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Re: [Qemu-block] [Qemu-devel] [PATCH v12 1/2] block/backup: fix max_tran

From: John Snow
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v12 1/2] block/backup: fix max_transfer handling for copy_range
Date: Wed, 18 Sep 2019 15:57:33 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0

On 9/17/19 12:07 PM, Vladimir Sementsov-Ogievskiy wrote:
Of course, QEMU_ALIGN_UP is a typo, it should be QEMU_ALIGN_DOWN, as we
are trying to find aligned size which satisfy both source and target.
Also, don't ignore too small max_transfer. In this case seems safer to
disable copy_range.

Fixes: 9ded4a0114968e
Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
  block/backup.c | 12 ++++++++----
  1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/block/backup.c b/block/backup.c
index 763f0d7ff6..d8fdbfadfe 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -741,12 +741,16 @@ BlockJob *backup_job_create(const char *job_id, 
BlockDriverState *bs,
      job->cluster_size = cluster_size;
      job->copy_bitmap = copy_bitmap;
      copy_bitmap = NULL;
-    job->use_copy_range = !compress; /* compression isn't supported for it */
      job->copy_range_size = MIN_NON_ZERO(blk_get_max_transfer(job->common.blk),
-    job->copy_range_size = MAX(job->cluster_size,
-                               QEMU_ALIGN_UP(job->copy_range_size,
-                                             job->cluster_size));
+    job->copy_range_size = QEMU_ALIGN_DOWN(job->copy_range_size,
+                                           job->cluster_size);
+    /*
+     * Compression is not supported for copy_range. Also, we don't want to
+     * handle small max_transfer for copy_range (which currently don't
+     * handle max_transfer at all).
+     */
+    job->use_copy_range = !compress && job->copy_range_size > 0;
/* Required permissions are already taken with target's blk_new() */
      block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,

I'm clear on the alignment fix, I'm not clear on the comment about max_transfer and how it relates to copy_range_size being non-zero.

"small max transfer" -- what happens when it's zero? we're apparently OK with a single cluster, but when it's zero, what happens?

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