RAID 0+1 –Possible With ZFS?

[Jayden67]

Through searching this forum I have discovered that creating striped mirrors simulates RAID 10 , but I have not seen anything about RAID 0+1. I intended to follow the same steps but in reverse to create two vDevs of striped disks then mirror them. That was until I read the informative presentation by Cyberjock, which states that when one vDev goes down the entire zPool is lost. If I understand the presentation correctly, mirroring two stripes in the same zPool would provide zero redundancy.

Is there a way to configure FreeNAS to simulate traditional RAID 0+1 which would allow a disk in either set to fail and have the other set continue to work until the bad disk is replaced (presuming that a disk in the good set doesn’t fail before the RAID is able to be rebuilt)? This volume will primarily be used for file sharing, not long term storage or any mission critical data, so I value transfer speed over redundancy. That’s why I didn’t opt for the striped mirror approach to create RAID 10 (but I require some level or redundancy).

Currently, I have two volumes each containing three striped disks mounted to different directories on my computer. I use a backup utility called SyncBack from 2 Bright Sparks to synchronize the volumes at scheduled intervals. When a disk in one stripe fails the data from the good set is copied to the newly build array after replacing the bad disk. The striped configuration provides the speed boost for writes, but because the data is not mirrored no increase in read speed.

If RAID 0+1 is not possible with ZFS I’m hoping at lease there is a way to have the FreeNAS server automatically copy data between volumes for increased redundancy and eliminate that processing load on the client machine.

 

[cyberjock]

Hehe, you can't "mirror stripes". You can "mirror disks" but there is no "mirroring stripes". Things are much more complex than you think and comparing RAID1 to mirroring and RAID10 (or even RAID 0+1) is not a good and fair comparison. ZFS does very unique things and it does them in a way that makes comparison to hardware RAID kind of stupid. ZFS is like a RAID 10 and RAID 0+1 at the same time. I mention them in my guide because most people are familiar with RAID1, RAID5, and RAID6. But the reality is that they aren't exactly the same except for the fact that they protect from x number of disk failures in a given arrangement. The layout of the data is not the same and the way that ZFS handles reads and writes is not the same.

Anyway, if you have 3 vdevs of mirrored disks you could theoretically lose one disk in ever mirror and still be okay. As long as you never lose 2 disks in the same vdev at the same time things will be "okay".

 

[Jayden67]

Thanks for explaining that mirroring and striping in ZFS is different than in hardware RAID and that I can't create a configuration to mirror the sets of striped disks. Using multiple mirrored vDevs in a zPool is something that I will consider in the future.

However, since speed and performance are more important than redundancy, in this case, I think I will go with my current configuration but with some tweaks. I plan to investigate using snapshots instead the external program to back up the data. If I can get the snapshots for striped zPool 1 to be stored on striped zPool 2 and visa versa than I could lose a disk in either stripe and still be "OK". The amount of time it will take to rebuild the RAID from the back up isn't really an issue. Also, by using multiple zPools attached to the computer as different drives I can set my DVR and internet download programs to different drives so each will have full access to its own set of disks, and when one zPool fails only one program would be affected.