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* QEMU Enhanced Disk Format L2 Cache
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* Copyright IBM, Corp. 2010
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* Anthony Liguori <aliguori@us.ibm.com>
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* This work is licensed under the terms of the GNU LGPL, version 2 or later.
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* See the COPYING.LIB file in the top-level directory.
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* L2 table cache usage is as follows:
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* An open image has one L2 table cache that is used to avoid accessing the
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* image file for recently referenced L2 tables.
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* Cluster offset lookup translates the logical offset within the block device
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* to a cluster offset within the image file. This is done by indexing into
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* the L1 and L2 tables which store cluster offsets. It is here where the L2
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* table cache serves up recently referenced L2 tables.
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* If there is a cache miss, that L2 table is read from the image file and
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* committed to the cache. Subsequent accesses to that L2 table will be served
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* from the cache until the table is evicted from the cache.
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* L2 tables are also committed to the cache when new L2 tables are allocated
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* in the image file. Since the L2 table cache is write-through, the new L2
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* table is first written out to the image file and then committed to the
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* Multiple I/O requests may be using an L2 table cache entry at any given
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* time. That means an entry may be in use across several requests and
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* reference counting is needed to free the entry at the correct time. In
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* particular, an entry evicted from the cache will only be freed once all
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* references are dropped.
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* An in-flight I/O request will hold a reference to a L2 table cache entry for
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* the period during which it needs to access the L2 table. This includes
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* cluster offset lookup, L2 table allocation, and L2 table update when a new
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* data cluster has been allocated.
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* An interesting case occurs when two requests need to access an L2 table that
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* is not in the cache. Since the operation to read the table from the image
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* file takes some time to complete, both requests may see a cache miss and
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* start reading the L2 table from the image file. The first to finish will
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* commit its L2 table into the cache. When the second tries to commit its
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* table will be deleted in favor of the existing cache entry.
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/* Each L2 holds 2GB so this let's us fully cache a 100GB disk */
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#define MAX_L2_CACHE_SIZE 50
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* Initialize the L2 cache
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void qed_init_l2_cache(L2TableCache *l2_cache)
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QTAILQ_INIT(&l2_cache->entries);
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l2_cache->n_entries = 0;
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void qed_free_l2_cache(L2TableCache *l2_cache)
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CachedL2Table *entry, *next_entry;
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QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next_entry) {
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qemu_vfree(entry->table);
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* Allocate an uninitialized entry from the cache
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* The returned entry has a reference count of 1 and is owned by the caller.
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* The caller must allocate the actual table field for this entry and it must
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* be freeable using qemu_vfree().
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CachedL2Table *qed_alloc_l2_cache_entry(L2TableCache *l2_cache)
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entry = g_malloc0(sizeof(*entry));
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trace_qed_alloc_l2_cache_entry(l2_cache, entry);
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* Decrease an entry's reference count and free if necessary when the reference
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* count drops to zero.
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void qed_unref_l2_cache_entry(CachedL2Table *entry)
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trace_qed_unref_l2_cache_entry(entry, entry->ref);
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if (entry->ref == 0) {
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qemu_vfree(entry->table);
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* Find an entry in the L2 cache. This may return NULL and it's up to the
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* caller to satisfy the cache miss.
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* For a cached entry, this function increases the reference count and returns
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CachedL2Table *qed_find_l2_cache_entry(L2TableCache *l2_cache, uint64_t offset)
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CachedL2Table *entry;
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QTAILQ_FOREACH(entry, &l2_cache->entries, node) {
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if (entry->offset == offset) {
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trace_qed_find_l2_cache_entry(l2_cache, entry, offset, entry->ref);
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* Commit an L2 cache entry into the cache. This is meant to be used as part of
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* the process to satisfy a cache miss. A caller would allocate an entry which
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* is not actually in the L2 cache and then once the entry was valid and
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* present on disk, the entry can be committed into the cache.
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* Since the cache is write-through, it's important that this function is not
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* called until the entry is present on disk and the L1 has been updated to
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* point to the entry.
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* N.B. This function steals a reference to the l2_table from the caller so the
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* caller must obtain a new reference by issuing a call to
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* qed_find_l2_cache_entry().
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void qed_commit_l2_cache_entry(L2TableCache *l2_cache, CachedL2Table *l2_table)
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CachedL2Table *entry;
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entry = qed_find_l2_cache_entry(l2_cache, l2_table->offset);
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qed_unref_l2_cache_entry(entry);
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qed_unref_l2_cache_entry(l2_table);
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if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) {
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entry = QTAILQ_FIRST(&l2_cache->entries);
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QTAILQ_REMOVE(&l2_cache->entries, entry, node);
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l2_cache->n_entries--;
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qed_unref_l2_cache_entry(entry);
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l2_cache->n_entries++;
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QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node);
added
removed
block/qed-l2-cache.c
