ZFS mismatched repli levels

aronjd

Member


Messages: 22

Hi guys,

I have in my NAS 5 drives in a raidz configuration. Everything works well. But i needed some more space. So i tried to add 3 more disks in a raidz in the 5 disk raidz pool. If you do so ZFS will complain that the replication level is mismatched and that you can use -f to force the add. If i make a dry-run it should work. I was reading about ZFS mismatched replication levels on multiple sites. But there is no definitive answer about the -f usage. I would like to know your experience or why it is bad to use the force option.

One guy wrote that it is there to warn you about redundancy. If you add a single disk to a raidz pool. That you will know that you will not have any redundancy left if that disk crashes. Another guy tells you that the speed will go down the drain. And multiple tutorials will tell you that it is ok to use the -f. Solaris only tells you "it is not recommended" but no explanation why?

And other people just say don't use a force switch on a production system, force usage is BAD!

Hope that the FreeBSD community will get the answer!

With Regards,

Aron
 

phoenix

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ZFS stripes reads/writes across all vdevs in the pool, similar to how a RAID0 setup works.

If all the vdevs in the pool are built the same (same size drives, same number of drives, same redundancy level) then the performance of the vdevs will be the same.

If the vdevs are different, especially if the redundancy and/or number of drives, then performance will be different across the vdevs, and the striping will be unbalanced (more data written to the faster vdev, for example).

It's not "harmful" or "dangerous", but it's not recommended. My home pool used to be a 3-disk raidz1 (SATA) vdev and a 2-disk mirror (IDE) vdev. Performance was fine for a home setup. I've since moved to a 3x 2-disk mirror setup using all SATA drives.

Just remember: YOU CANNOT REMOVE VDEVS FROM A POOL, so anything you force into the pool, you will be stuck with for the life of the pool.
 
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aronjd

Member


Messages: 22

Hi Phoenix,

Thank you for your reply.
I was wondering, with the unbalanced striping, will it have more downsides than you describe? Some page wrote that it would be possible that the largest vdev will not get filled to the top, because of the striping.

Thanks,

Aron
 
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