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Re: [PATCH v3 20/25] qapi: backup: disable copy_range by default

From: Max Reitz
Subject: Re: [PATCH v3 20/25] qapi: backup: disable copy_range by default
Date: Tue, 12 Jan 2021 15:05:31 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0

On 26.10.20 18:18, Vladimir Sementsov-Ogievskiy wrote:
Further commit will add a benchmark
(scripts/simplebench/bench-backup.py), which will show that backup
works better with async parallel requests (previous commit) and
disabled copy_range. So, let's disable copy_range by default.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
  qapi/block-core.json | 2 +-
  blockdev.c           | 2 +-
  2 files changed, 2 insertions(+), 2 deletions(-)

Uh, well, er, then why did you add it as true in patch 2?

Do you mean this patch as the basis for a discussion whether it should be true or false by default? I don’t have anything to contribute, though, ergo I don’t mind either way.

Do you have an idea why copy offloading isn’t better?


diff --git a/qapi/block-core.json b/qapi/block-core.json
index 5a21c24c1d..58eb2bcb86 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1376,7 +1376,7 @@
  # Optional parameters for backup. These parameters don't affect
  # functionality, but may significantly affect performance.
-# @use-copy-range: Use copy offloading. Default true.
+# @use-copy-range: Use copy offloading. Default false.
  # @max-workers: Maximum number of parallel requests for the sustained 
  #               copying process. Doesn't influence copy-before-write 
diff --git a/blockdev.c b/blockdev.c
index 0ed390abe0..1ac64d8ee2 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2788,7 +2788,7 @@ static BlockJob *do_backup_common(BackupCommon *backup,
      BlockJob *job = NULL;
      BdrvDirtyBitmap *bmap = NULL;
-    BackupPerf perf = { .use_copy_range = true, .max_workers = 64 };
+    BackupPerf perf = { .max_workers = 64 };
      int job_flags = JOB_DEFAULT;
if (!backup->has_speed) {

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