common/options: bluefs_buffered_io=true by default

Enable bluefs_buffered_io again because it makes a huge user-visible
improvement in metadata intensive scenarios, such as but not limited to
PG deletion.

In our environment, deleting PGs from 4 hybrid OSDs (sharing one SATA SSD block.db) saturates
the block.db at 350MB/s reads and causes slow reqs and flapping on the OSDs.
Those OSDs have 3GB osd_target_memory.
Enabling bluefs_buffered_io drops the SSD IO down to <1MBps and the OSDs
are performant again. (The underlying PG deletion inefficiency is being
solved separately, but the page cache is so much more effective than
the bluestore cache in this scenario).

Lastly, remove the comment about swap. We should separately advise
operators to disable swap on OSD machines, as it is much better in
our experience to OOM and restart than to chug along swapping.

Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>
Related-to: https://tracker.ceph.com/issues/45765
Related-to: https://tracker.ceph.com/issues/47044
(cherry picked from commit 5ec8e8e63d409860c35e24a192090ac2b70af8f6)

diff --git a/src/common/options.cc b/src/common/options.cc
index 06e7ee0dfb0bac3a3c325717b4e4c66c69049561..31706708166beec33ca59c75de3c1616599e9160 100644 (file)
--- a/src/common/options.cc
+++ b/src/common/options.cc
@@ -4386,9 +4386,9 @@ std::vector<Option> get_global_options() {
     .set_description(""),
 
     Option("bluefs_buffered_io", Option::TYPE_BOOL, Option::LEVEL_ADVANCED)
-    .set_default(false)
+    .set_default(true)
     .set_description("Enabled buffered IO for bluefs reads.")
-    .set_long_description("When this option is enabled, bluefs will in some cases perform buffered reads.  This allows the kernel page cache to act as a secondary cache for things like RocksDB compaction.  For example, if the rocksdb block cache isn't large enough to hold blocks from the compressed SST files itself, they can be read from page cache instead of from the disk.  This option previously was enabled by default, however in some test cases it appears to cause excessive swap utilization by the linux kernel and a large negative performance impact after several hours of run time.  Please exercise caution when enabling."),
+    .set_long_description("When this option is enabled, bluefs will in some cases perform buffered reads.  This allows the kernel page cache to act as a secondary cache for things like RocksDB compaction.  For example, if the rocksdb block cache isn't large enough to hold blocks from the compressed SST files itself, they can be read from page cache instead of from the disk."),
 
     Option("bluefs_sync_write", Option::TYPE_BOOL, Option::LEVEL_ADVANCED)
     .set_default(false)