On 14.12.2015 18:43, Vladimir Sementsov-Ogievskiy wrote: > The new feature for qcow2: storing dirty bitmaps. > > Only dirty bitmaps relative to this qcow2 image should be stored in it. > > Strings started from +# are RFC-strings, not to be commited of course. > > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> > --- > > docs/specs/qcow2.txt | 151 > ++++++++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 150 insertions(+), 1 deletion(-)
Overall: Looks better to me. Good enough for me to ACK it, but I still have some issues with it. Let's evaluate the main point of critique I had: I really want this not to be qemu-specific but potentially useful to all programs. Pretty good: You do implicitly describe what a (dirty) bitmap looks like by describing how to obtain the bit offset of a certain byte guest offset. So it's not an opaque binary data dump anymore. (Why only "pretty good"? I find the description to be a bit too "implicit", I think a separate section describing the bitmap structure would be better.) Good: The bitmap actually describes the qcow2 file. Not so good: While now any program knows how to read the bitmap and that it does refer to this qcow2 file, it's interpretation is not so easy still. Generally, a dirty bitmap has some reference point, that is the state of the disk when the bitmap was cleared or created. For instance, for incremental backups, whenever you create a backup based on a dirty bitmap, the dirty bitmap is cleared and the backup target is then said reference point. I think it would be nice to put that reference point (i.e. the name of an image file that contains the clean image) into the dirty bitmap header, if possible. (Note: I won't comment on orthography, because I feel like that is something a native speaker should do. O:-)) > diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt > index 121dfc8..3c89580 100644 > --- a/docs/specs/qcow2.txt > +++ b/docs/specs/qcow2.txt > @@ -103,7 +103,17 @@ in the description of a field. > write to an image with unknown auto-clear features if it > clears the respective bits from this field first. > > - Bits 0-63: Reserved (set to 0) > + Bit 0: Dirty bitmaps bit. > + This bit is responsible for Dirty bitmaps > + extension consistency. > + If it is set, but there is no Dirty bitmaps > + extensions, this should be considered as an > + error. > + If it is not set, but there is a Dirty > bitmaps > + extension, its data should be considered as > + inconsistent. > + > + Bits 1-63: Reserved (set to 0) > > 96 - 99: refcount_order > Describes the width of a reference count block entry > (width > @@ -123,6 +133,7 @@ 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 +177,31 @@ 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 an ability to > store > +dirty bitmaps in a qcow2 image. The data of this extension should be > considered > +as consistent only if corresponding auto-clear feature bit is set (see > +autoclear_features above). > +The fields of Dirty bitmaps extension are: > + > + 0 - 3: nb_dirty_bitmaps > + The number of dirty bitmaps contained in the image. Valid > + values: 1 - 65535. Again, I don't see a reason for why we should impose a strict upper limit here. I'd prefer "Note that qemu currently only supports up to 65535 dirty bitmaps per image." > +# Let's be strict, the feature should be deleted with deleting last bitmap. > + > + 4 - 7: dirty_bitmap_directory_size > + Size of the Dirty Bitmap Directory in bytes. It should be > + equal to sum of sizes of all (nb_dirty_bitmaps) dirty > bitmap > + headers. No, it "should" not be equal, it *must* be equal. But I think you can just omit that last sentence, that would be just as fine. > +# This field is necessary to effectively read Dirty Bitmap Directory, because > +# it's entries (which are dirty bitmap headers) may have different lengths. > + > + 8 - 15: dirty_bitmap_directory_offset > + Offset into the image file at which the Dirty Bitmap > + Directory 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 +396,116 @@ 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 dirty bitmaps in a qcow2 image. All dirty > bitmaps > +are relating to the virtual disk, stored in this image. > + > +=== Dirty Bitmap Directory === > + > +Each dirty bitmap saved in the image is described in a Dirty Bitmap Directory > +entry. Dirty Bitmap Directory is a contiguous area in the image file, whose > +starting offset and length are given by the header extension fields > +dirty_bitmap_directory_offset and dirty_bitmap_directory_size. The entries of > +the bitmap directory have variable length, depending on the length of the > +bitmap name. These entries are also called dirty bitmap headers. > + > +Dirty Bitmap Directory Entry: > + > + Byte 0 - 7: dirty_bitmap_table_offset > + Offset into the image file at which the Dirty Bitmap > Table > + (described below) for the bitmap starts. Must be aligned > to > + a cluster boundary. > + > + 8 - 11: dirty_bitmap_table_size > + Number of entries in the Dirty Bitmap Table of the > bitmap. > + > + 12 - 15: flags > + Bit > + 0: in_use > + The bitmap was not saved correctly and may be > + inconsistent. > + > + 1: auto > + The bitmap should be autoloaded as block dirty > bitmap > + and tracking should be started. Type of the bitmap > + should be 'Dirty Tracking Bitmap'. I find the wording a bit too qemu-specific. How about this: This bitmap is the default dirty bitmap for the virtual disk represented by this qcow2 image. It should track all write accesses immediately after the image has been opened. And I find the "should" in "Type of the bitmap should be..." a bit too weak. Maybe "This flag is valid only for Dirty Tracking Bitmaps" is better. > + > + Bits 2 - 31 are reserved and must be 0. > + > + 16 - 17: name_size > + Size of the bitmap name. Valid values: 1 - 1023. > + > + 18: type > + This field describes the sort of the bitmap. > + Values: > + 0: Dirty Tracking Bitmap If we allow different kinds of bitmaps, it should not be called "dirty bitmap" everywhere anymore. > + > + Values 1 - 255 are reserved. > +# This is mostly for error checking and information in qemu-img info output. > +# The other types may be, for example, "Backup Bitmap" - to make it possible > +# stop backup job on vm stop and resume it later. The another one is "Sector > +# Alloction Bitmap" (Fam, John, please comment). I'm waiting for their comments because that sounds like "refcount table with refcount_bits=1" to me. :-) > + 19: granularity_bits > + Granularity bits. Valid values are: 0 - 31. > +# Now, qemu allows creating bitmaps with granularity as a 32bit value. And > +# there are no reasons of increasing it. Good (implicit) question. I can't imagine any reason for wanting to have a coarser granularity than 2 GB, but I do think there may be a need in the future for some people. Once again, I think we should discriminate between what is generally a useful limitation and what is simply due to qemu not supporting anything else right now. Thus, I think it would be better to increase the range to 0 - 63 and make a note that qemu only supports values up to 31 right now. > + > + Granularity is calculated as > + granularity = 1 << granularity_bits > + > + Granularity of the bitmap is how many bytes of the image > + accounts for one bit of the bitmap. > +# To be closer to qcow2 and its reality, I've decided to use byte-granularity > +# here, not sector-granularity. I like that. But do note that qcow2 does align everything at least to 512 bytes, so having used sector granularity wouldn't have been too bad. > + > + variable: The name of the bitmap (not null terminated). Should be > + unique among all dirty bitmap names within the Dirty > + bitmaps extension. > + > + variable: Padding to round up the Dirty Bitmap Directory Entry size > + to the next multiple of 8. What I'd like here is variable additional information based on the bitmap type. For some types, this may be absolutely necessary; for dirty tracking bitmaps it depends on what we do about the reference point thing. The reference point thing is the following: As mentioned at the top, I'd like there to be some kind of description of what the clean state was. As far as I know, this is generally a backup in the form of a file. In that case, we could put that filename here. I don't think not having a reference point description is a serious show stopper. qemu itself does rely on the management layer to know which bitmap to use when. But I think it would be pretty nice to have it here. > + > +=== Dirty Bitmap Table === > + > +Dirty bitmaps are stored using a one-level (not two-level like refcounts and > +guest clusters mapping) structure for the mapping of bitmaps to host > clusters. > +It is called Dirty Bitmap Table. > + > +Each Dirty Bitmap Table has a variable size (stored in the Dirty Bitmap > +Directory Entry) and may use multiple clusters, however it must be contiguous > +in the image file. > + > +Given an offset (in bytes) into the bitmap, the offset into the image file > can > +be obtained as follows: > + > + byte_offset = > + dirty_bitmap_table[offset / cluster_size] + (offset % cluster_size) > + > +Taking into account the granularity of the bitmap, an offset in bits into the > +image file, corresponding to byte number byte_nr of the image can be > calculated > +like this: > + > + bit_offset = > + byte_offset(byte_nr / granularity / 8) * 8 + (byte_nr / granularity) > % 8 > + > +Note: the last formula is only for understanding the things, it is unlikely > for > +it to be useful in a program. I think this note is superfluous. All the pseudo-code in this file is only that, pseudo-code. ;-) Apart from that, I think this last formula should be in its own section ("Dirty Bitmaps" or simply "Bitmaps") which describes the structure of a bitmap. Putting this term there should basically suffice. I was about to say I'd like it to define the bit order, too (i.e. "bit offset 0 is the LSb"), but, well, it just uses the bit order used everywhere in qcow2. > + > +Dirty Bitmap Table entry: > + > + Bit 0: Reserved and should be zero if bits 9 - 55 are non-zero. > + If bits 9 - 55 are zero: > + 0: Cluster should be read as all zeros. > + 1: Cluster should be read as all ones. > + > + 1 - 8: Reserved and must be zero. > + > + 9 - 55: Bits 9 - 55 of host cluster offset. Must be aligned to a > + cluster boundary. If the offset is 0, the cluster is > + unallocated, see bit 0 description. > + > + 56 - 63: Reserved, must be 0. > Looks good. Max
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