From bfe8043e9214d2fc6572cc72b5f2218308747acd Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Fri, 27 Jul 2012 09:05:22 +0100 Subject: [PATCH] qcow2: implement lazy refcounts Lazy refcounts is a performance optimization for qcow2 that postpones refcount metadata updates and instead marks the image dirty. In the case of crash or power failure the image will be left in a dirty state and repaired next time it is opened. Reducing metadata I/O is important for cache=writethrough and cache=directsync because these modes guarantee that data is on disk after each write (hence we cannot take advantage of caching updates in RAM). Refcount metadata is not needed for guest->file block address translation and therefore does not need to be on-disk at the time of write completion - this is the motivation behind the lazy refcount optimization. The lazy refcount optimization must be enabled at image creation time: qemu-img create -f qcow2 -o compat=1.1,lazy_refcounts=on a.qcow2 10G qemu-system-x86_64 -drive if=virtio,file=a.qcow2,cache=writethrough Update qemu-iotests 031 and 036 since the extension header size changes when we add feature bit table entries. Signed-off-by: Stefan Hajnoczi Signed-off-by: Kevin Wolf --- block/qcow2-cluster.c | 5 ++- block/qcow2.c | 73 +++++++++++++++++++++++++++++++++++--- block/qcow2.h | 13 +++++++ block_int.h | 26 +++++++------- tests/qemu-iotests/031.out | 12 +++---- tests/qemu-iotests/036.out | 2 +- 6 files changed, 107 insertions(+), 24 deletions(-) diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index d7e0e19d9c..e179211c57 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -662,7 +662,10 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m) qcow2_cache_depends_on_flush(s->l2_table_cache); } - qcow2_cache_set_dependency(bs, s->l2_table_cache, s->refcount_block_cache); + if (qcow2_need_accurate_refcounts(s)) { + qcow2_cache_set_dependency(bs, s->l2_table_cache, + s->refcount_block_cache); + } ret = get_cluster_table(bs, m->offset, &l2_table, &l2_index); if (ret < 0) { goto err; diff --git a/block/qcow2.c b/block/qcow2.c index 7fe156712a..fd5e214431 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -214,6 +214,41 @@ static void report_unsupported_feature(BlockDriverState *bs, } } +/* + * Sets the dirty bit and flushes afterwards if necessary. + * + * The incompatible_features bit is only set if the image file header was + * updated successfully. Therefore it is not required to check the return + * value of this function. + */ +static int qcow2_mark_dirty(BlockDriverState *bs) +{ + BDRVQcowState *s = bs->opaque; + uint64_t val; + int ret; + + assert(s->qcow_version >= 3); + + if (s->incompatible_features & QCOW2_INCOMPAT_DIRTY) { + return 0; /* already dirty */ + } + + val = cpu_to_be64(s->incompatible_features | QCOW2_INCOMPAT_DIRTY); + ret = bdrv_pwrite(bs->file, offsetof(QCowHeader, incompatible_features), + &val, sizeof(val)); + if (ret < 0) { + return ret; + } + ret = bdrv_flush(bs->file); + if (ret < 0) { + return ret; + } + + /* Only treat image as dirty if the header was updated successfully */ + s->incompatible_features |= QCOW2_INCOMPAT_DIRTY; + return 0; +} + /* * Clears the dirty bit and flushes before if necessary. Only call this * function when there are no pending requests, it does not guard against @@ -752,6 +787,11 @@ static coroutine_fn int qcow2_co_writev(BlockDriverState *bs, goto fail; } + if (l2meta.nb_clusters > 0 && + (s->compatible_features & QCOW2_COMPAT_LAZY_REFCOUNTS)) { + qcow2_mark_dirty(bs); + } + cluster_offset = l2meta.cluster_offset; assert((cluster_offset & 511) == 0); @@ -994,6 +1034,11 @@ int qcow2_update_header(BlockDriverState *bs) .bit = QCOW2_INCOMPAT_DIRTY_BITNR, .name = "dirty bit", }, + { + .type = QCOW2_FEAT_TYPE_COMPATIBLE, + .bit = QCOW2_COMPAT_LAZY_REFCOUNTS_BITNR, + .name = "lazy refcounts", + }, }; ret = header_ext_add(buf, QCOW2_EXT_MAGIC_FEATURE_TABLE, @@ -1176,6 +1221,11 @@ static int qcow2_create2(const char *filename, int64_t total_size, header.crypt_method = cpu_to_be32(QCOW_CRYPT_NONE); } + if (flags & BLOCK_FLAG_LAZY_REFCOUNTS) { + header.compatible_features |= + cpu_to_be64(QCOW2_COMPAT_LAZY_REFCOUNTS); + } + ret = bdrv_pwrite(bs, 0, &header, sizeof(header)); if (ret < 0) { goto out; @@ -1289,6 +1339,8 @@ static int qcow2_create(const char *filename, QEMUOptionParameter *options) options->value.s); return -EINVAL; } + } else if (!strcmp(options->name, BLOCK_OPT_LAZY_REFCOUNTS)) { + flags |= options->value.n ? BLOCK_FLAG_LAZY_REFCOUNTS : 0; } options++; } @@ -1299,6 +1351,12 @@ static int qcow2_create(const char *filename, QEMUOptionParameter *options) return -EINVAL; } + if (version < 3 && (flags & BLOCK_FLAG_LAZY_REFCOUNTS)) { + fprintf(stderr, "Lazy refcounts only supported with compatibility " + "level 1.1 and above (use compat=1.1 or greater)\n"); + return -EINVAL; + } + return qcow2_create2(filename, sectors, backing_file, backing_fmt, flags, cluster_size, prealloc, options, version); } @@ -1485,10 +1543,12 @@ static coroutine_fn int qcow2_co_flush_to_os(BlockDriverState *bs) return ret; } - ret = qcow2_cache_flush(bs, s->refcount_block_cache); - if (ret < 0) { - qemu_co_mutex_unlock(&s->lock); - return ret; + if (qcow2_need_accurate_refcounts(s)) { + ret = qcow2_cache_flush(bs, s->refcount_block_cache); + if (ret < 0) { + qemu_co_mutex_unlock(&s->lock); + return ret; + } } qemu_co_mutex_unlock(&s->lock); @@ -1603,6 +1663,11 @@ static QEMUOptionParameter qcow2_create_options[] = { .type = OPT_STRING, .help = "Preallocation mode (allowed values: off, metadata)" }, + { + .name = BLOCK_OPT_LAZY_REFCOUNTS, + .type = OPT_FLAG, + .help = "Postpone refcount updates", + }, { NULL } }; diff --git a/block/qcow2.h b/block/qcow2.h index b5fefc08f7..b4eb65470e 100644 --- a/block/qcow2.h +++ b/block/qcow2.h @@ -118,6 +118,14 @@ enum { QCOW2_INCOMPAT_MASK = QCOW2_INCOMPAT_DIRTY, }; +/* Compatible feature bits */ +enum { + QCOW2_COMPAT_LAZY_REFCOUNTS_BITNR = 0, + QCOW2_COMPAT_LAZY_REFCOUNTS = 1 << QCOW2_COMPAT_LAZY_REFCOUNTS_BITNR, + + QCOW2_COMPAT_FEAT_MASK = QCOW2_COMPAT_LAZY_REFCOUNTS, +}; + typedef struct Qcow2Feature { uint8_t type; uint8_t bit; @@ -245,6 +253,11 @@ static inline int qcow2_get_cluster_type(uint64_t l2_entry) } } +/* Check whether refcounts are eager or lazy */ +static inline bool qcow2_need_accurate_refcounts(BDRVQcowState *s) +{ + return !(s->incompatible_features & QCOW2_INCOMPAT_DIRTY); +} // FIXME Need qcow2_ prefix to global functions diff --git a/block_int.h b/block_int.h index d72317fbe3..6c1d9cafb1 100644 --- a/block_int.h +++ b/block_int.h @@ -31,8 +31,9 @@ #include "qemu-timer.h" #include "qapi-types.h" -#define BLOCK_FLAG_ENCRYPT 1 -#define BLOCK_FLAG_COMPAT6 4 +#define BLOCK_FLAG_ENCRYPT 1 +#define BLOCK_FLAG_COMPAT6 4 +#define BLOCK_FLAG_LAZY_REFCOUNTS 8 #define BLOCK_IO_LIMIT_READ 0 #define BLOCK_IO_LIMIT_WRITE 1 @@ -41,16 +42,17 @@ #define BLOCK_IO_SLICE_TIME 100000000 #define NANOSECONDS_PER_SECOND 1000000000.0 -#define BLOCK_OPT_SIZE "size" -#define BLOCK_OPT_ENCRYPT "encryption" -#define BLOCK_OPT_COMPAT6 "compat6" -#define BLOCK_OPT_BACKING_FILE "backing_file" -#define BLOCK_OPT_BACKING_FMT "backing_fmt" -#define BLOCK_OPT_CLUSTER_SIZE "cluster_size" -#define BLOCK_OPT_TABLE_SIZE "table_size" -#define BLOCK_OPT_PREALLOC "preallocation" -#define BLOCK_OPT_SUBFMT "subformat" -#define BLOCK_OPT_COMPAT_LEVEL "compat" +#define BLOCK_OPT_SIZE "size" +#define BLOCK_OPT_ENCRYPT "encryption" +#define BLOCK_OPT_COMPAT6 "compat6" +#define BLOCK_OPT_BACKING_FILE "backing_file" +#define BLOCK_OPT_BACKING_FMT "backing_fmt" +#define BLOCK_OPT_CLUSTER_SIZE "cluster_size" +#define BLOCK_OPT_TABLE_SIZE "table_size" +#define BLOCK_OPT_PREALLOC "preallocation" +#define BLOCK_OPT_SUBFMT "subformat" +#define BLOCK_OPT_COMPAT_LEVEL "compat" +#define BLOCK_OPT_LAZY_REFCOUNTS "lazy_refcounts" typedef struct BdrvTrackedRequest BdrvTrackedRequest; diff --git a/tests/qemu-iotests/031.out b/tests/qemu-iotests/031.out index 297b4587e3..796c993df2 100644 --- a/tests/qemu-iotests/031.out +++ b/tests/qemu-iotests/031.out @@ -54,7 +54,7 @@ header_length 72 Header extension: magic 0x6803f857 -length 48 +length 96 data Header extension: @@ -68,7 +68,7 @@ No errors were found on the image. magic 0x514649fb version 2 -backing_file_offset 0xc8 +backing_file_offset 0xf8 backing_file_size 0x17 cluster_bits 16 size 67108864 @@ -92,7 +92,7 @@ data 'host_device' Header extension: magic 0x6803f857 -length 48 +length 96 data Header extension: @@ -155,7 +155,7 @@ header_length 104 Header extension: magic 0x6803f857 -length 48 +length 96 data Header extension: @@ -169,7 +169,7 @@ No errors were found on the image. magic 0x514649fb version 3 -backing_file_offset 0xe8 +backing_file_offset 0x118 backing_file_size 0x17 cluster_bits 16 size 67108864 @@ -193,7 +193,7 @@ data 'host_device' Header extension: magic 0x6803f857 -length 48 +length 96 data Header extension: diff --git a/tests/qemu-iotests/036.out b/tests/qemu-iotests/036.out index ca0fda13d3..063ca22d66 100644 --- a/tests/qemu-iotests/036.out +++ b/tests/qemu-iotests/036.out @@ -46,7 +46,7 @@ header_length 104 Header extension: magic 0x6803f857 -length 48 +length 96 data *** done