raidz2 a bit big

[Adam McDougall]

On Sat, May 23, 2009 at 06:04:04AM -0700, Randy Bush wrote:

  >>    NAME    STATE   READ WRITE CKSUM
  >>    tank    ONLINE    0   0   0
  >>     raidz2  ONLINE    0   0   0
  >>      da0s3  ONLINE    0   0   0
  >>      da1s3  ONLINE    0   0   0
  >>      da2s1  ONLINE    0   0   0
  >>      da3s1  ONLINE    0   0   0
  >>      da4s1  ONLINE    0   0   0
  >>      da5s1  ONLINE    0   0   0
  >>      da6s1  ONLINE    0   0   0
  >>      da7s1  ONLINE    0   0   0
  >>      da8s1  ONLINE    0   0   0
  >>      da9s1  ONLINE    0   0   0
  >>      da10s1 ONLINE    0   0   0
  >>      da11s1 ONLINE    0   0   0
  > 
  > Reads on such configurations are very slow.
  
  how are writes?
  
  > If one of your disk, for example, is capable of 100 IO per/sec, then:
  > with 12 disks in one raidz2 vdev you get only 100 IOPS
  > with 4 disks in raidz2 (total 3 raidz2 vdevs) you get 300 IOPS
  > with 2 disks in mirror (total 6 mirror vdevs) you can get 1200 IOPS
  
  ok.  sounds nice.  but then, don't i have six file systems and have to
  start playing lay-out design games?
  
  randy

For an example: (btw the read speed is fantastic in a mirror and the write
is notably faster than raidz, but if your I/O is all going to go through a
gig nic, then it may not matter such as if you are just using it for a
low concurrent user stash of large files)

zpool create tank mirror aacd0s1d aacd1s1d mirror aacd2s1d aacd3s1d mirror aacd4s1d aacd5s1d mirror 
aacd6s1d aacd7s1d

# zpool status
  pool: tank
 state: ONLINE
 scrub: none requested
config: 

        NAME          STATE     READ WRITE CKSUM
        tank          ONLINE       0     0     0
          mirror      ONLINE       0     0     0
            aacd0s1d  ONLINE       0     0     0
            aacd1s1d  ONLINE       0     0     0
          mirror      ONLINE       0     0     0
            aacd2s1d  ONLINE       0     0     0
            aacd3s1d  ONLINE       0     0     0
          mirror      ONLINE       0     0     0
            aacd4s1d  ONLINE       0     0     0
            aacd5s1d  ONLINE       0     0     0
          mirror      ONLINE       0     0     0
            aacd6s1d  ONLINE       0     0     0
            aacd7s1d  ONLINE       0     0     0