Re: Linux 2.6.29

Janne Grunau
2009-04-03T22:32:54Z

On Fri, Apr 03, 2009 at 05:57:05PM -0400, Jeff Garzik wrote:
> Janne Grunau wrote:
> > On Fri, Apr 03, 2009 at 03:57:52PM -0400, Jeff Garzik wrote:
> >> mythtv/libs/libmythtv/ThreadedFileWriter.cpp is a good place to start
> >> (Sync method... uses fdatasync if available, fsync if not).
> >>
> >> mythtv is definitely a candidate for sync_file_range() style output, IMO.
>
> > yeah, I'm on it.
>
> Just curious, does MythTV need fsync(), or merely to tell the kernel to
> begin asynchronously writing data to storage?

quoting the TheadedFileWriter comments

/*
* NOTE: This doesn't even try flush our queue of data.
* This only ensures that data which has already been sent
* to the kernel for this file is written to disk. This
* means that if this backend is writing the data over a
* network filesystem like NFS, then the data will be visible
* to the NFS server after this is called. It is also useful
* in preventing the kernel from buffering up so many writes
* that they steal the CPU for a long time when the write
* to disk actually occurs.
*/

> sync_file_range(..., SYNC_FILE_RANGE_WRITE) might be enough, if you do
> not need to actually wait for completion.
>
> This may be the case, if the idea behind MythTV's fsync(2) is simply to
> prevent the kernel from building up a huge amount of dirty pages in the
> pagecache [which, in turn, produces bursty write-out behavior].

see above, we care only about the write-out. The f{data}*sync calls are
already in a seperate thread doing nothing else.

Janne