On 06/08/2015 11:21 AM, Vladimir Sementsov-Ogievskiy wrote: > From: Vladimir Sementsov-Ogievskiy <vsement...@parallels.com> > > 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). > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> > --- > docs/specs/qcow2.txt | 66 > ++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 66 insertions(+) > > 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 > > @@ -166,6 +167,19 @@ the header extension data. Each entry look like this: > terminated if it has full length) > > > +== Dirty bitmaps == > + > +Dirty bitmaps is an optional header extension. It provides a possibility 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 bitmaps > table > + starts. Must be aligned to a cluster boundary. > + > + > == Host cluster management == > > qcow2 manages the allocation of host clusters by maintaining a reference > count > @@ -360,3 +374,55 @@ Snapshot table entry: > > variable: Padding to round up the snapshot table entry size to the > next multiple of 8. > + > + > +== Dirty bitmaps == > + > +The feature supports storing several dirty bitmaps in the qcow2 file. You could drop "several" from this sentence, we support an arbitrary number. > + > +=== Cluster mapping === > + > +Dirty bitmaps are stored using a ONE-level structure for the mapping of > +bitmaps to host clusters. There is only an L1 table. > + > +The L1 table has a variable size (stored in the Bitmap table entry) and 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 = l1_table[offset / cluster_size] + (offset % cluster_size) > + > +L1 table entry: > + > + Bit 0 - 61: Standard cluster descriptor > + > + 62 - 63: Reserved > + > +=== Bitmap table === > + > +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 header > fields > +dirty_bitmaps_offset and nb_dirty_bitmaps. The entries of the bitmap table > 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 for the > + bitmap starts. Must be aligned to a cluster boundary. > + > + 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. > Not yet being intricately familiar with qcow2, this looks good to me. --js
