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Date:      Tue, 15 Oct 2013 07:47:55 -0400 (EDT)
From:      Daniel Feenberg <>
To:        Polytropon <>
Cc:        FreeBSD Questions <>
Subject:   Re: SU+J Lost files after a power failure
Message-ID:  <>
In-Reply-To: <>
References:  <> <l3gc7e$c91$> <> <> <> <> <> <> <> <> <> <>

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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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