storage/posix: implement batched fsync in a single thread

Because of the extra fsync()s issued by AFR transaction, they
could potentially "clog" all the io-threads denying unrelated
operations from making progress.

This patch assigns a dedicated thread to issues fsyncs, as
an experimental feature to understand performance characteristics
with the approach.

As a basis, incoming individual fsync requests are grouped into
batches, falling in the same @batch-fsync-delay-usec window of
time. These windows can extend in practice, as processing of
the previous batch can take longer than @batch-fsync-delay-usec
while new requests are getting batched.

The feature support three modes (similar to the -S modes of fs_mark)

- syncfs: In this mode one syncfs() is issued per batch, instead
  of N fsync()s (one per file.)

- syncfs-single-fsync: In this mode one syncfs() is issued per
  batch (which, on Linux, guarantees the completion of write-out
  of dirty pages in the filesystem up to that point) and one single
  fsync() to synchronize or flush the controller/drive cache. This
  corresponds to -S 2 of fsmark.

- syncfs-reverse-fsync: In this mode, one syncfs() is issued per
  batch, and all the open files in that batch are fsync()'ed in
  the reverse order of the queue. This corresponds to -S 4 of
  fsmark.

- reverse-fsync: In this mode, no syncfs() is issued and all the
  files in the batch are fsync()'ed in the reverse order. This
  corresponds to -S 3 of fsmark.

Change-Id: Ia1e170a810c780c8d80e02cf910accc4170c4cd4
BUG: 927146
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4746
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/xlators/mgmt/glusterd/src/glusterd-volume-set.c b/xlators/mgmt/glusterd/src/glusterd-volume-set.c
index 6575e252889..245b29af940 100644
--- a/xlators/mgmt/glusterd/src/glusterd-volume-set.c
+++ b/xlators/mgmt/glusterd/src/glusterd-volume-set.c
@@ -1201,6 +1201,14 @@ struct volopt_map_entry glusterd_volopt_map[] = {
           .voltype     = "storage/posix",
           .op_version  = 1
         },
+        { .key         = "storage.batch-fsync-mode",
+          .voltype     = "storage/posix",
+          .op_version  = 3
+        },
+        { .key         = "storage.batch-fsync-delay-usec",
+          .voltype     = "storage/posix",
+          .op_version  = 3
+        },
         { .key         = "storage.owner-uid",
           .voltype     = "storage/posix",
           .option      = "brick-uid",