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%2BtpaK2Pr2po2cQ1yanQK9%2BwLp77SYqYHjxiXaU5FfXwHrkGow@mail.gmail.com> <CAFYkXjn-1wTJcQ4a_fyXCvwh9ukt3%2BdjM2qsMeaH1HhVJNvhiA@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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