Bob Friesenhahn wrote:
On Sat, 28 Mar 2009, Michael Shadle wrote:
Well this is for a home storage array for my dvds and such. If I have
to turn it off to swap a failed disk it's fine. It does not need to
be highly available and I do not need extreme performance like a
database for example. 45mb/sec would even be acceptable.
I can see that 14 disks costs a lot for a home storage array but to
you the data on your home storage array may be just as important as
data on some businesses enterprise storage array. In fact, it may be
even more critical since it seems unlikely that you will have an
effective backup system in place like large businesses do.
The main problem with raidz1 is that if a disk fails and you replace
it, that if a second disk substantially fails during resilvering
(which needs to successfully read all data on remaining disks) then
your ZFS pool (or at least part of the files) may be toast. The more
data which must be read during resilvering, the higher the probability
that there will be a failure. If 12TB of data needs to be read to
resilver a 1TB disk, then that is a lot of successful reading which
needs to go on.
This is a very good point for anyone following this and wondering why
RAIDZ2 is a good idea. I have seen over the years several large RAID 5
hardware arrays go belly
up as a 2nd drive fails during a rebuild with the end result of the
entire RAID set being rendered useless. If you can afford it then you
should use it. RAID6 or RAIDZ2 was
made for big SATA drives. If you do use it though, one should make sure
that you have reasonable CPU as it does require a bit more grunt to run
over RAIDZ.
The bigger the disks and the bigger the stripe the more likely you are
to encounter a issue during a rebuild of a failed drive. plain and simple.
In order to lessen risk, you can schedule a periodic zfs scrub via a
cron job so that there is less probabily of encountering data which
can not be read. This will not save you from entirely failed disk
drives though.
As far as Tim's post that NOBODY recommends using better than RAID5, I
hardly consider companies like IBM and NetApp to be "NOBODY". Only
Sun RAID hardware seems to lack RAID6, but Sun offers ZFS's raidz2 so
it does not matter.
Plenty of Sun hardware comes with RAID6 support out of the box these
days Bob. Certainly all of the 4140, 4150, 4240 and 4250 2 socket x86
/x64 systems have
hardware controllers for this. Also all of the 6140's, 6540 and 6780's
disk arrays do also have RAID 6 if they have Crystal firmware and of
course the Open Storage 7000 series
machines do as well being that they are Opensolaris and ZFS based.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us,
http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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_________________________________________________________________________
Scott Lawson
Systems Architect
Information Communication Technology Services
Manukau Institute of Technology
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