Re: usb flash drive removal (Re: Desktop NetBSD needs your help)

[Arnaud Lacombe <lacombar%gmail.com@localhost>]

On Mon, Feb 9, 2009 at 11:33 AM, Eric Haszlakiewicz 
<erh%nimenees.com@localhost> wrote:
> On Mon, Feb 09, 2009 at 10:51:32AM -0500, Arnaud Lacombe wrote:
>> Hi,
>>
>> On Mon, Feb 9, 2009 at 10:33 AM, Eric Haszlakiewicz 
>> <erh%nimenees.com@localhost> wrote:
>> > I think that there should not _be_ a wrong time to remove a USB drive.
>> > Yes, it may be hard to implement it in NetBSD, but it is entirely possible
>> > to have a system that doesn't crash, and even does something reasonable.
>> > (e.g. like providing an api for prompting the user to reattach the device)
>> >
>> You cannot ensure that the device is the same state than previously.
>> You cannot even ensure that the newly plugged device is the same that
>> the old one. The only sane thing to do is to clean-up everything.
>
> No, you're wrong.  You CAN ensure, with a reasonably high degree of certainty,
> that the device is the same as the old one.
Sorry to say that, but if it's not 100% of certainty you won't go far.

> Do all of the following:
> 1) A device is unplugged
>  a) If there is no unwritten data cached, gracefully unmount the device and
>  revoke any open file descriptors when the device is unplugged.
>  b) If there _is_ data cached, keep the mount around in a suspended state,
>     saving information about the device and in-flight data until later.
>     b.i) Issue an event that alerts the user that he should plug the device
>            back in.
> 2) When a device is reattached
>  a) If the device has a dependable hardware id of some kind, compare that
>        with what the previous id was.
in the mean time, I took a picture with my camera/got a phone call,
the file-system on the device changed, you replug-it, things are not
as they used to be... and boom! There is billion reasons for a device
to alter its file-system while it is supposedly doing nothing. In the
best case, we crash, in the worst case, we corrupt the remote
file-system. Not speaking that you'll need to keep track of every
possible unique ID that can exist, good luck.

 - Arnaud