O_SYNC
O_SYNC
Posted May 17, 2019 4:48 UTC (Fri) by andresfreund (subscriber, #69562)In reply to: O_SYNC by rweikusat2
Parent article: DAX semantics
> Dirty_writeback_centics is a /proc-file
What on earth made you think I didn't know that?
> which can be used to change the time interval between automatically performed sync-operation by the kernel. That's still sync and not fsync.
It's neither. Writeback triggered by those controls doesn't trigger data integrity flushes, but sync(1) and fsync() do.
> It's outstanding modifications to a certain open file, this referring to a per-process data structure referred to by a file descriptor. This can be as little or as much as an individual application desires it to be as it's up to the application to call fsync whenever this makes sense.
Well, fsync is synchronous, so constantly emitting it is less performant than the alternative I mentioned with controlling writeback via sync_file_range() (and less efficient than what I wished for) - needs to interact with the drive cache. And where have I doubted that one can also issue fsyncs to control the amount of outstanding data?
Posted May 17, 2019 17:28 UTC (Fri)
by rweikusat2 (subscriber, #117920)
[Link] (1 responses)
Why did you hack this sentence apart in order to insert a (wrong) conjecture about something I could have thought?
>> which can be used to change the time interval between automatically performed sync-operation by the kernel. That's
Indeed. The name is obviously different. Nevertheless, it refers to a timeout for periodically flushing all dirty page cache pages to stable storage, just like the sync but not the fsync system call would do, IOW, the combination "dirty_writeback_centisecs/fsync" makes no sense. For the case of a single process writing to a certain open file, the effects of O_SYNC are by definition (POSIX) identical to pairing each write(2) with an fsync(2) on the same file descriptor. As additional benefit, fsync is more portable and it enables application to batch several writes if so desired, eg, when creating a file, write the complete file content and then fsync it.
Posted May 21, 2019 23:10 UTC (Tue)
by nix (subscriber, #2304)
[Link]
O_SYNC
> What on earth made you think I didn't know that?
>> still sync and not fsync.
> It's neither.
O_SYNC
Nevertheless, it refers to a timeout for periodically flushing all dirty page cache pages to stable storage, just like the sync but not the fsync system call would do
Except that's not what it does. Every dirty_writeback_centisecs, dirty inodes that became dirty longer ago than dirty_writeback_centisecs are flushed. inodes that were dirtied more recently than that are *not* flushed. That's nothing like sync(), which flushes everything regardless of first dirty time, and you can't do it with fsync() either (even if you track the timing yourself, good luck with that) because there is extra behaviour regarding flushes that take too long which cannot be mimicked with fsync().