Re: [Qemu-devel] [PATCH 2/4] nbd/server: Advertise actual minimum block size

[Vladimir Sementsov-Ogievskiy]

17.08.2018 17:53, Eric Blake wrote:
> On 08/17/2018 08:49 AM, Vladimir Sementsov-Ogievskiy wrote:
>
> > > BDRV_SECTOR_SIZE : 1;
> > > +    /* minimum - 1 for back-compat, or actual if client will obey 
> > > it. */
> > > +    if (client->opt == NBD_OPT_INFO || blocksize) {
> > > +        sizes[0] = blk_get_request_alignment(exp->blk);
> > > +    } else {
> > > +        sizes[0] = 1;
> > > +    }
> > >       /* preferred - Hard-code to 4096 for now.
> > >        * TODO: is blk_bs(blk)->bl.opt_transfer appropriate? */
> > >       sizes[1] = 4096;
> >
> > Here, what about dirty bitmap export through BLOCK_STATUS? Could it's 
> > granularity be less than request_alignment? Shouldn't we handle it 
> > somehow?
>
> Can you create a dirty bitmap with a granularity smaller than 
> request_alignment?  I know you can configure dirty bitmap granularity 
> independently from cluster size (in both directions: either smaller or 
> larger than cluster size), but that it has a least a minimum lower 
> bounds of 512.  You're probably right that we also want it to have a 
> minimum lower bound of the request_alignment (if you're using a device 
> with 4k minimum I/O, request_alignment would be 4k, and having a dirty 
> bitmap any smaller than that granularity is wasted space).
>
> On the other hand, I also think we're safe for this patch: even if you 
> waste the space by creating a bitmap with too-small granularity, the 
> actions that write bits into the bitmap will still be aligned to 
> request_alignment,

Not quite right: nbd_export_bitmap searches through the backing chain 
for the bitmap, and it may correspond to the bds with smaller 
request_alignment.

> which means you always set a multiple of bits per action; when reading 
> back the dirty bitmap to report over NBD_CMD_BLOCK_STATUS, you'll 
> never encounter a change in bit status except on alignment boundaries 
> (based on how the bits were written), and thus what NBD reports will 
> still be aligned to the advertised minimum size, rather than the 
> smaller bitmap granularity.


--
Best regards,
Vladimir