"hot swap" storage devices

[Erik Fair <fair%netbsd.org@localhost>]

On Jun 25, 2011, at 11:57 , Johnny Billquist wrote:

> True. However, Unix have never really gracefully handled file systems or 
> devices that comes and goes. :-)

As Robert Elz wrote (and I'll echo): UNIX has always handled removable devices 
- you just have to tell UNIX that you're going to remove it *before* you do 
(e.g. umount(8)). There is a reason why someone wrote fuser(8), long ago:

http://miroirs.cesars.org/man/pages/SunOS-4.1.3/man8/fuser.8.txt


With regard to "hot swap" storage devices, we really have two choices which are 
not mutually exclusive:

1. Treat as now, but with some additional code in the kernel which yells, "hey! 
put that back! I have data to write on it!" when a device goes away without 
prior notice (umount), and hold on to (rather than discard) the data in the I/O 
buffer cache, in the hope that the user notices and heeds the directive. 
Timeout to discard? Probably depends upon how much RAM utilization pressure 
you're under. I think "minutes" would be a good unit here.

2. Do all writes to "hot swap" devices synchronously - no write-behind cache at 
all, so that the device is most likely in a consistent state when it "goes 
away." Read cache would still be OK, but would have to be marked read-only upon 
device disappearance. Probably want to discard the read cache if the device is 
reinserted/reattached - you never know where the device has been the interim.

I point out that this problem has been with us quite forcefully since Sun came 
up with NFS (oh, almost three decades ago), since networks and remote servers 
"go away" rather more randomly and frequently than the given attached hard disk.

And how long has MacOS had an "eject" paradigm ... ?

        Erik <fair%netbsd.org@localhost>