALLOCATED ZERO clusters in qcow2 and lseek-holes are reasonable to report as ZERO & HOLE,  because they reads as zero and "future writes to that area may cause fragmentation or encounter an NBD_ENOSPC"..

So, all combination are reasonable, we just need to fix Qemu NBD server to report correct statuses in all these cases.

It seems that ZERO/HOLE specification is a lot more reasonable than what we have with ZERO/DATA/ALLOCATED in Qemu, and may be true way is move internal block_status to use NBD terms.


And thanks for CCing me. Hmm, maybe, I'll suggest myself as co-maintainer for NBD?

-- 
Best regards,
Vladimir