| Message ID | 57cddd5db9c79406fba70a9bf46c436f6fc9ccd3.1572125022.git.berto@igalia.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Add subcluster allocation to qcow2 | expand |
On 26.10.19 23:25, Alberto Garcia wrote: > Subcluster allocation in qcow2 is implemented by extending the > existing L2 table entries and adding additional information to > indicate the allocation status of each subcluster. > > This patch documents the changes to the qcow2 format and how they > affect the calculation of the L2 cache size. > > Signed-off-by: Alberto Garcia <berto@igalia.com> > --- > docs/interop/qcow2.txt | 68 ++++++++++++++++++++++++++++++++++++++++-- > docs/qcow2-cache.txt | 19 +++++++++++- > 2 files changed, 83 insertions(+), 4 deletions(-) Sounds good, just a bit of bit nit-picking from my side. > diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt > index af5711e533..d34261f955 100644 > --- a/docs/interop/qcow2.txt > +++ b/docs/interop/qcow2.txt [...] > @@ -524,6 +535,57 @@ file (except if bit 0 in the Standard Cluster Descriptor is set). If there is > no backing file or the backing file is smaller than the image, they shall read > zeros for all parts that are not covered by the backing file. > > +== Extended L2 Entries == > + > +An image uses Extended L2 Entries if bit 3 is set on the incompatible_features > +field of the header. > + > +In these images standard data clusters are divided into 32 subclusters of the > +same size. They are contiguous and start from the beginning of the cluster. > +Subclusters can be allocated independently and the L2 entry contains information > +indicating the status of each one of them. Compressed data clusters don't have > +subclusters so they are treated like in images without this feature. > + > +The size of an extended L2 entry is 128 bits so the number of entries per table > +is calculated using this formula: > + > + l2_entries = (cluster_size / (2 * sizeof(uint64_t))) > + > +The first 64 bits have the same format as the standard L2 table entry described > +in the previous section, with the exception of bit 0 of the standard cluster > +descriptor. > + > +The last 64 bits contain a subcluster allocation bitmap with this format: > + > +Subcluster Allocation Bitmap (for standard clusters): > + > + Bit 0 - 31: Allocation status (one bit per subcluster) > + > + 1: the subcluster is allocated. In this case the > + host cluster offset field must contain a valid > + offset. > + 0: the subcluster is not allocated. In this case > + read requests shall go to the backing file or > + return zeros if there is no backing file data. > + > + Bits are assigned starting from the most significant one. > + (i.e. bit x is used for subcluster 31 - x) I seem to remember that someone proposed this bit ordering to you, but I wonder why. So far everything in qcow2 starts from the least significant bit, for example refcounts (“If refcount_bits implies a sub-byte width, note that bit 0 means the least significant bit in this context”), feature bits, and sub-byte structure descriptions in general (which you reference directly with “bit x”). Soo... What’s the reason for doing it the other way around here? > + 32 - 63 Subcluster reads as zeros (one bit per subcluster) > + > + 1: the subcluster reads as zeros. In this case the > + allocation status bit must be unset. The host > + cluster offset field may or may not be set. > + 0: no effect. > + > + Bits are assigned starting from the most significant one. > + (i.e. bit x is used for subcluster 63 - x) > + > +Subcluster Allocation Bitmap (for compressed clusters): Going from nit-picking to bike shedding: I’d drop the parentheses. Max
On Wed 30 Oct 2019 05:23:30 PM CET, Max Reitz wrote: >> +Subcluster Allocation Bitmap (for standard clusters): >> + >> + Bit 0 - 31: Allocation status (one bit per subcluster) >> + >> + 1: the subcluster is allocated. In this case the >> + host cluster offset field must contain a valid >> + offset. >> + 0: the subcluster is not allocated. In this case >> + read requests shall go to the backing file or >> + return zeros if there is no backing file data. >> + >> + Bits are assigned starting from the most significant one. >> + (i.e. bit x is used for subcluster 31 - x) > > I seem to remember that someone proposed this bit ordering to you, but > I wonder why. So far everything in qcow2 starts from the least > significant bit, for example refcounts (“If refcount_bits implies a > sub-byte width, note that bit 0 means the least significant bit in > this context”), feature bits, and sub-byte structure descriptions in > general (which you reference directly with “bit x”). > > Soo... What’s the reason for doing it the other way around here? The reason is that I thought that it would be better for debugging purposes. If I do an hexdump of the L2 table to see what's going on then starting from the most significant bit gives me a better visual image of what subclusters are allocated. In other words, if the first two subclusters are allocated I think this representation 11000000 00000000 00000000 00000000 (c0 00 00 00) is more natural than this one 00000000 00000000 00000000 00000011 (00 00 00 03) But I don't have a very strong opinion so I'm open to changing it. Berto
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index af5711e533..d34261f955 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -39,6 +39,9 @@ The first cluster of a qcow2 image contains the file header: as the maximum cluster size and won't be able to open images with larger cluster sizes. + Note: if the image has Extended L2 Entries then cluster_bits + must be at least 14 (i.e. 16384 byte clusters). + 24 - 31: size Virtual disk size in bytes. @@ -109,7 +112,12 @@ in the description of a field. An External Data File Name header extension may be present if this bit is set. - Bits 3-63: Reserved (set to 0) + Bit 3: Extended L2 Entries. If this bit is set then + L2 table entries use an extended format that + allows subcluster-based allocation. See the + Extended L2 Entries section for more details. + + Bits 4-63: Reserved (set to 0) 80 - 87: compatible_features Bitmask of compatible features. An implementation can @@ -437,7 +445,7 @@ cannot be relaxed without an incompatible layout change). Given an offset into the virtual disk, the offset into the image file can be obtained as follows: - l2_entries = (cluster_size / sizeof(uint64_t)) + l2_entries = (cluster_size / sizeof(uint64_t)) [*] l2_index = (offset / cluster_size) % l2_entries l1_index = (offset / cluster_size) / l2_entries @@ -447,6 +455,8 @@ obtained as follows: return cluster_offset + (offset % cluster_size) + [*] this changes if Extended L2 Entries are enabled, see next section + L1 table entry: Bit 0 - 8: Reserved (set to 0) @@ -487,7 +497,8 @@ Standard Cluster Descriptor: nor is data read from the backing file if the cluster is unallocated. - With version 2, this is always 0. + With version 2 or with extended L2 entries (see the next + section), this is always 0. 1 - 8: Reserved (set to 0) @@ -524,6 +535,57 @@ file (except if bit 0 in the Standard Cluster Descriptor is set). If there is no backing file or the backing file is smaller than the image, they shall read zeros for all parts that are not covered by the backing file. +== Extended L2 Entries == + +An image uses Extended L2 Entries if bit 3 is set on the incompatible_features +field of the header. + +In these images standard data clusters are divided into 32 subclusters of the +same size. They are contiguous and start from the beginning of the cluster. +Subclusters can be allocated independently and the L2 entry contains information +indicating the status of each one of them. Compressed data clusters don't have +subclusters so they are treated like in images without this feature. + +The size of an extended L2 entry is 128 bits so the number of entries per table +is calculated using this formula: + + l2_entries = (cluster_size / (2 * sizeof(uint64_t))) + +The first 64 bits have the same format as the standard L2 table entry described +in the previous section, with the exception of bit 0 of the standard cluster +descriptor. + +The last 64 bits contain a subcluster allocation bitmap with this format: + +Subcluster Allocation Bitmap (for standard clusters): + + Bit 0 - 31: Allocation status (one bit per subcluster) + + 1: the subcluster is allocated. In this case the + host cluster offset field must contain a valid + offset. + 0: the subcluster is not allocated. In this case + read requests shall go to the backing file or + return zeros if there is no backing file data. + + Bits are assigned starting from the most significant one. + (i.e. bit x is used for subcluster 31 - x) + + 32 - 63 Subcluster reads as zeros (one bit per subcluster) + + 1: the subcluster reads as zeros. In this case the + allocation status bit must be unset. The host + cluster offset field may or may not be set. + 0: no effect. + + Bits are assigned starting from the most significant one. + (i.e. bit x is used for subcluster 63 - x) + +Subcluster Allocation Bitmap (for compressed clusters): + + Bit 0 - 63: Reserved (set to 0) + Compressed clusters don't have subclusters, + so this field is not used. == Snapshots == diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index d57f409861..04eb4ce2f1 100644 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -1,6 +1,6 @@ qcow2 L2/refcount cache configuration ===================================== -Copyright (C) 2015, 2018 Igalia, S.L. +Copyright (C) 2015, 2018-2019 Igalia, S.L. Author: Alberto Garcia <berto@igalia.com> This work is licensed under the terms of the GNU GPL, version 2 or @@ -222,3 +222,20 @@ support this functionality, and is 0 (disabled) on other platforms. This functionality currently relies on the MADV_DONTNEED argument for madvise() to actually free the memory. This is a Linux-specific feature, so cache-clean-interval is not supported on other systems. + + +Extended L2 Entries +------------------- +All numbers shown in this document are valid for qcow2 images with normal +64-bit L2 entries. + +Images with extended L2 entries need twice as much L2 metadata, so the L2 +cache size must be twice as large for the same disk space. + + disk_size = l2_cache_size * cluster_size / 16 + +i.e. + + l2_cache_size = disk_size * 16 / cluster_size + +Refcount blocks are not affected by this.
Subcluster allocation in qcow2 is implemented by extending the existing L2 table entries and adding additional information to indicate the allocation status of each subcluster. This patch documents the changes to the qcow2 format and how they affect the calculation of the L2 cache size. Signed-off-by: Alberto Garcia <berto@igalia.com> --- docs/interop/qcow2.txt | 68 ++++++++++++++++++++++++++++++++++++++++-- docs/qcow2-cache.txt | 19 +++++++++++- 2 files changed, 83 insertions(+), 4 deletions(-)