safely remove USB hard drive

[Mikkel L. Ellertson]

Patrick O'Callaghan wrote:
> 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.)
> 
If I am reading this correctly, it is talking about the buffers on 
the drive itself, if you have write buffering enabled on the drive. 
To get around this, you would have to send a command to the drive 
telling it to write its buffers to disk, and have some way to know 
that it has done this. A work around would be to make sure that 
write caching is disabled on any removable hard drive.

> 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.
> 
For some file systems, you can use the sync option - but this does 
not help with NFS. Then again, any network file system is venerable 
it network problem...

As far as the man page goes, it may be in need or re-writing. You 
are correct that it does not say it say it writes the buffers, but 
it does not say it does not do this. So we are left guessing. It may 
be that that the person that wrote the man page didn't feel it 
needed to be covered. I have run into this before on other man pages.

Mikkel
-- 

   Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!

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