Date: Tue, 15 Oct 2013 07:30:48 +0200 From: Polytropon <freebsd@edvax.de> To: Charles Swiger <cswiger@mac.com> Cc: FreeBSD Questions <freebsd-questions@freebsd.org> Subject: Re: SU+J Lost files after a power failure Message-ID: <20131015073048.83d7bca4.freebsd@edvax.de> In-Reply-To: <2351E8C5-4FC0-4AE9-AC21-312DA46C0EE8@mac.com> 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>
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On Mon, 14 Oct 2013 14:04:45 -0700, Charles Swiger wrote: > First, unless you call fsync() before close() and your OS > and/or drive hardware isn't being deceptive when fsync() > returns about whether the bits have made it to permanent > storage, then you might be surprised at just how long the > unwritten buffers containing the last updates to the file > data take to get properly flushed to disk. The problem here is that it's hard to _predict_ the time needed to actually finish the desired writing operation. The library function will interact with the kernel, the kernel with the file system driver, which in turn will address the storage subsystem, which will address the hard disk driver to tell the disk what to do. Then the disk will also take some time to perform that operation. I assume unless you have a system in place that explcitely expects a "receipt" from the disk up through all those layers that the data has been stored, you can't fully be sure. So even if you call fsync() and immediately after this a power outage occurs, the disk might not have been noticed... 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. :-) Your characterization of the inner workings of a SSD (and how efficient its firmware is implemented) stengthens this belief. -- Polytropon Magdeburg, Germany Happy FreeBSD user since 4.0 Andra moi ennepe, Mousa, ...
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