From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: kwolf@redhat.com, qemu-devel@nongnu.org,
Vladimir Sementsov-Ogievskiy <vsementsov@parallels.com>,
stefanha@redhat.com, den@openvz.org, pbonzini@redhat.com,
jsnow@redhat.com
Subject: Re: [Qemu-devel] [PATCH 1/8] spec: add qcow2-dirty-bitmaps specification
Date: Wed, 10 Jun 2015 11:49:21 +0300 [thread overview]
Message-ID: <5577FA11.3080009@virtuozzo.com> (raw)
In-Reply-To: <5577F312.9040502@virtuozzo.com>
On 10.06.2015 11:19, Vladimir Sementsov-Ogievskiy wrote:
> On 09.06.2015 20:03, Stefan Hajnoczi wrote:
>> On Mon, Jun 08, 2015 at 06:21:19PM +0300, Vladimir
>> Sementsov-Ogievskiy wrote:
>>> From: Vladimir Sementsov-Ogievskiy <vsementsov@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 <vsementsov@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:
>>> +
>>> + 0 - 3: nb_dirty_bitmaps
>>> + Number of dirty bitmaps contained in the image
>> Is there a maximum?
> hmm. any proposals for this?
>>
>>> +
>>> + 4 - 11: dirty_bitmaps_offset
>>> + Offset into the image file at which the dirty
>>> bitmaps table
>>> + starts. Must be aligned to a cluster boundary.
>> The autoclear feature bit is undocumented.
>>
>>> == 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.
>>> +
>>> +=== 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.
>> The use of "L1 table" could be confusing. The refcount metadata uses
>> "refcount table" and "refcount block" to describe a one-level table.
> I agree. Hmm.. dirty bitmaps table? ok?
oh, no, bad idea. dirty bitmaps table is other thing))
>>
>>> +
>>> +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)
>> It might help to add granularity to this formula.
>>
>> Instead of "offset", "bit_number" or "bitnr" might be clearer since
>> "offset" means something different in other parts of the document.
> Hmm. In my opinion, the bitmap here is stored as raw data. And
> granularity is an additional parameter (for deserializing this data).
> So, it is an offset in bytes for this data. The format is not for
> accessing bitmap bits, it's only for loading the whole bitmap one time.
>>
>>> +
>>> +L1 table entry:
>>> +
>>> + Bit 0 - 61: Standard cluster descriptor
>>> +
>>> + 62 - 63: Reserved
>> Do you really want to use the standard cluster descriptor with it's zero
>> bit?
>>
>> Since bitmaps don't honor backing files there doesn't seem much point in
>> using the zero bit, things are simpler if just bits 9-55 are contain the
>> host cluster offset and 0 means the cluster is unallocated.
>>
>> By honoring the zero bit there are three states:
>> 1. Zero bit set, read zeroes
>> 2. Zero bit not set, host cluster offset != 0, bits valid
>> 3. Zero bit not set, host cluster offset == 0, unallocated
>>
>> State 1 is not useful.
>>
>>> +=== 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
>>> +
>>> + 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.
>> Referring to the internals of a C struct in QEMU is not appropriate for
>> a file format specification. Please document the fields fully including
>> their constraints, minimums, maximums, etc.
>
>
--
Best regards,
Vladimir
* now, @virtuozzo.com instead of @parallels.com. Sorry for this inconvenience.
next prev parent reply other threads:[~2015-06-10 8:49 UTC|newest]
Thread overview: 78+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-08 15:21 [Qemu-devel] [PATCH v2 RFC 0/8] block: persistent dirty bitmaps Vladimir Sementsov-Ogievskiy
2015-06-08 15:21 ` [Qemu-devel] [PATCH 1/8] spec: add qcow2-dirty-bitmaps specification Vladimir Sementsov-Ogievskiy
2015-06-09 16:01 ` John Snow
2015-06-09 17:03 ` Stefan Hajnoczi
2015-06-10 8:19 ` Vladimir Sementsov-Ogievskiy
2015-06-10 8:49 ` Vladimir Sementsov-Ogievskiy [this message]
2015-06-10 13:00 ` Eric Blake
2015-06-11 10:16 ` Vladimir Sementsov-Ogievskiy
2015-06-10 13:24 ` Stefan Hajnoczi
2015-06-11 10:19 ` Vladimir Sementsov-Ogievskiy
2015-06-11 13:03 ` Stefan Hajnoczi
2015-06-11 16:21 ` John Snow
2015-06-12 10:28 ` Stefan Hajnoczi
2015-06-12 15:19 ` John Snow
2015-06-10 15:34 ` Kevin Wolf
2015-06-11 10:25 ` Vladimir Sementsov-Ogievskiy
2015-06-11 16:30 ` John Snow
2015-06-12 8:33 ` Kevin Wolf
2015-08-24 10:46 ` Vladimir Sementsov-Ogievskiy
2015-08-24 13:30 ` Vladimir Sementsov-Ogievskiy
2015-08-24 14:08 ` Vladimir Sementsov-Ogievskiy
2015-08-24 14:04 ` Vladimir Sementsov-Ogievskiy
2015-08-31 22:21 ` Eric Blake
2015-08-31 22:24 ` John Snow
2015-06-08 15:21 ` [Qemu-devel] [PATCH 2/8] qcow2: add dirty-bitmaps feature Vladimir Sementsov-Ogievskiy
2015-06-09 16:52 ` Stefan Hajnoczi
2015-06-10 14:30 ` Stefan Hajnoczi
2015-06-12 19:02 ` John Snow
2015-06-15 14:42 ` Stefan Hajnoczi
2015-06-23 17:57 ` John Snow
2015-06-24 9:39 ` Stefan Hajnoczi
2015-08-14 17:14 ` Vladimir Sementsov-Ogievskiy
2015-08-26 9:09 ` Stefan Hajnoczi
2015-06-11 23:04 ` John Snow
2015-06-15 14:05 ` Vladimir Sementsov-Ogievskiy
2015-06-15 16:53 ` John Snow
2015-06-12 21:55 ` John Snow
2015-08-26 13:15 ` Vladimir Sementsov-Ogievskiy
2015-08-26 14:14 ` Vladimir Sementsov-Ogievskiy
2015-08-27 12:43 ` Vladimir Sementsov-Ogievskiy
2015-06-08 15:21 ` [Qemu-devel] [PATCH 3/8] block: store persistent dirty bitmaps Vladimir Sementsov-Ogievskiy
2015-06-08 15:21 ` [Qemu-devel] [PATCH 4/8] block: add bdrv_load_dirty_bitmap Vladimir Sementsov-Ogievskiy
2015-06-09 16:01 ` Stefan Hajnoczi
2015-06-10 22:33 ` John Snow
2015-06-11 10:41 ` Vladimir Sementsov-Ogievskiy
2015-06-08 15:21 ` [Qemu-devel] [PATCH 5/8] qcow2: add qcow2_dirty_bitmap_delete_all Vladimir Sementsov-Ogievskiy
2015-06-08 15:21 ` [Qemu-devel] [PATCH 6/8] qcow2: add autoclear bit for dirty bitmaps Vladimir Sementsov-Ogievskiy
2015-06-09 15:49 ` Stefan Hajnoczi
2015-06-09 15:50 ` Stefan Hajnoczi
2015-08-27 7:45 ` Vladimir Sementsov-Ogievskiy
2015-08-31 11:06 ` Vladimir Sementsov-Ogievskiy
2015-08-31 22:39 ` Eric Blake
2015-08-31 22:50 ` Eric Blake
2015-06-10 23:42 ` John Snow
2015-06-11 8:35 ` Kevin Wolf
2015-06-11 10:49 ` Vladimir Sementsov-Ogievskiy
2015-06-11 16:36 ` John Snow
2015-06-08 15:21 ` [Qemu-devel] [PATCH 7/8] qemu: command line option " Vladimir Sementsov-Ogievskiy
2015-06-11 20:57 ` John Snow
2015-06-12 21:49 ` John Snow
2015-06-08 15:21 ` [Qemu-devel] [PATCH 8/8] iotests: test internal persistent dirty bitmap Vladimir Sementsov-Ogievskiy
2015-06-09 16:17 ` Eric Blake
2015-06-10 15:27 ` [Qemu-devel] [PATCH v2 RFC 0/8] block: persistent dirty bitmaps Stefan Hajnoczi
2015-06-11 11:22 ` Vladimir Sementsov-Ogievskiy
2015-06-11 13:14 ` Stefan Hajnoczi
2015-06-11 20:06 ` Stefan Hajnoczi
2015-06-12 9:58 ` Denis V. Lunev
2015-06-12 10:36 ` Stefan Hajnoczi
2015-08-26 6:26 ` Vladimir Sementsov-Ogievskiy
2015-08-26 9:13 ` Stefan Hajnoczi
2015-06-12 19:34 ` John Snow
2015-06-17 14:29 ` Vladimir Sementsov-Ogievskiy
2015-06-24 0:21 ` John Snow
2015-07-08 12:24 ` Vladimir Sementsov-Ogievskiy
2015-07-08 15:21 ` John Snow
2015-08-27 10:08 ` Vladimir Sementsov-Ogievskiy
-- strict thread matches above, loose matches on Subject: below --
2015-01-13 17:02 [Qemu-devel] [PATCH 0/8] block: persistent dirty bitmaps (RFC) Vladimir Sementsov-Ogievskiy
2015-01-13 17:02 ` [Qemu-devel] [PATCH 1/8] spec: add qcow2-dirty-bitmaps specification Vladimir Sementsov-Ogievskiy
2015-01-27 15:39 ` Eric Blake
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