Date   : Fri, 12 Nov 2010 23:56:54 +0100
From   : rick@... (Rick Murray)
Subject: RFC: Need two error numbers

On 12/11/2010 21:10, J.G.Harston wrote:

> **ALL** filing systems buffer data in quantities of less than a whole
> sector until a whole sector can be read/written. How else does *ANY*
> filing system manage to read or write a single byte at a time to/from
> an open file?

I didn't want to overly complicate things, but this was hinted at when I 
mentioned the change-a-byte would be:
   read sector
   alter the byte
   write sector

The main gist, that I probably could have worded better, was that filing 
systems buffer data in memory; for the 8 bitters it can be one or more 
sectors (mmm, there'd only be a finite number on a FileStore type 
machine..?) while more recent developments could buffer considerably 
more. This isn't even accounting for the fact that some harddiscs 
optimise with potentially considerable write buffering, plus as touched 
upon, altering a little bit of a Flash will require a lot more to be 
erased, which will be read into a RAM during the erase, and written back 
after. Indeed, unexpected power loss can have the potential to cause 
considerable damage.


> If you open a file on DFS, write some data and not flush the buffer,
> then press Ctrl-Break, the data is simply lost, the open channel
> is abandoned.

On the other hand, while it isn't always perfect, some filesystem code 
aimed at removable media devices will take the paranoid approach and 
effectively *Dismount after each operation, so if the device is simply 
yanked out of the USB port, data corruption should be minimal. This 
explains, also, why you have to jump through hoops to get an external 
drive to run as NTFS. It's because NTFS keeps a lot of crap "in use" 
thus is not really suited to media that could be unplugged at any time.


The DFS problem manifests because the Beeb has no concept of a graceful 
shutdown handler. Many years ago when I was playing with an early Linux 
distro (Slackware?), hitting Ctrl-Alt-Del would invoke a shutdown rather 
than just reboot. I tried, to see what would happen. Let's say fsck 
(like chkdsk) was in a panic afterwards with numerous inconsistencies. ;-)


Even so - it is foolish to simply shut down or remove media, whether a 
BBC micro or an OSD PVR, whether a Master Compact or Ubuntu "Cranky 
Cougar" (or whatever daft name the current version has).


Best wishes,

Rick.

-- 
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...