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

[Max Reitz]

On 14.12.2015 21:45, John Snow wrote:
> 
> 
> On 12/14/2015 03:05 PM, 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 <address@hidden>
>>> ---
>>>
>>>  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.
>>
> 
> This starts to get a little spooky, because QEMU doesn't necessarily
> know where (or what) the reference is. QEMU doesn't even know where the
> last incremental is.
> 
> It might be hard to store something meaningful here.

Yes, I thought as much, that's where the "if possible" comes in.

I don't think it would hurt to include the field even if we're unsure
how much use we can make of it. If we can store something nice there,
great! If we can't, too bad.

> I suppose the management application could do it itself if it wants to.

Considering the qcow2 image is closed by qemu anyway just after the
bitmaps are written into it, this might indeed be possible. But I
wouldn't count on it.

>>
>> (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

[...]

>>> +# 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.
>>
> 
> It's informative, though. Just another clarifying detail that the bitmap
> directory is the collection of all the dirty bitmap headers.
> 
> Replacing "should" with "must" is sufficient.

Of course.

[...]

>>> +=== Dirty Bitmap Directory ===

[...]

>>> +        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.
>>
> 
> Closer; though we can certainly have more than one, so "a" may not be
> appropriate.
> 
> "This bitmap is a dirty tracking bitmap for the virtual disk represented
> by this qcow2 image."

Hm, aren't all dirty tracking bitmaps of this image... Well, dirty
tracking bitmaps of this image?

We currently don't have anything but dirty tracking bitmaps, and the
flag actually is explicitly valid only for that kind of bitmaps (for
now); also, in this revision, all dirty bitmaps do refer to this very
qcow2 file anyway.

Maybe I just interpreted the sentence wrong and put too much weight into
the first part ("should be autoloaded as block dirty bitmap") so I felt
the need to translate it to an explicit "is the default".

So I guess this bit simply means that the bitmap should be active in
that it tracks new writes?

> And to avoid "should" again:
> 
> "All writes to this file must also be represented in this bitmap."

:-)

I wasn't sure here. I felt "should" appropriate because maybe you do
have a very compelling reason not to do so and to let a certain write
access slip by on purpose.

So this is up to you, if it's "must", then it's "must". If it's
"should", then it's "should".

>> 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.
>>
> 
> Sure.
> 

[...]

>>> +# 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. :-)
>>
> 
> The idea is that we may allow for bitmaps to store other kinds of
> information, like "allocated in this layer." That information is not
> necessarily useful to qcow2, but it might be for other image formats. If
> we ever do add such subtypes, we can always add a new reserved entry:
> 
> # 1: Reserved - Invalid for qcow2
> # 2: Backup Progress Bitmap ...
> # 3-255: Reserved
> 
> Backup progress bitmaps may indeed be useful and sane information to
> store in a qcow2, though.

OK. I was wondering because I want to (again) make sure that these other
bitmaps are not plain dumps of some obscure bitmap qemu has, but that we
can imagine bitmaps that are generally useful.

First: OK, it doesn't really matter. Having this field definitely is
good, even if we wouldn't actually use it.

<offtopic reason="This series is about dirty bitmaps and not about other
types of bitmaps">
Now, my thoughts about backup progress bitmaps are the following:

If we put the backup progress into the source file, that's a bit
strange. You aren't modifying this file, so why would you put it here?
Also, no program but the one doing the backup can do anything with that
bitmap. Others will just say "Nice to know how far you're in your
backup, but, well, am I supposed do something about it?"

Instead, we could put it into the target. That would be nicer, because
then you open the target and you can see "Oh, there's a backup progress
bitmap! I guess I'll need to copy these clusters from the backup source
[given in the backup bitmap header]."

Strictly speaking, the same can be achived by combining the dirty bitmap
used for the backup (in case of an incremental backup) and the refcount
information for the backup target. However, finding the right dirty
bitmap may be a bit cumbersome, so I do think having such a backup
progress bitmap (in the target image) is justified.
</offtopic>

(If you've already decided to put the backup progress into the target
image, you may find that offtopic block not very helpful; however, it
may help you get an idea of when I consider some information to be
beneficial to a qcow2 file.)

> Fam may have more opinions as he's been working on this area of thought
> recently.
> 
>>> +             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.
>>
> 
> I suppose that won't hurt anything.
> 
> (I look forward to the future where the hard drives are so big and the
> network bandwidth so bountiful that 2GB granularity is seen as "too
> fine-grained!")

640k... ;-)

[...]

>>> +
>>> +        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.
>>
> 
> We may also have exported that backup to an NBD server and we're not
> sure (on the local end) where that data is anymore, though.
> 
> For local utility usage, when a reference is possible, we might be able
> to list it as an optional nice thing, but I think requiring it might be
> difficult.

You're right, we don't need to require it.

>> 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.
>>
> 
> I'm not opposed to listing some "nice" information when available.
> 
> last_backup /path/to/incremental.5.qcow2
> base_backup /path/to/reference.qcow2
> 

I don't think we need the base_backup since you can get that by walking
through the backing chain of the reference point (the backup target),
but it probably won't hurt if you can make a good general semantic
connection to the dirty bitmap.

Max

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