Date: Sat, 2 Apr 2011 12:55:15 -0400 From: David Magda <dmagda@ee.ryerson.ca> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: freebsd-stable@freebsd.org Subject: Re: Constant rebooting after power loss Message-ID: <1D1A4498-0CE0-4CE7-8DD3-6066B85C82AF@ee.ryerson.ca> In-Reply-To: <201104020335.p323Zp8Q018666@apollo.backplane.com> References: <87d3l6p5xv.fsf@cosmos.claresco.hr> <AANLkTi=kEyz-mKLzdV8LAf91ZhMTP8gLKs=3Eu5WD8mh@mail.gmail.com> <874o6ip0ak.fsf@cosmos.claresco.hr> <7b15d37d28f8ddac9eb81e4390231c96.HRCIM@webmail.1command.com> <AANLkTi=KEwmm1hM6Z=r_SWUAn9KhUrkTVzfF6VmqQauW@mail.gmail.com> <14c23d4bf5b47a7790cff65e70c66151.HRCIM@webmail.1command.com> <AANLkTi=6pqRwJ96Lg=603cYg_f8QUXkg8aXtbjbYpFrV@mail.gmail.com> <201104020335.p323Zp8Q018666@apollo.backplane.com>
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On Apr 1, 2011, at 23:35, Matthew Dillon wrote: > The solution to this first item is for the OS/filesystem to issue a > disk flush command to the drive at appropriate times. If I recall = the > ZFS implementation in FreeBSD *DOES* do this for transaction = groups, > which guarantees that a prior transaction group is fully synced = before > a new ones starts running (HAMMER in DragonFly also does this). > (Just getting an 'ack' from the write transaction over the SATA bus = only > means the data made it to the drive's cache, not that it made it to > the platter). It should also be noted that some drives ignore or lie about these flush = commands: i.e., they say they flushed the buffers but did not in fact do = so. This is sometimes done on cheap SATA drives, but also on expensive = SANS. If the former's case it's often to help with benchmark numbers. In = the latter's case, it's usually okay because the buffers are actually = NVRAM, and so are safe across power cycles. There are also some = USB-to-SATA chipsets that don't handle flush commands and simply ACK = them without passing them to the drive, so yanking a drive can cause = problems. There has been quite a bit of discussion on the zfs-discuss list on this = topic of the years, especially when it comes to (consumer) SSDs.
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