| Message ID | ad59735f252161ec7ed2f08b30cd517cbfb1d360.1588699789.git.berto@igalia.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Add subcluster allocation to qcow2 | expand |
On 5/5/20 12:38 PM, 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> > Reviewed-by: Max Reitz <mreitz@redhat.com> > --- > docs/interop/qcow2.txt | 68 ++++++++++++++++++++++++++++++++++++++++-- > docs/qcow2-cache.txt | 19 +++++++++++- > 2 files changed, 83 insertions(+), 4 deletions(-) > > @@ -547,7 +557,8 @@ 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. In your cover letter, you said you changed things to tolerate this bit being set even with extended L2 entries. Does this sentence need a tweak? > > 1 - 8: Reserved (set to 0) > > @@ -584,6 +595,57 @@ 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 4 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 the same as 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. Also this sentence. > + > +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) Why two spaces after '-'? I understand it in situations like '0 - 3' in the same list as '16 - 19', to make for a right-justified column, but here, everything in the second column is two digits, so the extra padding doesn't add anything useful. Or did you mean to have '64 - 95' and '96 - 127', making it obvious that these are the second set of bits on top of the existing bits in the first 8 bytes? > + > + 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 least significant > + one (i.e. bit x is used for subcluster 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 least significant > + one (i.e. bit x is used for subcluster x - 32). Of course, if you change to 64-95 and 96-127, the two sentences mapping bit x to subcluster y need adjusting by 64 as well. > + > +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. I can live with the wording as-is (since you did call out the "second 64 bits" or with the adjusted bit numberings.
On Tue 05 May 2020 09:35:06 PM CEST, Eric Blake wrote: > On 5/5/20 12:38 PM, 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> >> Reviewed-by: Max Reitz <mreitz@redhat.com> >> --- >> docs/interop/qcow2.txt | 68 ++++++++++++++++++++++++++++++++++++++++-- >> docs/qcow2-cache.txt | 19 +++++++++++- >> 2 files changed, 83 insertions(+), 4 deletions(-) >> > >> @@ -547,7 +557,8 @@ 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. > > In your cover letter, you said you changed things to tolerate this bit > being set even with extended L2 entries. Does this sentence need a > tweak? Not a bad idea. >> +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) > > Why two spaces after '-'? No reason, I guess I just didn't notice. I'll fix it. Berto
On Wed 06 May 2020 05:02:52 PM CEST, Alberto Garcia wrote: >>> - With version 2, this is always 0. >>> + With version 2 or with extended L2 entries (see the next >>> + section), this is always 0. >> >> In your cover letter, you said you changed things to tolerate this >> bit being set even with extended L2 entries. Does this sentence need >> a tweak? > > Not a bad idea. I just had a look at what happens with v2 images. There the bit is actually checked and the image is marked corrupt, but that only seems to happen in qcow2_get_host_offset(). You can write to a cluster that has that bit set and QEMU won't complain (the bit is however cleared). 'qemu-img check' does not detect any inconsistency either. As I said in my cover letter maybe it's worth preparing a separate series that adds a QCOW2_CLUSTER_INVALID type and includes all these cases. Berto
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index 298a031310..f08e43f228 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -42,6 +42,9 @@ 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. @@ -117,7 +120,12 @@ the next fields through header_length. clusters. The compression_type field must be present and not zero. - Bits 4-63: Reserved (set to 0) + Bit 4: 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 5-63: Reserved (set to 0) 80 - 87: compatible_features Bitmask of compatible features. An implementation can @@ -497,7 +505,7 @@ 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 @@ -507,6 +515,8 @@ 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) @@ -547,7 +557,8 @@ 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) @@ -584,6 +595,57 @@ 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 4 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 the same as 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 least significant + one (i.e. bit x is used for subcluster 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 least significant + one (i.e. bit x is used for subcluster x - 32). + +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..5f763aa6bb 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, 2018-2020 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.