Re: test_fsync label adjustments

On Jan 18, 2011, at 5:16 PM, Bruce Momjian wrote:

> A.M. wrote:
>> 
>> On Jan 18, 2011, at 3:55 PM, Bruce Momjian wrote:
>> 
>>> I have modified test_fsync to use test labels that match wal_sync_method
>>> values, and and added more tests for open_sync with different sizes. 
>>> This should make the program easier for novices to understand.  Here is
>>> a test run for Ubuntu 11.04:
>>> 
>>> 	$ ./test_fsync
>>> 	2000 operations per test
>>> 	
>>> 	Compare file sync methods using one 8k write:
>>> 	(in wal_sync_method preference order, except fdatasync
>>> 	is Linux's default)
>>> 	        open_datasync (non-direct I/O)*    85.127 ops/sec
>>> 	        open_datasync (direct I/O)         87.119 ops/sec
>>> 	        fdatasync                          81.006 ops/sec
>>> 	        fsync                              82.621 ops/sec
>>> 	        fsync_writethrough                            n/a
>>> 	        open_sync (non-direct I/O)*        84.412 ops/sec
>>> 	        open_sync (direct I/O)             91.006 ops/sec
>>> 	* This non-direct I/O mode is not used by Postgres.
>> 
>> I am curious how this is targeted at novices. A naive user might enable
>> the "fastest" option which could be exactly wrong. For this to be useful
>> to novices, I suspect the tool will need to generate platform-specific
>> suggestions, no?
> 
> Uh, why isn't the fastest option right for them?  It is hardware/kernel
> specific when you run it --- how could it be better?

Because the fastest option may not be syncing to disk. For example, the only option that makes sense on OS X is fsync_writethrough- it would be helpful if the tool pointed that out (on OS X only, obviously).

Cheers,
M