performance/read-ahead: stricter adherence to force-atime-update

Throwaway read-ahead cache in fstat only if force-atime-update is
set. Note that fstat flushes read-ahead cache only for atime
consistency. However if atime consistency is needed user is required
to set force-atime-update which updates atime on backend fs even
though application reads are served from read-ahead cache. So, if user
has not set force-atime-update, atime won't be accurate and there is
no point in flushing read-ahead cache in fstats. mounts
requiring atime consistency have to mandatorily set
force-atime-update.

Also note that normally kernel interspers reads with fstat. So,
read-ahead is not effective as fstats flush read-ahead-cache. Instead
it regresses performance due to wasted network reads. It is
recommended to turn off read-ahead if applications require atime
consistency.

This patch is aimed at applications which don't require atime
consistency. Without atime consistency required, read-ahead cache is
effective and increases performance of sequential reads.

Change-Id: I122bbc410cee96661823f9c4b934383495c18446
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Fixes: bz#1601166

1 files changed, 18 insertions, 12 deletions

diff --git a/xlators/performance/read-ahead/src/read-ahead.c b/xlators/performance/read-ahead/src/read-ahead.c
index 1185c6e4183..652b001129b 100644
--- a/xlators/performance/read-ahead/src/read-ahead.c
+++ b/xlators/performance/read-ahead/src/read-ahead.c
@@ -867,6 +867,9 @@ ra_fstat (call_frame_t *frame, xlator_t *this, fd_t *fd, dict_t *xdata)
         inode_t   *inode    = NULL;
         uint64_t   tmp_file = 0;
         int32_t    op_errno = EINVAL;
+        ra_conf_t *conf     = NULL;
+
+        conf = this->private;
 
         GF_ASSERT (frame);
         GF_VALIDATE_OR_GOTO (frame->this->name, this, unwind);
@@ -874,20 +877,23 @@ ra_fstat (call_frame_t *frame, xlator_t *this, fd_t *fd, dict_t *xdata)
 
         inode = fd->inode;
 
-        LOCK (&inode->lock);
-        {
-                list_for_each_entry (iter_fd, &inode->fd_list, inode_list) {
-                        tmp_file = 0;
-                        fd_ctx_get (iter_fd, this, &tmp_file);
-                        file = (ra_file_t *)(long)tmp_file;
-
-                        if (!file)
-                                continue;
-                        flush_region (frame, file, 0,
-                                      file->pages.prev->offset + 1, 0);
+        if (conf->force_atime_update) {
+                LOCK (&inode->lock);
+                {
+                        list_for_each_entry (iter_fd, &inode->fd_list,
+                                             inode_list) {
+                                tmp_file = 0;
+                                fd_ctx_get (iter_fd, this, &tmp_file);
+                                file = (ra_file_t *)(long)tmp_file;
+
+                                if (!file)
+                                        continue;
+                                flush_region (frame, file, 0,
+                                              file->pages.prev->offset + 1, 0);
+                        }
                 }
+                UNLOCK (&inode->lock);
         }
-        UNLOCK (&inode->lock);
 
         STACK_WIND (frame, ra_attr_cbk, FIRST_CHILD (this),
                     FIRST_CHILD (this)->fops->fstat, fd, xdata);