Re: [Qemu-devel] [PATCH v1 01/13] qcow2: alloc space for COW in one chunk

[Denis V. Lunev]

On 05/26/2017 11:11 AM, Kevin Wolf wrote:
>
> No. This is what you bypass:
some analysis for the record.

> * All sanity checks that the block layer does
>
> * bdrv_inc/dec_in_flight(), which is required for drain to work
>   correctly. Not doing this will cause crashes.
Should not be  a problem. We are in the request, which
is accounted for the parent. Drain waits parent first and after that
switched to child.

> * tracked_request_begin/end(), mark_request_serialising() and
>   wait_serialising_requests(), which are required for serialising
>   requests to work correctly
should not cause a problem as we are protected by meta
and this area is not exposed to read operation until
fully complete. But yes, not that good.
 
> * Ensuring correct request alignment for file. This means that e.g.
>   qcow2 with cluster size 512 on a host with a 4k native disk will
>   break.
we have already checked that clusters are aligned.

> * blkdebug events
yes
> * before_write_notifiers. Not calling these will cause corrupted backups
>   if someone backups file.
no as there is no user visible data here, we are
writing zeroes where there are zeroes from guest
point of view

> * Dirty bitmap updates
same. The data from the guest point of view is not changed.
This is even good that we do not mark this areas as dirty.

> * Updating write_gen, wr_highest_offset and total_sectors
write_gen is not a problem. It will be set on actual write.
total_sectors is changed manually. Agree that this is not
really elegant.

> * Ensuring that bl.max_pwrite_zeroes and bl.pwrite_zeroes_alignment are
>   respected
yes
> And these are just the obvious things. I'm sure I missed some.
anyway, this is a good list of things to take into account
within bdrv_allocate. But it is important, that from the
guest point of view the content of COW areas is not
changed and thus we can have some important shortcuts.

Den