On Nov 11, 2009, at 4:30 PM, Rob Logan wrote:
from a two disk (10krpm) mirror layout to a three disk raidz-1.
wrights will be unnoticeably slower for raidz1 because of parity
calculation
and latency of a third spindle. but reads will be 1/2 the speed
of the mirror because it can split the reads between two disks.
... where "speed" is "latency." For bandwidth, a 3-device RAIDZ
should be approximately the same as a 2-way mirror. For larger
RAIDZ sets, bandwidth/space can scale.
another way to say the same thing:
a raidz will be the speed of the slowest disk in the array, while a
mirror will be x(Number of mirrors) time faster for reads or
the the speed of the slowest disk for wrights.
The model I use for a pool with no cache or log devices, is that
the number of small, random reads (IOPS) is approximately
RAIDZ IOPS = IOPS of one device * N/(N-1)
where N is the number of disks in in the RAIDZ set.
This model completely falls apart for workloads other than
small, random reads or if a cache or log device exists. It also
explains why using a SSD for a cache device for workloads
which make small, random reads can be a huge win :-)
-- richard
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