I just set up a freenas zfs raid-z2 with 4 drives sata enterprise drives and doing some performance tests. Right now I'm pushing and pulling linux images into the storage. My notebook has a samsung 840pro ssd with 400MB/s local read write speed. Samba4 is used.

I can write with avg 105 MB/s in an continuous stream. I'm impressed, this is is really fine thinking of a 1Gb/s lan.

However reading is pretty slow and network io is jumping from a few kB to 30MB/s probably in avg. about 10MB/s. Adding a l2arc doesn't help.

Any ideas, why the reading performance is so poor? is this normal?

2 Answers 2


You may want to read this.

Essentially, in a single RAID Z group, read performance is equal to the performance of a single disk. RAID Z is great for write performance and poor for read performance. Given the slow low-end disks you're using, the numbers you've posted seem reasonable.

If you want to use RAID Z and still have reasonable read performance, you'll have to create multiple RAID Z groups (which you don't have disks for) and stripe across them.

With four disks, you may be best off creating two mirrors and striping data across them.

  • My sata drives seem to be not to bad. just run xdd on the server. I can write up to 2000MB/s random access. Reading single files I end up with 600MB/s. Multiple reads up to 1200MB/s My problem was my local virus scanner, checking incoming traffic. Deactivating it I ended up with 80MB/s incoming over samba. Reading your link raidz2 should rise performance, however adding more devices wouldn't increase performance much. Keeping in mind that most files are >1MB and my lan is limited to 2Gb/s mirroring wouldn't increase performance a lot. If I had small junks I'd agree to mirroring.
    – Manuel
    Mar 24, 2014 at 11:49
  • Do you have any experiences with l2arc?
    – Manuel
    Mar 24, 2014 at 11:49

Even thought this post is rather old, I came accross it while I was looking for the solution to the exact same problem. So maybe others can benefit from my experience;
I have a FreeNas setup where I can push up to a 110MB/s to it (write), but reading from it was twice as slow (50MB/s). Couldn't figure out why. Read a few articles where an expert user says that in general FreeNas shouldn't need tuneups or special ZIL's and L2ARC's and stuff and still should be able to hit the limits of one's gigabit network in both directions. That same dude was telling about Realtec NICs not always performing fine. I did have a Realtec NIC in my machine, and thought if it can pull data in with over 100MB/s it should also be able to push data out with 100MB/s. Just because I had a spare Intel server based NIC (PRO/1000) lying around I swapped it with the Realtec NIC and did the exact same test. My FreeNas now has syncronious read and write speeds. The both hit 110MB/s. Simple as that!

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