Bob - thanks, that makes sense. The classroom book refers to "top-level virtual devices," and were referred to as TLDs throughout the class (Top-Level Devices). As you noted, those are either the base LUN, mirror, raidz, or raidz2.
So there's no limit to the number of TLDs/vdevs we can have, the only recommendation is that we have no more than 3-9 LUNs per RAIDZ vdev? Chris On Tue, Dec 1, 2009 at 12:25 PM, Bob Friesenhahn < bfrie...@simple.dallas.tx.us> wrote: > 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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