Re: Flames over -- Re: Which is simpler?

[Kyle Moffett <mrmacman_g4@mac.com>]

On Feb 14, 2006, at 01:27, Phillip Susi wrote:
> Kyle Moffett wrote:
> > No, that's _exactly_ what the spec says (well, not verbatim but  
> > close enough).  When you disconnect, both the master and slave  
> > devices are perfectly free to assume that the connection is  
> > completely broken and no state is maintained.  Anything that  
> > breaks that assumption is against the spec and likely to break in  
> > odd scenarios.
>
> Perfectly free to != required to.

In this case they _are_ equivalent due to symmetry.  If the other  
device _may_ assume that the connection is broken, then you _must_  
assume that the connection is broken.  Since either device _may_  
assume that, both devices therefore _must_ according to spec.

> > Which causes worse data-loss, writing out cached pages and  
> > filesystem metadata to a filesystem that has changed in the mean- 
> > time (possibly allocating those for metadata, etc) or forcibly  
> > unmounting it as though the user pulled the cable?  Most  
> > filesystems are designed to handle the latter (it's the same as a  
> > hard-shutdown), whereas _none_ are designed to handle the former.
>
> So you condemn the common correct use case to always suffer data  
> loss to give _slightly_ better protection to the uncommon and  
> incorrect use case?

No, as I said before, a good set of USB suspend scripts can solve  
this problem.  All of the ones I am aware of *now* already sync all  
data, which is good enough to prevent data-loss in _every_ case where  
the device is spontaneously unplugged.  On the other hand, this is  
_never_ good enough if the device is accidentally switched underneath  
us while suspended (and that's not so terribly uncommon, I know a lot  
of people who would do that accidentally, myself included).

> I think most users prefer a system that works right when you use it  
> right to one that doesn't break quite as badly when you do  
> something stupid.

I think you just proved my point.  Running the "sync" command a  
couple times then unplugging the USB stick basically never results in  
data loss even if the filesystem is mounted.  Spontaneously switching  
block devices under a mounted filesystem is guaranteed to either  
panic the machine or cause massive data corruption or both.

> Also why is this argument more valid for USB than SCSI?  I am just  
> as free to unplug a scsi disk and replace it with a different one  
> while hibernated, yet I don't suffer data loss when I don't do such  
> foolishness.

SCSI != USB.  Users generally don't expect to hotplug SCSI devices  
while booted and running (with the exception of some _really_  
expensive hotplug-bays where we expect the admin to know what the  
hell they're doing).  On the other hand, users _do_ expect to hotplug  
random USB devices whenever they feel like it.

Cheers,
Kyle Moffett

--
Q: Why do programmers confuse Halloween and Christmas?
A: Because OCT 31 == DEC 25.



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