On Tue, 2 Dec 2008, Carsten Aulbert wrote: > > Questions: > (a) Why the first vdev does not get an equal share of the load
You may have one or more "slow" disk drives which slow down the whole vdev due to long wait times. If you can identify those slow disk drives and replace them, then overall performance is likely to improve. The problem is that under severe load, the vdev with the highest backlog will be used the least. One or more slow disks in the vdev will slow down the whole vdev. It takes only one slow disk to slow down the whole vdev. > (b) Why is a large raidz2 so bad? When I use a standard Linux box with > hardware raid6 over 16 disks I usually get more bandwidth and at least > about the same small file performance ZFS commits the writes to all involved disks in a raidz2 before proceeding with the next write. With so many disks, you are asking for quite a lot of fortuitous luck in that everything must be working optimally. Compounding the problem is that I understand that when the stripe width exceeds the number of segmented blocks from the data to be written (ZFS is only willing to dice to a certain minimum size), then only a subset of the disks will be used, wasting potiential I/O bandwidth. Your stripes are too wide. > (c) Would the use of several smaller vdev would help much? And which > layout would be a good compromise for getting space as well as > performance and reliability? 46 disks have so few prime factors Yes, more vdevs should definitely help quite a lot for dealing with real-world muti-user loads. One raidz/raidz2 vdev provides (at most) the IOPs of a single disk. There is a point of diminishing returns and your layout has gone far beyond this limit. Bob ====================================== Bob Friesenhahn [EMAIL PROTECTED], http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
