From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45991) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aIbTL-0000lU-Q5 for qemu-devel@nongnu.org; Mon, 11 Jan 2016 07:21:21 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aIbTH-0001g2-A7 for qemu-devel@nongnu.org; Mon, 11 Jan 2016 07:21:19 -0500 Received: from relay.parallels.com ([195.214.232.42]:48199) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aIbTG-0001dh-Rb for qemu-devel@nongnu.org; Mon, 11 Jan 2016 07:21:15 -0500 Message-ID: <56939E25.9000309@virtuozzo.com> Date: Mon, 11 Jan 2016 15:20:53 +0300 From: Vladimir Sementsov-Ogievskiy MIME-Version: 1.0 References: <1450892961-6495-1-git-send-email-vsementsov@virtuozzo.com> <568AFD50.9030006@redhat.com> In-Reply-To: <568AFD50.9030006@redhat.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v6] spec: add qcow2 bitmaps extension specification List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: John Snow , qemu-devel@nongnu.org, Eric Blake Cc: kwolf@redhat.com, den@openvz.org, famz@redhat.com, stefanha@redhat.com, mreitz@redhat.com On 05.01.2016 02:16, John Snow wrote: > Since Max didn't offer a grammatical review, here's my attempt at some > suggestions. > > On 12/23/2015 12:49 PM, Vladimir Sementsov-Ogievskiy wrote: >> The new feature for qcow2: storing bitmaps. >> >> This patch adds new header extension to qcow2 - Bitmaps Extension. It >> provides an ability to store virtual disk related bitmaps in a qcow2 >> image. For now there is only one type of such bitmaps: Dirty Tracking >> Bitmap, which just tracks virtual disk changes from some moment. >> >> Note: Only bitmaps, relative to the virtual disk, stored in qcow2 file, >> should be stored in this qcow2 file. The size of each bitmap >> (considering its granularity) is equal to virtual disk size. >> >> Signed-off-by: Vladimir Sementsov-Ogievskiy >> --- >> >> v6: >> >> - reword bitmap_directory_size description >> - bitmap type: make 0 reserved >> - extra_data_size: resize to 4bytes >> Also, I've marked this field as "must be zero". We can always change >> it, if we decide allowing managing app to specify any extra data, by >> defining some magic value as a top of user extra data.. So, for now >> non zeor extra_data_size should be considered as an error. >> - swap name and extra_data to give good alignment to extra_data. >> >> >> v5: >> >> - 'Dirty bitmaps' renamed to 'Bitmaps', as we may have several types of >> bitmaps. >> - rewordings >> - move upper bounds to "Notes about Qemu limits" >> - s/should/must somewhere. (but not everywhere) >> - move name_size field closer to name itself in bitmap header >> - add extra data area to bitmap header >> - move bitmap data description to separate section >> >> docs/specs/qcow2.txt | 161 ++++++++++++++++++++++++++++++++++++++++++++++++++- >> 1 file changed, 160 insertions(+), 1 deletion(-) >> >> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt >> index 121dfc8..b23966a 100644 >> --- a/docs/specs/qcow2.txt >> +++ b/docs/specs/qcow2.txt >> @@ -103,7 +103,19 @@ 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: Bitmaps extension bit. > For consistency, no period after this. > >> + This bit is responsible for Bitmaps extension >> + consistency. >> + > I might phrase it as: "This bit indicates consistency for the Bitmaps > extension data." > >> + If it is set, but there is no Bitmaps >> + extension, this should be considered as an >> + error. >> + > "This should be considered as an error" can be shortened to just "This > is an error." This makes the sentence top-heavy though, so how about: > > "It is an error if this bit is set without the Bitmaps extension present." > >> + If it is not set, but there is a Bitmaps >> + extension, its data should be considered as >> + inconsistent. >> + > Let's remove "considered" here. The data *is* inconsistent if this has > happened. > > "If the Bitmaps extension is present but this bit is unset, the Bitmaps > extension data is inconsistent." > >> + Bits 1-63: Reserved (set to 0) >> >> 96 - 99: refcount_order >> Describes the width of a reference count block entry (width >> @@ -123,6 +135,7 @@ 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 - Bitmaps extension >> other - Unknown header extension, can be safely >> ignored >> >> @@ -166,6 +179,34 @@ the header extension data. Each entry look like this: >> terminated if it has full length) >> >> >> +== Bitmaps extension == >> + >> +Bitmaps extension is an optional header extension. It provides an ability to >> +store virtual disk related bitmaps in a qcow2 image. For now there is only one >> +type of such bitmaps: Dirty Tracking Bitmap, which just tracks virtual disk >> +changes from some moment. >> + > I think "Bitmaps extension" is awkward without "The" leading it, so: > > "The Bitmaps extension is an optional header extension." > > And from Eric's suggestion: > > "It provides the ability to store bitmaps related to a virtual disk. > For now, there is only one bitmap type: Dirty Tracking Bitmap, which > tracks virtual disk changes from some moment." > >> +The data of the extension should be considered as consistent only if >> +corresponding auto-clear feature bit is set (see autoclear_features above). >> + > I might remove the parenthetical. > > "The data in this extension should be considered consistent only if the > corresponding auto-clear feature bit is set, see autoclear_features above." > >> +The fields of Bitmaps extension are: >> + > Adding 'the' again: > > "The fields of the Bitmaps extension are:" > >> + 0 - 3: nb_bitmaps >> + The number of bitmaps contained in the image. Must be >> + greater or equal to 1. >> + > "Greater than or equal to" is a common phrasing -- adding the 'than'. > >> + Note: Qemu currently only supports up to 65535 bitmaps per >> + image. >> + >> + 4 - 7: bitmap_directory_size >> + Size of the Bitmap Directory in bytes. It is a cumulative >> + size of all (nb_bitmaps) bitmap headers. >> + >> + 8 - 15: bitmap_directory_offset >> + Offset into the image file at which the 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 +401,121 @@ Snapshot table entry: >> >> variable: Padding to round up the snapshot table entry size to the >> next multiple of 8. >> + >> + >> +== Bitmaps == >> + >> +The feature supports storing bitmaps in a qcow2 image. All bitmaps are related >> +to the virtual disk, stored in this image. >> + > Maybe like Eric's earlier suggestion, we can avoid "in a qcow2 image" > here. We can probably just repeat some of the earlier description here. > >> +=== Bitmap Directory === >> + >> +Each bitmap saved in the image is described in a Bitmap Directory entry. Bitmap > "The Bitmap Directory" > >> +Directory is a contiguous area in the image file, whose starting offset and >> +length are given by the header extension fields bitmap_directory_offset and >> +bitmap_directory_size. The entries of the bitmap directory have variable >> +length, depending on the length of the bitmap name and extra data. These >> +entries are also called bitmap headers. >> + >> +Bitmap Directory Entry: >> + >> + Byte 0 - 7: bitmap_table_offset >> + Offset into the image file at which the Bitmap Table >> + (described below) for the bitmap starts. Must be aligned to >> + a cluster boundary. >> + >> + 8 - 11: bitmap_table_size >> + Number of entries in the 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 must reflect all changes of the virtual >> + disk by any application that would write to this qcow2 >> + file (including writes, snapshot switching, etc.). The >> + type of this bitmap must be 'Dirty Tracking Bitmap'. >> + >> + Bits 2 - 31 are reserved and must be 0. >> + >> + 16: type >> + This field describes the sort of the bitmap. >> + Values: >> + 1: Dirty Tracking Bitmap >> + >> + Values 0, 2 - 255 are reserved. >> + >> + 17: granularity_bits >> + Granularity bits. Valid values are: 0 - 63. >> + >> + Note: Qemu currently doesn't support granularity_bits >> + greater than 31. >> + >> + 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. >> + >> + 18 - 19: name_size >> + Size of the bitmap name. Valid values: 1 - 1023. >> + >> + 20 - 23: extra_data_size >> + Size of type-specific extra data. >> + >> + For now, as no extra data is defined, extra_data_size is >> + reserved and must be zero. >> + >> + variable: Type-specific extra data for the bitmap. >> + > And I guess we don't have any definitions for the "Dirty Tracking" type, > so this is zero. > > Should we add a table of the bitmap types and their corresponding > definitions for the "Type-specific extra data"? > > e.g. > > === Bitmap Types === > > 0: Reserved > extra_data_size: 0 > > This bitmap type is invalid and must not be used. > > 1: Dirty Tracking Bitmap > extra_data_size: 0 Are you sure? What about creation\last change dates, file links, user data, etc? For now, formally, current "For now, as no extra data is defined, extra_data_size is reserved and must be zero." is equal to such table, but provides more flexibility for future.. > > This bitmap type represents data changed since a point in time > and has no extra data. > > 2: Lorem Ipsum Bitmap > extra_data_size: 4 > > Lorem ipsum, dolor sit amet... > This bitmap type has four byte of extra data: > > 0-1: Identifier > 2-3: Padding > >> + variable: The name of the bitmap (not null terminated). Must be >> + unique among all bitmap names within the Bitmaps extension. >> + >> + variable: Padding to round up the Bitmap Directory Entry size to the >> + next multiple of 8. >> + >> +=== Bitmap Table === >> + >> +Bitmaps are stored using a one-level (not two-level like refcounts and guest >> +clusters mapping) structure for the mapping of bitmaps data to host clusters. >> +It is called Bitmap Table. >> + > I might add "the" before Bitmap Table above. > >> +Each Bitmap Table has a variable size (stored in the Bitmap Directory Entry) >> +and may use multiple clusters, however it must be contiguous in the image file. >> + >> +Bitmap Table entry: >> + >> + Bit 0: Reserved and must 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 and must be zero. >> + >> +=== Bitmap Data === >> + >> +As noted above, bitmap data is stored in several (or may be one, exactly >> +bitmap_table_size) separate clusters, described by Bitmap Table. Given an >> +offset (in bytes) into the bitmap data, the offset into the image file can be >> +obtained as follows: >> + >> + image_offset = >> + bitmap_table[bitmap_data_offset / cluster_size] + >> + (bitmap_data_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 virtual disk can be >> +calculated like this: >> + >> + bit_offset = >> + image_offset(byte_nr / granularity / 8) * 8 + >> + (byte_nr / granularity) % 8 >> > That's all the language commentary I have on this. Hopefully not long > before consensus. > > Thank you, > --John -- Best regards, Vladimir * now, @virtuozzo.com instead of @parallels.com. Sorry for this inconvenience.