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. ;-) 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%). 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. -B -- Brandon High : bh...@freaks.com
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