Hello all, in September I sent an e-mail with a proposal to allow reducing the size of entries in the qcow2 L2 cache in order to improve performance and make a more efficient use of the cache memory. You can follow this link for the full details:
https://lists.gnu.org/archive/html/qemu-block/2017-09/msg00635.html This series is the implementation of this feature. It adds a new parameter for the qcow2 driver named l2-cache-entry-size (related to the existing l2-cache-size). With this, the qcow2 driver no longer needs to read full L2 tables for cluster lookups but smaller chunks which I call L2 slices, following John Snow's suggestion. The L2 slice size has the same restrictions as the cluster size: it has to be a power of wo between 512 bytes and the image's cluster size. I tried to be conservative in general so the slice size is equal to the cluster size by default (i.e. there should be no user-visible changes if you don't enable this). I believe that 4KB is probably the best default, but I'd like to test a bit with different scenarios and cluster sizes, and we can decide on a default value in the next revision of the series, or later. About the patches themselves, please do not be discouraged by the size of the series :) Many of the changes are trivial and don't introduce any semantic changes but instead simply rename variable names or remove obsolete parameters. The patches that do introduce semantic changes are -I believe- easy to follow. The ones that seem more complicated are because they have a new nested loop. I suggest that you read them with 'diff -w' in order to hide the effects of the indentation changes. I also tried to keep each patch touching one single function. The nature of the changes allows for that, and should make the code easier to review. Here's a general overview of the series: - Patch 1 is a documentation fix. - Patch 2 adds a field to Qcow2Cache to store the table size. - Patches 3-11 remove obsolete BDS parameters from the API as a result of the changes in patch 2. - Patches 12-14 add new auxiliary functions to calculate offsets. - Patches 15-26 update existing functions to handle L2 slices correctly (the important part of the series is here). - Patches 27-30 rename variables but don't change semantics. - Patch 31 adds the new l2-cache-entry-size parameter. As usual, feedback is very welcome. Thanks in advance, Berto Alberto Garcia (31): qcow2: Fix documentation of get_cluster_table() qcow2: Add table size field to Qcow2Cache qcow2: Remove BDS parameter from qcow2_cache_get_table_addr() qcow2: Remove BDS parameter from qcow2_cache_get_table_idx() qcow2: Remove BDS parameter from qcow2_cache_table_release() qcow2: Remove BDS parameter from qcow2_cache_entry_mark_dirty() qcow2: Remove BDS parameter from qcow2_cache_put() qcow2: Remove BDS parameter from qcow2_cache_destroy() qcow2: Remove BDS parameter from qcow2_cache_clean_unused() qcow2: Remove BDS parameter from qcow2_cache_discard() qcow2: Remove BDS parameter from qcow2_cache_is_table_offset() qcow2: Add offset_to_l1_index() qcow2: Add l2_slice_size field to BDRVQcow2State qcow2: Add offset_to_l2_slice_index() qcow2: Update l2_load() to support L2 slices qcow2: Update l2_allocate() to support L2 slices qcow2: Update get_cluster_table() to support L2 slices qcow2: Update qcow2_get_cluster_offset() to support L2 slices qcow2: Update qcow2_alloc_cluster_link_l2() to support L2 slices qcow2: Update handle_copied() to support L2 slices qcow2: Update handle_alloc() to support L2 slices qcow2: Update discard_single_l2() to support L2 slices qcow2: Update zero_single_l2() to support L2 slices qcow2: Update qcow2_update_snapshot_refcount() to support L2 slices qcow2: Update expand_zero_clusters_in_l1() to support L2 slices qcow2: Update qcow2_truncate() to support L2 slices qcow2: Rename l2_table in qcow2_alloc_compressed_cluster_offset() qcow2: Rename l2_table in count_contiguous_clusters() qcow2: Rename l2_table in count_contiguous_clusters_unallocated() qcow2: Rename l2_table in count_cow_clusters() qcow2: Allow configuring the L2 slice size block/qcow2-cache.c | 78 ++++---- block/qcow2-cluster.c | 493 ++++++++++++++++++++++++++----------------------- block/qcow2-refcount.c | 200 ++++++++++---------- block/qcow2.c | 59 ++++-- block/qcow2.h | 29 ++- 5 files changed, 469 insertions(+), 390 deletions(-) -- 2.11.0
