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RaidZ2 vs Mirrors

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chipmunkofdoom2
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RaidZ2 vs Mirrors

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Post by chipmunkofdoom2 » 12 Jul 2019 16:34

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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