osd/scrub: decrease default deep scrub chunk size

The previous default of 25 objects per chunk proved to take too long
(many hundreds of milliseconds) on a busy cluster. As the scrubber
locks all objects in the chunk for the duration, a large chunk size
can cause a significant impact on the client ops' latencies.

Fixes: https://tracker.ceph.com/issues/68057

Signed-off-by: Ronen Friedman <[email protected]>

Author: ronen-fr

src/common/options/osd.yaml.in

@@ -349,8 +349,10 @@ options:
   type: int
   level: advanced
   desc: Maximum number of objects to deep-scrub in a single chunk
-  fmt_desc: The maximum number of object store chunks to scrub during single operation.
-  default: 25
+  fmt_desc: The maximum number of objects to deep-scrub during single internal
+    scrub operation. Large values would improve scrubbing performance but
+    may adversely affect client operations' latency.
+  default: 15
   see_also:
   - osd_scrub_chunk_min
   with_legacy: true