safely remove USB hard drive

[Patrick O'Callaghan]

On Thu, 2008-04-24 at 10:28 -0500, Mikkel L. Ellertson wrote:
> Patrick O'Callaghan wrote:
> > 
> > You've stopped using the filesystem, you sync to make sure everything's
> > hunky dory, then you unmount. If you can't be sure that activity has
> > stopped (i.e. there are no open files or current directories on the fs)
> > you have to hope that unmount will fail and let you know, but there is a
> > potential race condition if you're not careful. The only way to avoid
> > the race would be if 'unmount' included a 'sync' within the kernel, but
> > the docs don't say that. I imagine the reason it's that way is that
> > unmount can take an indeterminate amount of time, especially for
> > remotely mounted filesystems. Just speculating of course.
> > 
> > poc
> > 
> By design, the default action of umount is to fail if there are any 
> open files on the filesystem. I am not sure what potential race 
> condition you are talking about - I would think that proper unmount 
> code would first make sure the file system in not in use, then block 
> opening of files on the file system, write any dirty buffers, and 
> then unmount the file system.

The potential race is when the last process closes the last file, then
unmount goes into action. Unmount will find the fs 'unused' because
there are no open files, but the buffer system may not have finished
writing blocks to the device. Also, when the 'sync' command returns, all
you can be sure of is that the buffer flushing action has been started
in the kernel, *not* that the data is physically on the drive.

>From the BUGS section of sync(2):

        According to the standard specification (e.g., POSIX.1-2001),
        sync() schedules the writes, but may return before the actual
        writing is done.  However, since version 1.3.20 Linux does
        actually wait. (This still does not guarantee data integrity:
        modern disks have large caches.)

Also, as I've said before, it may be "reasonable" to assume that unmount
flushes the buffers, but the docs don't actually say this. We all
imagine sync(2) and umount(2) in the context of disks or pendrives, but
they are also supposed to work for other kinds of filesystem, including
NFS and so on.

poc