On 5/5/2012 8:04 AM, Bob Friesenhahn wrote:
On Fri, 4 May 2012, Erik Trimble wrote:
predictable, and the backing store is still only giving 1 disk's IOPS. The RAIDZ* may, however, give you significantly more throughput (in MB/s) than a single disk if you do a lot of sequential read or write.

Has someone done real-world measurements which indicate that raidz* actually provides better sequential read or write than simple mirroring with the same number of disks? While it seems that there should be an advantage, I don't recall seeing posted evidence of such. If there was a measurable advantage, it would be under conditions which are unlikely in the real world.

The only thing totally clear to me is that raidz* provides better storage efficiency than mirroring and that raidz1 is dangerous with large disks.

Provided that the media reliability is sufficiently high, there are still many performance and operational advantages obtained from simple mirroring (duplex mirroring) with zfs.


I'll see what I can do about actual measurements. Given that we're really recommending a minimum of RAIDZ2 nowdays (with disks > 1TB), that means, for N disks, you get N-2 data disks in a RAIDZ2, and N/2 disks in a standard striped mirror. My brain says that even with the overhead of parity calculation, for doing sequential read/write of at least the slab size (i.e. involving all the data drives in a RAIDZ2), performance for the RAIDZ2 should be better for N >= 6. But, that's my theoretical brain, and we should do some decent benchmarking, to put some hard fact to that.

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