Re: [PATCH 3/6] mirror: Skip writing zeroes when target is already zero

[Eric Blake]

On Mon, Apr 21, 2025 at 09:15:33AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 17.04.25 00:51, Eric Blake wrote:
> > (a write zeroes that fails AND causes the disk to no longer read as
> > zero should not happen)
> 
> I don't know, is there such a contract? write-zeroes may fallback to write(), 
> which only state that:
> 
>        An error return value while performing write() using direct I/O
>        does not mean the entire write has failed.  Partial data may be
>        written and the data at the file offset on which the write() was
>        attempted should be considered inconsistent.
> 
> So, I used to think that on failed write nothing is guaranteed.
> 
> What do we lose if we just unset the bitmap before write-zeroes, and set it 
> again in case of success?
>

I still don't see the point.  Either the cluster was already non-zero
before the failed write-zero (so there's no bit to pre-clear); or the
cluster was already zero before the failed write-zero, and any failure
that corrupts the disk by actually turning zeroes into non-zero is not
worth worrying about, so pre-clearing the bit is not going to make
things any better.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org