Bit of a loaded question and It depends on your systems. As I understand it. A HDD can do 32kB packet in about 8ms - 125 iops or 4MB/s. But it can do 1MB packet in about the same time say 20ms - 50 iops or 50MB/s. With a mirror set you can increase the seq write rate to the pool on both counts. But you'd need a 10 vdev mirror to get 40MB/s for 32kB packets.
This is the value of hybrid storage. At 1600 iops for 32kB slog device and a 1 vdev HDD storage pool is the same as a 10 vdev storage pool for performance. So, 1 SSD + 2 SATA "is the same as" 20 SATA for performance. Given the SSD device is only three times the cost of SATA disk and the form factor of the whole set is smaller it is an overall win. This example is pretty simplistic, but you get the idea. Nicholas On Wed, May 20, 2009 at 4:36 PM, Ellis, Mike <[email protected]> wrote: > and how quickly things can be destaged onto the main pool.... > > hard to model since its so workload/hardware dependent. > >
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