On the "safe filesystem" and write() topic

[Vipin Malik]

At 11:25 AM 7/7/2001 +0200, Bjorn Wesen wrote:

> > HeHe, well, maybe the fs can (will or may?) block, but in all realistic
> > situations it's unacceptable for a real world embedded app to block for
> > multiple seconds while the fs is "busy". Where does the app store any data
>
>You might not like it but you cannot have it any other way :)
>
>Fact: flash chip sectors takes long to erase (1-2 seconds)
>
>Fact: you need to erase to make room for new data
>
>Hence, if you need the app to do synchronous writing, it will need to
>wait.
>
>
> > time a task updating variables on the FS will block. The question is: How
> > long a block is acceptable? IMHO, anything more than a few hundred ms will
> > be unacceptable to a reasonable percentage of embedded applications. I 
> know
>
>Then you can't use flash chips in your embedded application :)
>
> > > > caching layer that will allow the transaction log to be put on 
> *another*
> > > > non-volatile medium if such is available in your system. The big 
> advantage
> > >
> > >Why would this be necessary ?
> >
> > To provide for 0 latency writes for tasks updating data values, when the
> > underlying fs is blocked and cannot accept any more writes for another
> > "few" (at the moment >40) seconds.
>
>So what happens when that gets full and need to be erased ? All you'd do
>is interleave the writes and postpone the problem a bit. If you mean that
>the transactional log will "never" get full and require erasing, then yes,
>that would work but I doubt the "never" constraint :)

That's why if 0 latency writes are important to a design, they must put 
this cache on a 0-erase-latency non-volatile medium like a battery backed 
RAM or FRAM.
Then a simple equation will help one size it for one's particular need, namely:

C_KB = size_of_nonvolatile_cache_device_required;
t1 = max_block_time_of_flash_FS_sec;
t2 = time_to_xfer_C_KB_to_flash_FS_sec;
NEW_KB_PER_SEC = max_new_data_generating_rate_KB_per_sec;


C_KB = (t1+t2) * NEW_KB_PER_SEC;

That's what the 0 latency write, transaction protected, embedded database 
project on the dev list is for.

Vipin