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网站视觉设计方案,广州网络推广定制,做跨境电商哪个平台好,长春网站建设开发的有哪些本文拟对压缩qcow2镜像所带来的虚拟机性能损失进行简单分析背景生产中发现使用压缩镜像启动的虚拟机开机总是会慢一些。 qcow2镜像的压缩方式为：qemu-img convert -p -c -O qcow2 zero_disk.qcow2 compress_disk.qcow2 分析 qemu代码：https://down…

本文拟对压缩qcow2镜像所带来的虚拟机性能损失进行简单分析

背景

生产中发现使用压缩镜像启动的虚拟机开机总是会慢一些。
qcow2镜像的压缩方式为：qemu-img convert -p -c -O qcow2 zero_disk.qcow2 compress_disk.qcow2

分析

qemu代码：https://download.qemu.org/qemu-9.2.0.tar.xz
主要看下使用qcow2后端的guest在读操作时后端操作qcow2文件的一个流程。

block/qcow2.c
后端driver指定读写回调：qcow2_co_preadv_part、qcow2_co_pwritev_part

BlockDriver bdrv_qcow2 = {.format_name                        = "qcow2",.instance_size                      = sizeof(BDRVQcow2State),.bdrv_probe                         = qcow2_probe,.bdrv_open                          = qcow2_open,.bdrv_close                         = qcow2_close,.bdrv_reopen_prepare                = qcow2_reopen_prepare,.bdrv_reopen_commit                 = qcow2_reopen_commit,.bdrv_reopen_commit_post            = qcow2_reopen_commit_post,.bdrv_reopen_abort                  = qcow2_reopen_abort,.bdrv_join_options                  = qcow2_join_options,.bdrv_child_perm                    = bdrv_default_perms,.bdrv_co_create_opts                = qcow2_co_create_opts,.bdrv_co_create                     = qcow2_co_create,.bdrv_has_zero_init                 = qcow2_has_zero_init,.bdrv_co_block_status               = qcow2_co_block_status,.bdrv_co_preadv_part                = qcow2_co_preadv_part,.bdrv_co_pwritev_part               = qcow2_co_pwritev_part,.bdrv_co_flush_to_os                = qcow2_co_flush_to_os,.bdrv_co_pwrite_zeroes              = qcow2_co_pwrite_zeroes,.bdrv_co_pdiscard                   = qcow2_co_pdiscard,.bdrv_co_copy_range_from            = qcow2_co_copy_range_from,.bdrv_co_copy_range_to              = qcow2_co_copy_range_to,.bdrv_co_truncate                   = qcow2_co_truncate,.bdrv_co_pwritev_compressed_part    = qcow2_co_pwritev_compressed_part,.bdrv_make_empty                    = qcow2_make_empty,.bdrv_snapshot_create               = qcow2_snapshot_create,.bdrv_snapshot_goto                 = qcow2_snapshot_goto,.bdrv_snapshot_delete               = qcow2_snapshot_delete,.bdrv_snapshot_list                 = qcow2_snapshot_list,.bdrv_snapshot_load_tmp             = qcow2_snapshot_load_tmp,.bdrv_measure                       = qcow2_measure,.bdrv_co_get_info                   = qcow2_co_get_info,.bdrv_get_specific_info             = qcow2_get_specific_info,.bdrv_co_save_vmstate               = qcow2_co_save_vmstate,.bdrv_co_load_vmstate               = qcow2_co_load_vmstate,.is_format                          = true,.supports_backing                   = true,.bdrv_co_change_backing_file        = qcow2_co_change_backing_file,.bdrv_refresh_limits                = qcow2_refresh_limits,.bdrv_co_invalidate_cache           = qcow2_co_invalidate_cache,.bdrv_inactivate                    = qcow2_inactivate,.create_opts                        = &qcow2_create_opts,.amend_opts                         = &qcow2_amend_opts,.strong_runtime_opts                = qcow2_strong_runtime_opts,.mutable_opts                       = mutable_opts,.bdrv_co_check                      = qcow2_co_check,.bdrv_amend_options                 = qcow2_amend_options,.bdrv_co_amend                      = qcow2_co_amend,.bdrv_detach_aio_context            = qcow2_detach_aio_context,.bdrv_attach_aio_context            = qcow2_attach_aio_context,.bdrv_supports_persistent_dirty_bitmap =qcow2_supports_persistent_dirty_bitmap,.bdrv_co_can_store_new_dirty_bitmap = qcow2_co_can_store_new_dirty_bitmap,.bdrv_co_remove_persistent_dirty_bitmap =qcow2_co_remove_persistent_dirty_bitmap,
};

block/qcow2.c
读取时走读回调:qcow2_co_preadv_task_entry

static int coroutine_fn GRAPH_RDLOCK
qcow2_co_preadv_part(BlockDriverState *bs, int64_t offset, int64_t bytes,QEMUIOVector *qiov, size_t qiov_offset,BdrvRequestFlags flags)
{BDRVQcow2State *s = bs->opaque;int ret = 0;unsigned int cur_bytes; /* number of bytes in current iteration */uint64_t host_offset = 0;QCow2SubclusterType type;AioTaskPool *aio = NULL;while (bytes != 0 && aio_task_pool_status(aio) == 0) {/* prepare next request */cur_bytes = MIN(bytes, INT_MAX);if (s->crypto) {cur_bytes = MIN(cur_bytes,QCOW_MAX_CRYPT_CLUSTERS * s->cluster_size);}qemu_co_mutex_lock(&s->lock);ret = qcow2_get_host_offset(bs, offset, &cur_bytes,&host_offset, &type);qemu_co_mutex_unlock(&s->lock);if (ret < 0) {goto out;}if (type == QCOW2_SUBCLUSTER_ZERO_PLAIN ||type == QCOW2_SUBCLUSTER_ZERO_ALLOC ||(type == QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN && !bs->backing) ||(type == QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC && !bs->backing)){qemu_iovec_memset(qiov, qiov_offset, 0, cur_bytes);} else {if (!aio && cur_bytes != bytes) {aio = aio_task_pool_new(QCOW2_MAX_WORKERS);}ret = qcow2_add_task(bs, aio, qcow2_co_preadv_task_entry, type,host_offset, offset, cur_bytes,qiov, qiov_offset, NULL);if (ret < 0) {goto out;}}bytes -= cur_bytes;offset += cur_bytes;qiov_offset += cur_bytes;}out:if (aio) {aio_task_pool_wait_all(aio);if (ret == 0) {ret = aio_task_pool_status(aio);}g_free(aio);}return ret;
}

qcow2_co_preadv_task

/** This function can count as GRAPH_RDLOCK because qcow2_co_preadv_part() holds* the graph lock and keeps it until this coroutine has terminated.*/
static int coroutine_fn GRAPH_RDLOCK qcow2_co_preadv_task_entry(AioTask *task)
{Qcow2AioTask *t = container_of(task, Qcow2AioTask, task);assert(!t->l2meta);return qcow2_co_preadv_task(t->bs, t->subcluster_type,t->host_offset, t->offset, t->bytes,t->qiov, t->qiov_offset);
}

之后会判断当前读的cluster是什么类型的，如果是压缩的镜像，会走到QCOW2_SUBCLUSTER_COMPRESSED

static int coroutine_fn GRAPH_RDLOCK
qcow2_co_preadv_task(BlockDriverState *bs, QCow2SubclusterType subc_type,uint64_t host_offset, uint64_t offset, uint64_t bytes,QEMUIOVector *qiov, size_t qiov_offset)
{BDRVQcow2State *s = bs->opaque;switch (subc_type) {case QCOW2_SUBCLUSTER_ZERO_PLAIN:case QCOW2_SUBCLUSTER_ZERO_ALLOC:/* Both zero types are handled in qcow2_co_preadv_part */g_assert_not_reached();case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:assert(bs->backing); /* otherwise handled in qcow2_co_preadv_part */BLKDBG_CO_EVENT(bs->file, BLKDBG_READ_BACKING_AIO);return bdrv_co_preadv_part(bs->backing, offset, bytes,qiov, qiov_offset, 0);case QCOW2_SUBCLUSTER_COMPRESSED:return qcow2_co_preadv_compressed(bs, host_offset,offset, bytes, qiov, qiov_offset);case QCOW2_SUBCLUSTER_NORMAL:if (bs->encrypted) {return qcow2_co_preadv_encrypted(bs, host_offset,offset, bytes, qiov, qiov_offset);}BLKDBG_CO_EVENT(bs->file, BLKDBG_READ_AIO);return bdrv_co_preadv_part(s->data_file, host_offset,bytes, qiov, qiov_offset, 0);default:g_assert_not_reached();}g_assert_not_reached();
}

qcow2_co_preadv_compressed

static int coroutine_fn GRAPH_RDLOCK
qcow2_co_preadv_compressed(BlockDriverState *bs,uint64_t l2_entry,uint64_t offset,uint64_t bytes,QEMUIOVector *qiov,size_t qiov_offset)
{BDRVQcow2State *s = bs->opaque;int ret = 0, csize;uint64_t coffset;uint8_t *buf, *out_buf;int offset_in_cluster = offset_into_cluster(s, offset);qcow2_parse_compressed_l2_entry(bs, l2_entry, &coffset, &csize);buf = g_try_malloc(csize);if (!buf) {return -ENOMEM;}out_buf = qemu_blockalign(bs, s->cluster_size);BLKDBG_CO_EVENT(bs->file, BLKDBG_READ_COMPRESSED);ret = bdrv_co_pread(bs->file, coffset, csize, buf, 0);if (ret < 0) {goto fail;}if (qcow2_co_decompress(bs, out_buf, s->cluster_size, buf, csize) < 0) {ret = -EIO;goto fail;}qemu_iovec_from_buf(qiov, qiov_offset, out_buf + offset_in_cluster, bytes);fail:qemu_vfree(out_buf);g_free(buf);return ret;
}

最后调用解压缩

/** qcow2_co_decompress()** Decompress some data (not more than @src_size bytes) to produce exactly* @dest_size bytes using the compression method defined by the image* compression type** @dest - destination buffer, @dest_size bytes* @src - source buffer, @src_size bytes** Returns: 0 on success*          a negative error code on failure*/
ssize_t coroutine_fn
qcow2_co_decompress(BlockDriverState *bs, void *dest, size_t dest_size,const void *src, size_t src_size)
{BDRVQcow2State *s = bs->opaque;Qcow2CompressFunc fn;switch (s->compression_type) {case QCOW2_COMPRESSION_TYPE_ZLIB:fn = qcow2_zlib_decompress;break;#ifdef CONFIG_ZSTDcase QCOW2_COMPRESSION_TYPE_ZSTD:fn = qcow2_zstd_decompress;break;
#endifdefault:abort();}return qcow2_co_do_compress(bs, dest, dest_size, src, src_size, fn);
}