On Mon, Jan 26, 2015 at 09:20:00PM +0800, Zhang Haoyu wrote: > Hi, all > > Regarding too large qcow2 image, e.g., 2TB, > so long disruption happened when performing snapshot, > which was caused by cache update and IO wait.
I have CCed Kevin Wolf, the qcow2 maintainer.
> perf top data shown as below,
> PerfTop: 2554 irqs/sec kernel: 0.4% exact: 0.0% [4000Hz cycles],
> (target_pid: 34294)
> ------------------------------------------------------------------------------------------------------------------------
>
> 33.80% qemu-system-x86_64 [.] qcow2_cache_do_get
> 27.59% qemu-system-x86_64 [.] qcow2_cache_put
> 15.19% qemu-system-x86_64 [.] qcow2_cache_entry_mark_dirty
> 5.49% qemu-system-x86_64 [.] update_refcount
> 3.02% libpthread-2.13.so [.] pthread_getspecific
> 2.26% qemu-system-x86_64 [.] get_refcount
> 1.95% qemu-system-x86_64 [.] coroutine_get_thread_state
> 1.32% qemu-system-x86_64 [.] qcow2_update_snapshot_refcount
> 1.20% qemu-system-x86_64 [.] qemu_coroutine_self
> 1.16% libz.so.1.2.7 [.] 0x0000000000003018
> 0.95% qemu-system-x86_64 [.] qcow2_update_cluster_refcount
> 0.91% qemu-system-x86_64 [.] qcow2_cache_get
> 0.76% libc-2.13.so [.] 0x0000000000134e49
> 0.73% qemu-system-x86_64 [.] bdrv_debug_event
> 0.16% qemu-system-x86_64 [.] pthread_getspecific@plt
> 0.12% [kernel] [k] _raw_spin_unlock_irqrestore
> 0.10% qemu-system-x86_64 [.] vga_draw_line24_32
> 0.09% [vdso] [.] 0x000000000000060c
> 0.09% qemu-system-x86_64 [.] qcow2_check_metadata_overlap
> 0.08% [kernel] [k] do_blockdev_direct_IO
>
> If expand the cache table size, the IO will be decreased,
> but the calculation time will be grown.
> so it's worthy to optimize qcow2 cache get and put algorithm.
>
> My proposal:
> get:
> using ((use offset >> cluster_bits) % c->size) to locate the cache entry,
> raw implementation,
> index = (use offset >> cluster_bits) % c->size;
> if (c->entries[index].offset == offset) {
> goto found;
> }
>
> replace:
> c->entries[use offset >> cluster_bits) % c->size].offset = offset;
> ...
>
> put:
> using 64-entries cache table to cache
> the recently got c->entries, i.e., cache for cache,
> then during put process, firstly search the 64-entries cache,
> if not found, then the c->entries.
>
> Any idea?
>
> Thanks,
> Zhang Haoyu
>
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