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Disc failure during expansion
Hi all. I have been running a ReadyNAS with 3 drives in for a while, (2+2+4 Gig respectively). I ended up with 3.6TB of capacity, and hit 80%+ full the other day. Accordingly, I put an 8Gig dr...
robinwatts wrote:
If I had an array with 4+8+8, could I safely add another 4TB drive later to get it to the maximum 16?
The usual guidance is that you can only add 8 TB (or bigger). Taking the expansion limits into account, that means upgrading the 4 TB to 8 TB.
Some experiments posted here show that OS-6 NAS would let you add a 4 TB (since there is a 4 TB already in the array). However, I don't know if OS 5 will allow that (though it is technically possible).
Under the covers, a 4+8+8 setup has two RAID groups - a 3x4TB RAID-5 group that covers all drives, and a 2x4 RAID-1 group that covers the two larger drives. These are assembled into one volume.
Adding a 2 TB drive requires that the 3x4TB group be split into two 3x2TB groups (followed by adding the new drive to one of those groups). XRAID won't split an existing group that way (and I don't think it can be done easily w/o data loss).
> The last step in option (2) won't work, since you can't add a 2 TB disk to an array
> with larger drives. If you leave that step out, you could upgrade the 4 TB to 8 TB
> when the time comes.
Ah, so what is the rule here?
If I had an array with 4+8+8, could I safely add another 4TB drive later to get it to the maximum 16?
Thanks.
robinwatts wrote:
If I had an array with 4+8+8, could I safely add another 4TB drive later to get it to the maximum 16?
The usual guidance is that you can only add 8 TB (or bigger). Taking the expansion limits into account, that means upgrading the 4 TB to 8 TB.
Some experiments posted here show that OS-6 NAS would let you add a 4 TB (since there is a 4 TB already in the array). However, I don't know if OS 5 will allow that (though it is technically possible).
Under the covers, a 4+8+8 setup has two RAID groups - a 3x4TB RAID-5 group that covers all drives, and a 2x4 RAID-1 group that covers the two larger drives. These are assembled into one volume.
Adding a 2 TB drive requires that the 3x4TB group be split into two 3x2TB groups (followed by adding the new drive to one of those groups). XRAID won't split an existing group that way (and I don't think it can be done easily w/o data loss).
Just to close out this thread with how I resolved it in the end.
Firstly, huge thanks for StephenB for his patience and help throughout this.
I ended up copying all the data off, and going for an 8+8+4 setup.
I tried the 8+8+4+2 setup initially, but the way the NAS works, means that this is immediately populated by striping across the bottom 2TB of all the discs (so 2+2+2+ 2parity gives 6TB of storage).
When the NAS finishes setting up, on the next reboot, you'll then get another 2+2+2parity stripe, giving a further 4TB of storage.
Then on the next reboot you get another 4+4parity stripe, giving another 4TB, for a total of 14.
The drawback with this, of course is that because we can never expand by more than 8TB from where we started means that I'd be forever limited to that 14TB of storage. (As we started from 6TB initially). And as the 2TB drive is aging, I thought it better to leave it out for now.
Accordingly, I went for the 8+8+4 setup. This starts by using 4+4+4parity (for an 8TB data region), that then expands by another 4+4parity = 4TB for a total of 12.
This leaves me free to either add another 4TB later, or replace the 4TB with an 8TB for a total of 16TB.
(All these figures are slightly less of course due to the con of disc sizes being specified in Trillions of bytes rather than TeraBytes, but hey...)
So, it's been a long painful process, but I'm back up and running now.
Thanks again!
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