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Date:      Tue, 15 Oct 2013 07:47:55 -0400 (EDT)
From:      Daniel Feenberg <feenberg@nber.org>
To:        Polytropon <freebsd@edvax.de>
Cc:        FreeBSD Questions <freebsd-questions@freebsd.org>
Subject:   Re: SU+J Lost files after a power failure
Message-ID:  <alpine.LRH.2.03.1310150742370.29524@nber.org>
In-Reply-To: <20131015073048.83d7bca4.freebsd@edvax.de>
References:  <525A6831.5070402@gmail.com> <l3gc7e$c91$1@ger.gmane.org> <20131014133953.58f74659@gumby.homeunix.com> <525C1D1C.9050708@gmail.com> <CA+tpaK2Pr2po2cQ1yanQK9+wLp77SYqYHjxiXaU5FfXwHrkGow@mail.gmail.com> <CAFYkXjn-1wTJcQ4a_fyXCvwh9ukt3+djM2qsMeaH1HhVJNvhiA@mail.gmail.com> <525C2554.7080203@pchotshots.com> <CAFYkXjm8y0Br31_pqRZc0sNFbqCNtKHhjeQuiLXkGT2zxSu0GA@mail.gmail.com> <525C2FBC.4080808@cran.org.uk> <alpine.LRH.2.03.1310141447520.16969@nber.org> <2351E8C5-4FC0-4AE9-AC21-312DA46C0EE8@mac.com> <20131015073048.83d7bca4.freebsd@edvax.de>

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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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