On Wed, Apr 17, 2013 at 7:38 AM, Edward Ned Harvey (openindiana) < [email protected]> wrote:
> You also said the raidz2 will offer more protection against failure, > because you can survive any two disk failures (but no more.) I would argue > this is incorrect (I've done the probability analysis before). Mostly > because the resilver time in the mirror configuration is 8x to 16x faster > (there's 1/8 as much data to resilver, and IOPS is limited by a single > disk, not the "worst" of several disks, which introduces another factor up > to 2x, increasing the 8x as high as 16x), so the smaller resilver window > means lower probability of "concurrent" failures on the critical vdev. > We're talking about 12 hours versus 1 week, actual result of my machines > in production. > Did you also compare the probability of bit errors causing data loss without a complete pool failure? 2-way mirrors, when one device completely dies, have no redundancy on that data, and the copy that remains must be perfect or some data will be lost. On the other hand, raid-z2 will still have available redundancy, allowing every single block to have a bad read on any single component disk, without losing data. I haven't done the math on this, but I seem to recall some papers claiming that this is the more likely route to lost data on modern disks, by comparing bit error rate and capacity. Of course, a second outright failure puts raid-z2 in a much worse boat than 2-way mirrors, which is a reason for raid-z3, but this may already be a less likely case. Also, as for time to resilver, I'm guessing that depends largely on where bottlenecks are (it has to read effectively all of the remaining disks in the vdev either way, but can do so in parallel, so ideally it could be the same speed), and how busy the pool is with other things - how does the bandwidth sharing during resilver work? Tim _______________________________________________ OpenIndiana-discuss mailing list [email protected] http://openindiana.org/mailman/listinfo/openindiana-discuss
