Re: UBIFS robustness questions

[Adrian Hunter <adrian.hunter@nokia.com>]

Hunter Adrian (Nokia-D/Helsinki) wrote:
> Charles Manning wrote:
>> This is probably documented somewhere but I could not find it...
>>
>> What operations in UBIFS are robust to power failure and which are not?
> 
> Only sync operations guarantee that changes have reached the flash.
> There are all the usual ways to sync:
> 	fsync/fdatasync a file/directory
> 	open a file as synchronous
> 	mark a file with the sync flag
> 	sync the filesystem
> 	mount the file system as synchronous
> 
>> I know for example that writing a file into flash does not mean it has been 
>> completely written to flash until after a sync, but what about other 
>> operations such as mv?
> 
> After mv, the containing directory must be sync'd to be sure the change reaches the
> flash.  But rename is atomic so there will always be either the old
> naming or the new naming
> 
>> The reasonn I'm asking this is that I want to be able to "hot-swap" a 
>> directory of files without losing any file state.
> 
> Should be no problem if you sync correctly.
> 
>> What I'm considerings doing is something like:
>>
>> Start with ~/runtime having a sane set of files
>>
>> untar etc into ~/updated
>> sync
>> mv ~/updated ~/run-time
>> sync
>>
>> What is unacceptable is that, at any time, a power failure/reboot results in 
>> ~/runtime having a non-sane set of files.
>>
>> * Does the above sequence look safe?
> 
> Yes

Well, safe but not possible. You cannot rename over the top
of a non-empty directory. Sorry I was misleading.

>> * Is the second sync required?
> 
> It is required to guarantee that the mv has reached the flash at that
> point in time i.e. power loss before the second sync => same as if mv
> was not done