is RAIDZ3 really necessary?

[Plato]

Hi,

I plan to build a big array from 6 TB disks ( about 24-25 disks ). But I heard there are many concerns about big arrays.

• If a disk fails resilvering takes too much time ( people tend to say it doesn't finish )
• it's better to build 2 raidz2s instead of one big raidz3.
• IOPS is too bad..

So is that really true. I'll use this at home, so no one will access this other than me, and I'll be using it as media storage only. Should I be concerned?

What if I use raidz2 instead of raidz3? While I know that if one disk fails, generally another disk will also fail when resilvering and three backup is always better than two, if the data is not really that imported is it really necessary?

 

[joeschmuck]

Should I be concerned?

Of course you should if the data is important to you. If the data is recoverable and you accept that risk, then it's fine.

Over time what we have learned is any hard drive over 4TB in a large array should be built as a RAIDZ3. This is strictly due to the amount of time it takes to rebuild one replaced drive which can be almost a week for 6TB drives that are fairly full. You should think about that, what could go wrong in 7 consecutive calendar days? Of course speed on the rebuild depends on several factors, the RAIDZ type, If there are mirrors, CPU, RAM, and drive throughput. Lastly how busy the NAS is during the rebuild process. You are looking to create a very large pool of 100TB (T25 6TB drives in RAIDZ2 - 20%). You loose only 6TB if you make this a RAIDZ3 and gain a significant safety net.

 

[Jailer]

Are you considering 24 or 25 disks in a single vdev? If so don't.

 

[joeschmuck]

Are you considering 24 or 25 disks in a single vdev? If so don't.

I think he/she is. I agree that it's a bad idea too. You could get away with two vdevs but three vdevs would be more sound.