On Tue, 1 Dec 2009, Cindy Swearingen wrote:
The TLD terminology is confusing so let's think about this way:
Performance is best when only 3-9 physical devices are included in
each mirror or RAIDZ grouping as shown in the configuration guide.
It seems that these "TLDs" are what the rest of us call "vdev"s.
Performance improves with more "vdev"s since zfs load-shares across
them. The load-sharing helps defend against performance loss due to
poorly-performing devices.
Each write to a vdev produces one I/O operation to each of the devices
in the vdev. While adding more devices to raidz1/raidz2 vdevs
increases storage capacity, the increase is at a loss to the number of
IOPS which can be sustained. Variability in device performance allows
a poorly-performing device to dominate the performance of the whole
vdev, and particulary for raidz1/raidz2, which need to be written to
synchronously. The loss of IOPS, and the risk of performance loss due
to imperfectly-matched hardware, results in increased risk of
performance loss with too many devices in a raidz1/raidz2 vdev.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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