qemu/block/qed-l2-cache.c

2  * QEMU Enhanced Disk Format L2 Cache
15  * L2 table cache usage is as follows:
17  * An open image has one L2 table cache that is used to avoid accessing the
23  * table cache serves up recently referenced L2 tables.
25  * If there is a cache miss, that L2 table is read from the image file and
26  * committed to the cache.  Subsequent accesses to that L2 table will be served
27  * from the cache until the table is evicted from the cache.
29  * L2 tables are also committed to the cache when new L2 tables are allocated
30  * in the image file.  Since the L2 table cache is write-through, the new L2
32  * cache.
34  * Multiple I/O requests may be using an L2 table cache entry at any given
37  * particular, an entry evicted from the cache will only be freed once all
40  * An in-flight I/O request will hold a reference to a L2 table cache entry for
46  * is not in the cache.  Since the operation to read the table from the image
47  * file takes some time to complete, both requests may see a cache miss and
49  * commit its L2 table into the cache.  When the second tries to commit its
50  * table will be deleted in favor of the existing cache entry.
58 /* Each L2 holds 2GB so this let's us fully cache a 100GB disk */
62  * Initialize the L2 cache
71  * Free the L2 cache
84  * Allocate an uninitialized entry from the cache
123  * Find an entry in the L2 cache.  This may return NULL and it's up to the
124  * caller to satisfy the cache miss.
146  * Commit an L2 cache entry into the cache.  This is meant to be used as part of
147  * the process to satisfy a cache miss.  A caller would allocate an entry which
148  * is not actually in the L2 cache and then once the entry was valid and
149  * present on disk, the entry can be committed into the cache.
151  * Since the cache is write-through, it's important that this function is not
172     /* Evict an unused cache entry so we have space.  If all entries are in use  in qed_commit_l2_cache_entry()
173      * we can grow the cache temporarily and we try to shrink back down later.  in qed_commit_l2_cache_entry()