On Mar 20, 2010, at 10:12 PM, Brandon High wrote: > On Sat, Mar 20, 2010 at 1:35 PM, Richard Elling <richard.ell...@gmail.com> > wrote: > For those disinclined to click, data retention when mirroring wins over raidz > when looking at the problem from the perspective of number of drives > available. Why? Because 5+1 raidz survives the loss of any disk, but 3 sets > of 2-way mirrors can survive the loss of 3 disks, as long as 2 of those disks > are not in the same set. The rest is just math. > > The one dimension left out in your comparison is the portion of space that's > available for use vs. redundancy overhead. I'm sure you just never thought of > it. ;-)
There are two dimensions missing: space and performance. > For 12 disks using a 4-way mirror, you'd have 75% overhead but the best > MTTDL. raidz3 is only 25% overhead, but provides a better MTTDL than 3-way > mirrors (at 66% overhead). raidz2 (16% overhead) has better MTTDL than 2-way > mirrors (at 50%). The "all-in" post puts all three on one chart, but in this case it is for 46 disks, not 12. http://blogs.sun.com/relling/entry/zfs_raid_recommendations_space_performance1 > So clearly, if fault tolerance is the absolute most important factor, a > really big mirror is best. This will also give very good read performance. I > imagine a 12-way mirror would last a while (2.09E+57 years according to > Richard's formula) but it's also at high cost. > > I think the only real route to follow is to determine how much space you > need, and then optimize MTTDL and performance around that constraint. If you > determine that you need 10 TB available, then (using 1.5T drives) you need to > use at least 7 disks for data. That means a 12-disk raidz3 (13.5 TB), or 2x > 6-disk raidz2 (12 TB). The raidz3 will have higher fault tolerance, but lower > performance. Indeed. Space, performance, dependability: pick two -- richard ZFS storage and performance consulting at http://www.RichardElling.com ZFS training on deduplication, NexentaStor, and NAS performance Las Vegas, April 29-30, 2010 http://nexenta-vegas.eventbrite.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
