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From: Eric Blake <eblake@redhat.com>
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Date: Wed, 20 Jun 2018 10:43:52 -0500
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Subject: Re: [Qemu-devel] [PATCH v5 4/6] nbd/server: implement dirty bitmap
 export
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To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>, qemu-block@nongnu.org, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, pbonzini@redhat.com, den@openvz.org, armbru@redhat.com, mreitz@redhat.com

On 06/20/2018 06:24 AM, Eric Blake wrote:

>> +/* Set several extents, describing region of given @length with given=
=20
>> @flags.
>> + * Do not set more than @nb_extents, return number of set extents.
>> + */
>> +static unsigned add_extents(NBDExtent *extents, unsigned nb_extents,
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 uint64_t length, uint32_t flags)
>> +{
>> +=C2=A0=C2=A0=C2=A0 unsigned i =3D 0;
>> +=C2=A0=C2=A0=C2=A0 uint32_t max_extent =3D QEMU_ALIGN_DOWN(INT32_MAX,=
 BDRV_SECTOR_SIZE);
>=20
> This is too small of a granularity wrong when the server advertised 4k=20
> alignment during NBD_OPT_GO; it should probably refer to=20
> bs->bl.request_alignment.

In fact, we can just use INT32_MAX. The dirty bitmap has a granularity=20
at least as large as the sector size, but no smaller than the=20
request_alignment. We don't have to worry about alignment here, as the=20
extents will already be naturally aligned when converting from the=20
bitmap into extents in the caller.

--=20
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org