From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42261) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z2LyM-0006M7-NV for qemu-devel@nongnu.org; Tue, 09 Jun 2015 12:01:58 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z2LyG-0005VW-Kl for qemu-devel@nongnu.org; Tue, 09 Jun 2015 12:01:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57300) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z2LyG-0005V2-DX for qemu-devel@nongnu.org; Tue, 09 Jun 2015 12:01:48 -0400 Message-ID: <55770DEA.6080907@redhat.com> Date: Tue, 09 Jun 2015 12:01:46 -0400 From: John Snow MIME-Version: 1.0 References: <1433776886-27239-1-git-send-email-vsementsov@virtuozzo.com> <1433776886-27239-2-git-send-email-vsementsov@virtuozzo.com> In-Reply-To: <1433776886-27239-2-git-send-email-vsementsov@virtuozzo.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH 1/8] spec: add qcow2-dirty-bitmaps specification List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org Cc: kwolf@redhat.com, pbonzini@redhat.com, Vladimir Sementsov-Ogievskiy , stefanha@redhat.com, den@openvz.org On 06/08/2015 11:21 AM, Vladimir Sementsov-Ogievskiy wrote: > From: Vladimir Sementsov-Ogievskiy >=20 > Persistent dirty bitmaps will be saved into qcow2 files. It may be used > as 'internal' bitmaps (for qcow2 drives) or as 'external' bitmaps for > other drives (there may be qcow2 file with zero disk size but with > several dirty bitmaps for other drives). >=20 > Signed-off-by: Vladimir Sementsov-Ogievskiy > --- > docs/specs/qcow2.txt | 66 ++++++++++++++++++++++++++++++++++++++++++++= ++++++++ > 1 file changed, 66 insertions(+) >=20 > diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt > index 121dfc8..0fffba2 100644 > --- a/docs/specs/qcow2.txt > +++ b/docs/specs/qcow2.txt > @@ -123,6 +123,7 @@ be stored. Each extension has a structure like the = following: > 0x00000000 - End of the header extension area > 0xE2792ACA - Backing file format name > 0x6803f857 - Feature name table > + 0x23852875 - Dirty bitmaps > other - Unknown header extension, can be = safely > ignored > =20 > @@ -166,6 +167,19 @@ the header extension data. Each entry look like th= is: > terminated if it has full length) > =20 > =20 > +=3D=3D Dirty bitmaps =3D=3D > + > +Dirty bitmaps is an optional header extension. It provides a possibili= ty of > +storing dirty bitmaps in qcow2 image. The fields are: > + I would say "It provides the ability to store dirty bitmaps in a qcow2 image." We have more than the possibility now :) > + 0 - 3: nb_dirty_bitmaps > + Number of dirty bitmaps contained in the image > + Most fields seem to be documented as complete sentences, so maybe "The number of dirty bitmaps contained in the image." and add the period. > + 4 - 11: dirty_bitmaps_offset > + Offset into the image file at which the dirty bitma= ps table > + starts. Must be aligned to a cluster boundary. > + > + > =3D=3D Host cluster management =3D=3D > =20 > qcow2 manages the allocation of host clusters by maintaining a referen= ce count > @@ -360,3 +374,55 @@ Snapshot table entry: > =20 > variable: Padding to round up the snapshot table entry size = to the > next multiple of 8. > + > + > +=3D=3D Dirty bitmaps =3D=3D > + > +The feature supports storing several dirty bitmaps in the qcow2 file. You could drop "several" from this sentence, we support an arbitrary numb= er. > + > +=3D=3D=3D Cluster mapping =3D=3D=3D > + > +Dirty bitmaps are stored using a ONE-level structure for the mapping o= f > +bitmaps to host clusters. There is only an L1 table. > + > +The L1 table has a variable size (stored in the Bitmap table entry) an= d may > +use multiple clusters, however it must be contiguous in the image file= . > + > +Given an offset into the bitmap, the offset into the image file can be > +obtained as follows: > + > + offset =3D l1_table[offset / cluster_size] + (offset % cluster_siz= e) > + > +L1 table entry: > + > + Bit 0 - 61: Standard cluster descriptor > + > + 62 - 63: Reserved > + > +=3D=3D=3D Bitmap table =3D=3D=3D > + > +A directory of all bitmaps is stored in the bitmap table, a contiguous= area in > +the image file, whose starting offset and length are given by the head= er fields > +dirty_bitmaps_offset and nb_dirty_bitmaps. The entries of the bitmap t= able have > +variable length, depending on the length of name and extra data. > + > +Bitmap table entry: > + > + Byte 0 - 7: Offset into the image file at which the L1 table f= or the > + bitmap starts. Must be aligned to a cluster bounda= ry. > + > + 8 - 11: Number of entries in the L1 table of the bitmap > + > + 12 - 15: Bitmap granularity in bytes > + > + 16 - 23: Bitmap size in sectors > + > + 24 - 25: Size of the bitmap name > + This is, believe it or not, the first place in code that I am aware of that actually places a limit on how big a bitmap name can be (64K!) -- we should probably clamp this value to something even lower (1024 is probably graciously sufficient) and enforce that in the various bitmap add/create routines. > + variable: The name of the bitmap (not null terminated) > + > + variable: Padding to round up the bitmap table entry size to= the > + next multiple of 8. > + > +The fields "size", "granularity" and "name" are corresponding with the= fields > +in struct BdrvDirtyBitmap. >=20 Not yet being intricately familiar with qcow2, this looks good to me. --js