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

From: Kyle Moffett
Date: Tue Feb 14 2006 - 11:23:02 EST

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.

Kyle Moffett

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

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