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