Re: [Qemu-devel] [PATCH v4 RFC] spec: add qcow2-dirty-bitmaps specification

[Kevin Wolf <kwolf@redhat.com>]

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Am 14.12.2015 um 21:05 hat Max Reitz geschrieben:
> 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 <vsementsov@virtuozzo.com>

First of all, I think we may need some improvemnts on details and wording
here and there, but the format in general looks quite reasonable to me
(at the first sight at least).

> >  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.

I don't think it's a valid assumption that the reference point has a
file name. Which makes me wonder... How do dirty bitmaps and internal
snapshots play together?

What I guess could be done is storing a creation date if you think this
would be useful.

> (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.

...unless an internal snapshot is loaded?

> 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.

Why 63 and not 255 (the maximum for this one-byte field)? As soon as we
pick an arbitrary value, we can just take whatever qemu actually
supports. We can always increase the limit later.

> > +
> > +                    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.

No 512 byte units in the image format, please. It's an implementation
artifact of qemu and has nothing to do with physical disk sectors. Those
happen to be 4k in most cases nowadays.

> > +
> > +        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.

This could turn out useful, yes.

I also think some extensibility mechanism would be good, like extra_data
in internal snapshots.

> 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.

Who says that it's a local file? Or that it hasn't been moved since we
created the bitmap?

> 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.

Kevin

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