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
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