Can an existing RAID-Z2 be extended by adding disks?

[CDuv]

Hello,

I'm in the process of building a NAS with 6 4TB disks and after all I've read on RAID5 reliability when rebuilding, I'm going for ZFS and RAID-Z2 (offering ~14TB of storage).

It will be used in a professional environment as "Windows shares" provider and workstations backup location. Everything will be replicated on an offsite server.

I'm a bit concerned with how I could eventually gain more space in the future. Server case can house 8 disks, so I'm thinking I might buy 2 other 4TB (or 6TB, who knows...) in a few year if ZFS volume gets filled.
But could the existing, data-containing RAID-Z2 volume be "extended" with theses 2 new disks (as if I had created the RAID-Z2 volume on 8 disks from the begining)?

 

[joeschmuck]

You need to read Cyberjock's presentation on this topic, but in short you can extend it however there are huge risks if you do this wrong. The risk is loosing all your data due to a single drive failure. Read the presentation here.

 

[danb35]

But could the existing, data-containing RAID-Z2 volume be "extended" with theses 2 new disks (as if I had created the RAID-Z2 volume on 8 disks from the begining)?

No. You can add disks to the volume, but they will be a separate RAID group, striped with your first. If you're unfortunate enough to add a single disk, that disk becomes critical--its failure would result in the loss of all data in your pool.

 

[CDuv]

I see, http://forums.freenas.org/index.php...ning-vdev-zpool-zil-and-l2arc-for-noobs.7775/ is really helpful: especially the part where it explains why adding a single disk/vdev to a pool is suicidal.

I guess I'll create another pool for the new disk when (if) they arrive or depend on the fact data are already backups (data exists elsewhere) that are replicated off-site (another place where theses data exists) to destroy and recreate the pool with 8 disks.

 

[joeschmuck]

If you feel you will be expanding your pool in the next 2 years, I'd add the extra drives in right now and for go the hassle of that upgrade in the future. If it's a production type system, the cost may easily be justified.

 

[CDuv]

Not so sure about the "2 years": it might be 4 or 5 :)
It's just that it's quite difficult to plan for future with workstation backups such as:
At which rate will they "update" (resulting in a new file write) all their work files?
How will compression help me gain space?

 

[joeschmuck]

Compression will depend on the files stored but I can say you should design the system based on zero compression factor. 14 TB of data is about right for the six 4TB drives but it would be just over 21TB of you added the two extra drives up front.

I have no idea if you are replacing a current system in which you hopefully know the growth rate of your data, or if this is a new system and you are taking an educated guess on the storage space required. Since this is a work place use I would definitely build the system properly using quality server grade components. I would also factor in growth and if there will be heavy usage of the server, like 5 people using it at once. Will folks be saving music files to their personal share space? I know at my company, I know of folks who have gigabytes of music and video files that have nothing to do with work, on the company servers.

In the future you could add another six 4TB drives, or you could start replacing the 4TB drives with 6TB drives to expand your storage as well when they start to fail, and the 6TB drives once all replaced would give you 21TB of storage as well. Heck, in 5 years you might want new hardware overall but at least you could move the drives.

Good Luck, hope this helps.

 

[CDuv]

I'm replacing a 4x 2TB RAID5 NAS that is almost full. I cannot say I have a clear view on how fast disk where filled (new system will keep track of such info) so I'm taking a guess but I do know users cannot have more than 100Gb of files on their computer (limitation set at partition level on computers) and that non-work-related won't be stored on that server (backup tool enforces that).

Replacing with 6TB when 4TB are failing is a good idea :) However disks are WD enterprise-grade with 5 years warranty: I'm sure WD will provide 4TB disks as RMA. But it could still work (4Tb fails => buy 6TB and use RMA'ed 4TB for other stuff).

Thanks for the infos.

 

[gpsguy]

Ditto everything that joeschmuck has said.

How much RAM can you put on the server? If you haven't already bought it, consider one that will hold 64Gb or more. If you need to grow your storage and replace the 4Tb drives with 10Tb drives, you'll be able to add RAM to the server to accomodate the larger drives.

 

[CDuv]

Server has only 8GB of RAM but has 6 slots so I can go up to 48GB (or 96GB if using 16GB DIMM): Double that if I add a second CPU. I'm sure that part won't be an issue (as long as it's upgraded along side with disks).

 

[joeschmuck]

Just ensure, if you are not already doing so, to run daily Short SMART tests and weekly Long SMART tests on each drive and have the results setup to me emailed to you. On the Long SMART tests, I'd take and do one drive a day during non-working hours as this test will take longer if done while in active use and will slow the system down a little bit, might not even be noticeable depending on what you plan to do with the system.

As for RMA drives... Typically I never see businesses RMA'ing hard drives because they contain work product, privacy act data, personnel records, or maybe HIPPA (medical) data, even if the drive has failed it is not hard to imagine the data could be retrieved by a hard drive manufacturing company. And I'm not saying WD would want to retrieve the data but it's possible someone else could if WD sells the defective hard drives as bulk sales to some third party. I don't know if they do that but I can't think of what would stop them from doing that. Of course if the drives are encrypted then you should be fine.

And if the drives are encrypted, hopefully you have taken proper steps to backup the proper data needed to recover properly from a drive replacement.

Lastly, if you are only doing a periodic backup of data with the server then 8GB of RAM should be fine however if you ever use the server as something people can access files all the time, you can likely make your system work better by adding the 48GB more or up to 96GB if you like, depends on how much money you can toss at it. Of course there are other things to consider as well like adding another VDEV to speed access up overall.

Attached is something I created for myself and you might customize to your specific needs should you choose to use it. The Cheat Sheet is printed out and then attached to the server for later use when it's needed. It's in PDF format because the forums do not allow .rtf files to be attached. If you wanted this in .rtf or a Word .doc file just visit my shared server. There are a few other files there as well however my upload speed is limited to 2Mbps, not a problem for documents but the UBCD could take a little longer, but you don't have a bunch of crap to go through to get it either.

To access the FTP server you must use the following:
URL: ftp://joeschmuckatelli.homeftp.org:1025
Name: GuestFTP
Password: guestftp

I hope all the info has been helpful.

EDIT: The FTP Server was shut down on 31 December 2014. If you would like the file just toss me a message with your email address and I'll forward it along.