苗木 网站模板,搜索引擎培训班,专业影视广告制作公司,WordPress头像ssl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#xff1a;qemu-img convert -p -c -O qcow2 zero_disk.qcow2 compress_disk.qcow2
分析
qemu代码#xff1a;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);
}总结
从上面的流程分析可以看出当qcow2被压缩后guest每次需要读取原qcow2文件中的内容时都会让host后端进行一次解压缩这会消耗cpu
参考
https://github.com/qemu/qemu
