zfs discuss - Apr 2008 - Copies Option: Anyone actually seen the end result?

I''ve been trying to figure out how the copies command works and have
been experimenting, but I haven''t really seen any results (both with 5
physical drives I will soon add to my data pool as a 2nd RAIDZ and on a virtual
machine with two RAIDZ in a pool). First: Is data copied across physical devices
or virtual devices in a pool (thus could it work with a single RAIDZ, or would I
need multiple virtual devices in the pool)? I''ve been trying to add
some redundancy to my personal files in a RAID-Z array so that if two drives go
down my important data will still be safe, but experimentation has led to me
seeing no redundancy for filesystems set with copies=3 before copying dat to
them. I have tried downing two out of 5 drives in a single RAIDZ pool and
downing 2/3 drives in one RAIDZ in a dual-RAIDZ pool.
 
 
This message posted from opensolaris.org

Austin wrote:
> I''ve been trying to figure out how the copies command works and
have been experimenting, but I haven''t really seen any results (both
with 5 physical drives I will soon add to my data pool as a 2nd RAIDZ and on a
virtual machine with two RAIDZ in a pool). First: Is data copied across physical
devices or virtual devices in a pool (thus could it work with a single RAIDZ, or
would I need multiple virtual devices in the pool)? I''ve been trying to
add some redundancy to my personal files in a RAID-Z array so that if two drives
go down my important data will still be safe, but experimentation has led to me
seeing no redundancy for filesystems set with copies=3 before copying dat to
them. I have tried downing two out of 5 drives in a single RAIDZ pool and
downing 2/3 drives in one RAIDZ in a dual-RAIDZ pool.
>   
I made some pretty pictures describing copies at:
    http://blogs.sun.com/relling/entry/zfs_copies_and_data_protection

The most common failure mode is unrecoverable read, not complete
disk failure.  Copies is a good technique to help protect you from
unrecoverable reads.
 -- richard