On Mon, 12/14 21:05, Max Reitz wrote:
> 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.
Do you mean unsetting the auto-clear feature bit? Yes, I think that makes sense.
> > +
> > + 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. :-)
Allocation information for qcow2 can be obtained from L1/L2/refcount tables, so
maybe it's not worth a type here. I am not quite sure about the "backup
bitmap" type either, because it is QEMU specific; alternatively we can add an
"enabled" flag to each dirty bitmap, so that the "drive-backup" bitmap can be
saved as a disabled dirty tracking bitmap.
But the idea of having the type field makes sense to me in general.
>
> > + 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.
I would then suggest limiting the valid values to 9 - 63, at least as a note
for QEMU support.
>
> > +
> > + 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.
>
Apart from all above, is it worth documenting what will happen with all the
existing dirty bitmaps when internal snapshots are created/removed/switched?
Fam
