ZFS on RAID 5 - WTF?

[time]

Met a guy planning to buy a $500 LSI controller so he could get decent write performance out of ZFS running on RAID 5 under Free BSD. Looks like he has space for 6 drives but currently only 3 or 4.

WTF?

I thought ZFS already enabled error correction on a random collection of disks, but I admit I wasn't paying attention. What's the reality here?

 

[Mercutio]

He's going to run softraid on top of hardware raid. His write performance will be every kind of absymal and he clearly doesn't belong anywhere near a $500 RAID controller.

 

[Chewy509]

As Merc said, he's in for a world of trouble...

I can find the reference for you, but there is a white paper (from back in Sun days) that clearly details setting up ZFS for performance applications, and one of the key points was that ZFS must be able to interact with the disks directly to ensure all the error correction and load distribution occurs correctly. Running ZFS on top of traditional RAID is a no-no. Even Sun's massive 48/2 drive Thumper storage array, has no RAID (either hard or soft) capability, as ZFS does it all. (** 48x 15K SAS + 2x SSD for L2-ARC)

 

[Mercutio]

I once ran in to a server that a "techie" DBA had set up that RAID10 implemented on the hardware controller and software RAID5 for the main storage volume. The guy could NOT understand why his writes were topping out at 2MB/sec with 15k SCSI drives.