Josh Berkus wrote:
> Greg,
>
> > This is interesting, because test_fsync consistently reported a rate of
> > about half this when using open_datasync instead of the equal
> > performance I'm getting from the database. I'll see if I can reproduce
> > that further, but it's no reason to be concerned about the change that's
> > been made I think. Just more evidence that test_fsync has quirks left
> > to be sorted out. But that's not backbranch material, it should be part
> > of 9.1 only refactoring, already in progress via the patch Josh
> > submitted. There's a bit of time left to get that done.
>
> Did you rerun test_sync with O_DIRECT entabled, using my patch? The
> figures you had from test_fsync earlier were without O_DIRECT.
I have modified test_fsync with the attached, applied patch to report
cases where we are testing without O_DIRECT when only O_DIRECT would be
used by the server, and cases where O_DIRECT fails because of the file
system type. Josh Berkus wanted the first case kept in case we decide
to offer non-direct-io options on machines that support direct i/o.
The new messages are:
* This non-direct I/O option is not used by Postgres.
** This file system and its mount options do not support direct
I/O, e.g. ext4 in journaled mode.
You can see the first one below in my output from Ubuntu:
$ ./test_fsync
Ops-per-test = 2000
Simple non-sync'ed write:
8k write 58.175 ops/sec
Compare file sync methods using one write:
(in wal_sync_method preference order, except fdatasync
is Linux's default)
open_datasync n/a
8k write, fdatasync 68.425 ops/sec
8k write, fsync 63.932 ops/sec
fsync_writethrough n/a
open_sync 8k write* 73.785 ops/sec
open_sync 8k direct I/O write 82.929 ops/sec
* This non-direct I/O option is not used by Postgres.
Compare file sync methods using two writes:
(in wal_sync_method preference order, except fdatasync
is Linux's default)
open_datasync n/a
8k write, 8k write, fdatasync 42.728 ops/sec
8k write, 8k write, fsync 43.625 ops/sec
fsync_writethrough n/a
2 open_sync 8k writes* 37.150 ops/sec
2 open_sync 8k direct I/O writes 43.722 ops/sec
* This non-direct I/O option is not used by Postgres.
Compare open_sync with different sizes:
(This is designed to compare the cost of one large
sync'ed write and two smaller sync'ed writes.)
open_sync 16k write 46.428 ops/sec
2 open_sync 8k writes 38.703 ops/sec
Test if fsync on non-write file descriptor is honored:
(If the times are similar, fsync() can sync data written
on a different descriptor.)
8k write, fsync, close 65.744 ops/sec
8k write, close, fsync 63.077 ops/sec
I believe test_fsync now matches the backend code. If we decide to
change things, it can be adjusted.
--
Bruce Momjian <[email protected]> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
diff --git a/src/tools/fsync/test_fsync.c b/src/tools/fsync/test_fsync.c
index cd2b1f2..562da66 100644
*** /tmp/8wSNdd_test_fsync.c Sat Jan 15 18:42:58 2011
--- src/tools/fsync/test_fsync.c Sat Jan 15 18:38:57 2011
*************** void
*** 163,169 ****
test_sync(int writes_per_op)
{
int tmpfile, ops, writes;
!
if (writes_per_op == 1)
printf("\nCompare file sync methods using one write:\n");
else
--- 163,170 ----
test_sync(int writes_per_op)
{
int tmpfile, ops, writes;
! bool fs_warning = false;
!
if (writes_per_op == 1)
printf("\nCompare file sync methods using one write:\n");
else
*************** test_sync(int writes_per_op)
*** 176,184 ****
*/
#ifdef OPEN_DATASYNC_FLAG
if (writes_per_op == 1)
! printf(LABEL_FORMAT, "open_datasync 8k write");
else
! printf(LABEL_FORMAT, "2 open_datasync 8k writes");
fflush(stdout);
if ((tmpfile = open(filename, O_RDWR | O_DSYNC, 0)) == -1)
--- 177,193 ----
*/
#ifdef OPEN_DATASYNC_FLAG
if (writes_per_op == 1)
! printf(LABEL_FORMAT, "open_datasync 8k write"
! #if PG_O_DIRECT != 0
! "*"
! #endif
! );
else
! printf(LABEL_FORMAT, "2 open_datasync 8k writes"
! #if PG_O_DIRECT != 0
! "*"
! #endif
! );
fflush(stdout);
if ((tmpfile = open(filename, O_RDWR | O_DSYNC, 0)) == -1)
*************** test_sync(int writes_per_op)
*** 201,207 ****
*/
#if PG_O_DIRECT != 0
if ((tmpfile = open(filename, O_RDWR | O_DSYNC | PG_O_DIRECT, 0)) == -1)
! printf(NA_FORMAT, "o_direct", "n/a on this filesystem\n");
else
{
if (writes_per_op == 1)
--- 210,219 ----
*/
#if PG_O_DIRECT != 0
if ((tmpfile = open(filename, O_RDWR | O_DSYNC | PG_O_DIRECT, 0)) == -1)
! {
! printf(NA_FORMAT, "o_direct", "n/a**\n");
! fs_warning = true;
! }
else
{
if (writes_per_op == 1)
*************** test_sync(int writes_per_op)
*** 321,329 ****
*/
#ifdef OPEN_SYNC_FLAG
if (writes_per_op == 1)
! printf(LABEL_FORMAT, "open_sync 8k write");
else
! printf(LABEL_FORMAT, "2 open_sync 8k writes");
fflush(stdout);
if ((tmpfile = open(filename, O_RDWR | OPEN_SYNC_FLAG, 0)) == -1)
--- 333,349 ----
*/
#ifdef OPEN_SYNC_FLAG
if (writes_per_op == 1)
! printf(LABEL_FORMAT, "open_sync 8k write"
! #if PG_O_DIRECT != 0
! "*"
! #endif
! );
else
! printf(LABEL_FORMAT, "2 open_sync 8k writes"
! #if PG_O_DIRECT != 0
! "*"
! #endif
! );
fflush(stdout);
if ((tmpfile = open(filename, O_RDWR | OPEN_SYNC_FLAG, 0)) == -1)
*************** test_sync(int writes_per_op)
*** 352,358 ****
fflush(stdout);
if ((tmpfile = open(filename, O_RDWR | OPEN_SYNC_FLAG | PG_O_DIRECT, 0)) == -1)
! printf(NA_FORMAT, "o_direct", "n/a on this filesystem\n");
else
{
gettimeofday(&start_t, NULL);
--- 372,381 ----
fflush(stdout);
if ((tmpfile = open(filename, O_RDWR | OPEN_SYNC_FLAG | PG_O_DIRECT, 0)) == -1)
! {
! printf(NA_FORMAT, "o_direct", "n/a**\n");
! fs_warning = true;
! }
else
{
gettimeofday(&start_t, NULL);
*************** test_sync(int writes_per_op)
*** 375,380 ****
--- 398,414 ----
#else
printf(NA_FORMAT, "open_sync", "n/a\n");
#endif
+
+ #if defined(OPEN_DATASYNC_FLAG) || defined(OPEN_SYNC_FLAG)
+ if (PG_O_DIRECT != 0)
+ printf("* This non-direct I/O option is not used by Postgres.\n");
+ #endif
+
+ if (fs_warning)
+ {
+ printf("** This file system and its mount options do not support direct\n");
+ printf("I/O, e.g. ext4 in journaled mode.\n");
+ }
}
void
*************** test_open_syncs(void)
*** 389,394 ****
--- 423,430 ----
printf("(This is designed to compare the cost of one large\n");
printf("sync'ed write and two smaller sync'ed writes.)\n");
+ /* XXX no PG_O_DIRECT */
+
/*
* Test open_sync with different size files
*/
--
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