From 298800cae72eb2a549374189e87615b2b0b79262 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Mon, 6 Dec 2010 16:08:01 +0000 Subject: qed: Table, L2 cache, and cluster functions This patch adds code to look up data cluster offsets in the image via the L1/L2 tables. The L2 tables are writethrough cached in memory for performance (each read/write requires a lookup so it is essential to cache the tables). With cluster lookup code in place it is possible to implement bdrv_is_allocated() to query the number of contiguous allocated/unallocated clusters. Signed-off-by: Stefan Hajnoczi Signed-off-by: Kevin Wolf --- block/qed-l2-cache.c | 173 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 173 insertions(+) create mode 100644 block/qed-l2-cache.c (limited to 'block/qed-l2-cache.c') diff --git a/block/qed-l2-cache.c b/block/qed-l2-cache.c new file mode 100644 index 0000000000..57518a4e7f --- /dev/null +++ b/block/qed-l2-cache.c @@ -0,0 +1,173 @@ +/* + * QEMU Enhanced Disk Format L2 Cache + * + * Copyright IBM, Corp. 2010 + * + * Authors: + * Anthony Liguori + * + * This work is licensed under the terms of the GNU LGPL, version 2 or later. + * See the COPYING.LIB file in the top-level directory. + * + */ + +/* + * L2 table cache usage is as follows: + * + * An open image has one L2 table cache that is used to avoid accessing the + * image file for recently referenced L2 tables. + * + * Cluster offset lookup translates the logical offset within the block device + * to a cluster offset within the image file. This is done by indexing into + * the L1 and L2 tables which store cluster offsets. It is here where the L2 + * table cache serves up recently referenced L2 tables. + * + * If there is a cache miss, that L2 table is read from the image file and + * committed to the cache. Subsequent accesses to that L2 table will be served + * from the cache until the table is evicted from the cache. + * + * L2 tables are also committed to the cache when new L2 tables are allocated + * in the image file. Since the L2 table cache is write-through, the new L2 + * table is first written out to the image file and then committed to the + * cache. + * + * Multiple I/O requests may be using an L2 table cache entry at any given + * time. That means an entry may be in use across several requests and + * reference counting is needed to free the entry at the correct time. In + * particular, an entry evicted from the cache will only be freed once all + * references are dropped. + * + * An in-flight I/O request will hold a reference to a L2 table cache entry for + * the period during which it needs to access the L2 table. This includes + * cluster offset lookup, L2 table allocation, and L2 table update when a new + * data cluster has been allocated. + * + * An interesting case occurs when two requests need to access an L2 table that + * is not in the cache. Since the operation to read the table from the image + * file takes some time to complete, both requests may see a cache miss and + * start reading the L2 table from the image file. The first to finish will + * commit its L2 table into the cache. When the second tries to commit its + * table will be deleted in favor of the existing cache entry. + */ + +#include "trace.h" +#include "qed.h" + +/* Each L2 holds 2GB so this let's us fully cache a 100GB disk */ +#define MAX_L2_CACHE_SIZE 50 + +/** + * Initialize the L2 cache + */ +void qed_init_l2_cache(L2TableCache *l2_cache) +{ + QTAILQ_INIT(&l2_cache->entries); + l2_cache->n_entries = 0; +} + +/** + * Free the L2 cache + */ +void qed_free_l2_cache(L2TableCache *l2_cache) +{ + CachedL2Table *entry, *next_entry; + + QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next_entry) { + qemu_vfree(entry->table); + qemu_free(entry); + } +} + +/** + * Allocate an uninitialized entry from the cache + * + * The returned entry has a reference count of 1 and is owned by the caller. + * The caller must allocate the actual table field for this entry and it must + * be freeable using qemu_vfree(). + */ +CachedL2Table *qed_alloc_l2_cache_entry(L2TableCache *l2_cache) +{ + CachedL2Table *entry; + + entry = qemu_mallocz(sizeof(*entry)); + entry->ref++; + + trace_qed_alloc_l2_cache_entry(l2_cache, entry); + + return entry; +} + +/** + * Decrease an entry's reference count and free if necessary when the reference + * count drops to zero. + */ +void qed_unref_l2_cache_entry(CachedL2Table *entry) +{ + if (!entry) { + return; + } + + entry->ref--; + trace_qed_unref_l2_cache_entry(entry, entry->ref); + if (entry->ref == 0) { + qemu_vfree(entry->table); + qemu_free(entry); + } +} + +/** + * Find an entry in the L2 cache. This may return NULL and it's up to the + * caller to satisfy the cache miss. + * + * For a cached entry, this function increases the reference count and returns + * the entry. + */ +CachedL2Table *qed_find_l2_cache_entry(L2TableCache *l2_cache, uint64_t offset) +{ + CachedL2Table *entry; + + QTAILQ_FOREACH(entry, &l2_cache->entries, node) { + if (entry->offset == offset) { + trace_qed_find_l2_cache_entry(l2_cache, entry, offset, entry->ref); + entry->ref++; + return entry; + } + } + return NULL; +} + +/** + * Commit an L2 cache entry into the cache. This is meant to be used as part of + * the process to satisfy a cache miss. A caller would allocate an entry which + * is not actually in the L2 cache and then once the entry was valid and + * present on disk, the entry can be committed into the cache. + * + * Since the cache is write-through, it's important that this function is not + * called until the entry is present on disk and the L1 has been updated to + * point to the entry. + * + * N.B. This function steals a reference to the l2_table from the caller so the + * caller must obtain a new reference by issuing a call to + * qed_find_l2_cache_entry(). + */ +void qed_commit_l2_cache_entry(L2TableCache *l2_cache, CachedL2Table *l2_table) +{ + CachedL2Table *entry; + + entry = qed_find_l2_cache_entry(l2_cache, l2_table->offset); + if (entry) { + qed_unref_l2_cache_entry(entry); + qed_unref_l2_cache_entry(l2_table); + return; + } + + if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) { + entry = QTAILQ_FIRST(&l2_cache->entries); + QTAILQ_REMOVE(&l2_cache->entries, entry, node); + l2_cache->n_entries--; + qed_unref_l2_cache_entry(entry); + } + + l2_cache->n_entries++; + QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node); +} -- cgit v1.2.1