On Mon, Mar 1, 2010 at 12:58 PM, Eduardo Bragatto <edua...@bragatto.com>wrote:
> Hi everyone, > > I just joined the list after finding an unanswered message from Ray Van > Dolson in the archives. > > I'm reproducing his question here, as I'm wondering about the same issue > and did not find an answer for it anywhere yet. > > Can anyone shed any light on this subject? > > -- Original Message -- > > What are the technical reasons to not have mismatched replication > levels? > > For example, I am creating a zpool with three raidz vdevs. Two with 8 > disks and one with only 7. zpool allows me to do this with -f of > course, but I can't find much documentation on why I shouldn't other > than it's not recommended. > > I can understand why, perhaps, for situations where you add new vdevs > to your pool later and accidentally use some that aren't redundant to > the same degree others are -- you might unknowingly compromise your > vpool in that way... > > But as long as we're aware, is there any performance other other > technical reason I shouldn't set up my vdevs as I have above? > > Thanks, > Ray > > -- End of Original Message -- > > According to the documentation, here: > > http://docs.sun.com/app/docs/doc/819-5461/gavwn?a=view > > "(..)The command also warns you about creating a mirrored or RAID-Z pool > using devices of different sizes. While this configuration is allowed, > mismatched levels of redundancy result in unused space on the larger > device(..)" > > However, when I create a pool with two raidz groups with 7 vdevs each, and > two raidz groups with 7 and 8 vdevs each, I do get more space, indicating > the extra space in the largest raidz set is available (which I wouldn't > expect to happen based on the statement above): > > 2 x raidz ( 7 + 8 ) using 1TB disks: > > backup2.nbg:~ root# zfs list benchpool78 > NAME USED AVAIL REFER MOUNTPOINT > benchpool 155K 11.5T 31.0K /benchpool78 > backup2.nbg:~ root# zpool list benchpool78 > NAME SIZE USED AVAIL CAP HEALTH ALTROOT > benchpool 13.6T 354K 13.6T 0% ONLINE - > > 2 x raidz ( 7 + 7 ) using 1TB disks: > > backup2.nbg:~ root# zfs list benchpool77 > NAME USED AVAIL REFER MOUNTPOINT > benchpool 117K 10.6T 1.70K /benchpool77 > backup2.nbg:~ root# zpool list benchpool > NAME SIZE USED AVAIL CAP HEALTH ALTROOT > benchpool 12.6T 146K 12.6T 0% ONLINE - > > So, is there any real reason for not using mismatched replication levels? > Is there any performance penalty? > > Thanks, > Eduardo > The primary concern as I understand it is performance. If they're close in size, it shouldn't be a big deal, but when you've got mismatched rg's it can cause quite the performance troubleshooting nightmare. It's the same reason you don't want to make a pool that has one raid group that's entirely 15kRPM SAS drives and the other 5400RPM SATA. --Tim
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