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RaidZ2 vs Mirrors

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RaidZ2 vs Mirrors


Post by chipmunkofdoom2 »

Hi all, I currently a have server that stores media and documents, and has a relatively low load. It has 6 x 3TB drives, all configured in mirror vdevs. I'm wondering what everyone's thoughts are regarding re-configuring the server to contain a single 6 x 3TB raidZ2 vdev. I feel like I have a pretty good understanding of the pros and cons of each, I would just like to get some opinions from other users.

The reason I'm considering the change to raidz2 is partially because I have enough disks now, but mostly because of fault tolerance. A two-disk mirror is protected from a single disk failure, but that's it. If the surviving disk fails during resilver, the vdev and pool are both destroyed. I enjoy the simplicity and relative flexibility of mirrors, but I always am nervous about the surviving disk when resilvering a mirror. With raidZ2, resilvering might be a bit more intense, but I can lose any other random disk from the vdev and still be safe (so long as no more fail). Plus, I'll get a bit more storage efficiency: three mirrors of two disks will give me about 1/2 usable storage (9TB in my case), while Raidz2 with six disks will give me about 2/3 usable storage (12TB).

I am concerned about the lack of flexibility with a raidz2 vdev though. If I want to add more storage, I have to replace every single disk in the vdev with a larger one, which can get expensive. I could add a new two-disk mirror if my main pool gets full, but that only has a single-disk fault tolerance. Resilvering time is also a concern. As of now, I'm at 70% capacity and relatively low fragmentation, but if my pool becomes more full because I can't upgrade the storage, I worry that resilvering might be a concern and could lead to additional disk failures.

How does everyone else feel about mirrored vdevs vs raidz2? What do you choose to run for your server?

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Lee Sharp
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Re: RaidZ2 vs Mirrors


Post by Lee Sharp »

The only reason to go with striped mirrors is iops. If you NEED a lot of iops, striped mirrors can simply not be beat. But if you do not, raidz2 means you can lose any two drives and still have all your data.

As for capacity, I generally do forklift upgrades of all the disks when the time comes. Often by putting the old nas on a bench and building a new one in the old case. :) Then I can copy the data and still have a archive copy.

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