SU+J Lost files after a power failure

Daniel Feenberg feenberg at
Tue Oct 15 11:48:01 UTC 2013

On Tue, 15 Oct 2013, Polytropon wrote:

> On Mon, 14 Oct 2013 14:04:45 -0700, Charles Swiger wrote:
> Unmounting a disk will usually make sure all remaining
> buffers have been written. Given the previously described
> stack of involved layers, it might still be good to be a
> little bit paranoid regarding this assumption. :-)

We have a disk-to-disk backup that runs continuously. I have found that if 
I unmount the backup disk and remove it without delay, then sometimes it 
requires an fsck to remount. So I believe that umount does not wait for 
all writes to complete. I understand that buffers are written out to disk 
at least every 30 seconds, but it may take some time for the buffers to be 
flushed and the disk itself has a buffer and I have never seen any 
discussion of the maximum delay there. So the minimum wait for a safe 
removal remains unknown.

Perhaps some trick like unmounting, then remounting with fsync, then 
unmounting would ensure all buffers were at least sent to the disk, but
I have never seen that suggested.

Daniel Feenberg

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