How to sync a dos file system externally

[Chris Johns]

On 03/10/2018 04:56, Cliff Geschke wrote:
> I have implemented a dos file system (msdos_*) on a flash device.

Is the flash device a chip you have direct access too? The reason I ask is if
possible using JFFS2 (a journaling file system) or even YAFFS (commercial
license maybe needed) is a better solution.

> Because it is possible for the user to power down the system unexpectedly, I
> want to sync the file system to the flash device after a 3 second idle time. 
> How do I externally force a sync on msdos from another thread?

The following test ...

 https://git.rtems.org/rtems/tree/testsuites/fstests/fsdosfssync01/init.c

shows how to purge a disk at the block layer. Wrap something like this is the
way to purge the cache.

> A related problem is that I use FTP (ftpd.c) to externally read/write files on
> the msdos formatted flash.  I have the idle timeout set to 3 minutes for the FTP
> connection.  After 3 minutes, ftpd issues a chdir("/") which eventually calls
> msdos_free_node_info() that calls fat_file_close() and may try to write out data
> to the flash.  This is a problem because if the power is turned off at that time
> the flash is corrupted.
> 
> So whatever method I use to sync msdos needs to update and write out the fat so
> that the subsequent msdos_free_node_info() does nothing.

I should point out there is a finite chance the disk can still become corrupted.
How small the window becomes depends on the system design, for example how often
the disk is updated, power supply power down storage vs media write time,
non-bricking read-only partitions, etc.

I suspect triggering a timer to purge the cache as shown above is no different
to lowering the `swapout` task's period. It has been a while since I looked over
this code. The libblock cache implements separate threads to "sync" the cache to
the media on a periodic basis. The cache's configuration lets you set the
period. The values are documented here:

https://docs.rtems.org/doxygen/branches/master/group__rtems__bdbuf.html#define-members

This however raises a difficult question which is documented in the code in a
comment:

 https://git.rtems.org/rtems/tree/cpukit/libblock/src/bdbuf.c#n1013

It is difficult when using a timer to know if the purge is 100% safe. If you can
arrange the purge call to happen after you know the update has finished it
lowers the chance the disk maybe become corrupted.

Chris