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Re: [Qemu-block] [PATCH v8 8/9] qcow2: skip writing zero buffers to empt
Re: [Qemu-block] [PATCH v8 8/9] qcow2: skip writing zero buffers to empty COW areas
Fri, 16 Mar 2018 18:09:22 +0300
Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0
On 16/3/2018 4:58 PM, Alberto Garcia wrote:
On Mon 12 Mar 2018 11:16:57 AM CET, Anton Nefedov wrote:
@@ -2584,6 +2584,8 @@
# @cor_write: a write due to copy-on-read (since 2.11)
+# @cluster_alloc_space: an allocation of a cluster file space (since 2.12)
now it's 2.13 I believe
That doesn't sound like correct English to me.
"an allocation of file space for a cluster"
+static bool is_unallocated(BlockDriverState *bs, int64_t offset, int64_t bytes)
+ int64_t nr;
+ return !bytes ||
+ (!bdrv_is_allocated_above(bs, NULL, offset, bytes, &nr) && nr ==
Is this really more efficient than the previous is_zero() call ?
It seems that in both cases the code ends up calling
the difference is that is_allocated passes 'want_zero = 0'.
It hints drivers to skip thorough seeks for zeroes (e.g. file-posix
returns (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) right away).
Moreover, bdrv_co_block_status() even skips the check in local_file
unless the protocol driver returned BDRV_BLOCK_RAW.
+static bool is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
+ /* content with false negatives, giving is_allocated() is faster than
+ * a proper zero detection with possible actual image seeks, which is
+ * performed by is_zero() */
It took me a bit to understand this sentence, maybe some native English
speaker can suggest an alternate wording?
"is_allocated() is not as accurate as is_zero() and can give us some
false negatives, but it is much more efficient so let's use it here"
wording is the hardest part of it all :)
/* This check is designed for optimization shortcut so it must be
* Instead of is_zero(), use is_unallocated() as it is faster (but not
* as accurate and can result in false negatives). */
+ return is_unallocated(bs, m->offset + m->cow_start.offset,
+ m->cow_start.nb_bytes) &&
+ is_unallocated(bs, m->offset + m->cow_end.offset,
- [Qemu-block] [PATCH v8 4/9] block: introduce BDRV_REQ_ALLOCATE flag, (continued)
- [Qemu-block] [PATCH v8 4/9] block: introduce BDRV_REQ_ALLOCATE flag, Anton Nefedov, 2018/03/12
- [Qemu-block] [PATCH v8 3/9] quorum: set supported write/zero flags, Anton Nefedov, 2018/03/12
- [Qemu-block] [PATCH v8 9/9] iotest 134: test cluster-misaligned encrypted write, Anton Nefedov, 2018/03/12
- [Qemu-block] [PATCH v8 6/9] file-posix: support BDRV_REQ_ALLOCATE, Anton Nefedov, 2018/03/12
- [Qemu-block] [PATCH v8 8/9] qcow2: skip writing zero buffers to empty COW areas, Anton Nefedov, 2018/03/12
- [Qemu-block] [PATCH v8 5/9] block: treat BDRV_REQ_ALLOCATE as serialising, Anton Nefedov, 2018/03/12
- [Qemu-block] [PATCH v8 7/9] block: support BDRV_REQ_ALLOCATE in passthrough drivers, Anton Nefedov, 2018/03/12