Rob Logan wrote: > > which is better 8+2 or 8+1+spare? > > 8+2 is safer for the same speed > 8+2 requires alittle more math, so its slower in theory. (unlikely seen) > (4+1)*2 is 2x faster, and in theory is less likely to have wasted space > in transaction group (unlikely seen) > (4+1)*2 is cheaper to upgrade in place because of its fewer elements > > so, Mr (no scale on the time access) Elling: so what's the MTTDL > between theses three?
All things equal, an 8+2 raidz2 has an MTTDL about 4-5 orders of magnitude greater than 8+1+spare raidz1. Bigger MTTDL is better. The reason to recommend spares is to reduce the time when the system is vulnerable to a second failure. This time can be quite large, especially when you defer maintenance. Looking at this another way, in the 8+2 raidz2 case, the mirror is already sync'ed so you won't have the vulnerable resync time where you could lose data due to a second failure. Another reason to recommend spares is when you have multiple top-level vdevs and want to amortize the spare cost over multiple sets. For example, if you have 19 disks then 2x 8+1 raidz + spare amortizes the cost of the spare across two raidz sets. -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
