Data Domain 620 appliances come in two flavors, a 12TB version and a 7TB version, for both versions the chassis is the same only the drive count is different. For the 12TB version you get 12x1TB drives in RAID 6, and for the 7TB version you get 7x1TB drives in RAID6 (both versions use 1x1Tb drive for a hotspare). Usable capacity according to the DD Hardware guide is about 7.8TB for the 12 drive model and 3.3TB for the 7 drive model. The interesting part is when you want to upgrade your 7TB model to the 12TB model….
I was reading through the DD620 hardware guide the other day when I noticed a 2TB difference in the capacity of a DD620 that was purchased with all 12 drives from the factory versus a unit that was purchased with 7 drives and later upgraded to 12 drives. (As you can see this same issue applies to DD160 boxes that are upgraded as well but you only give up about 1TB instead of 2TB)
Notice anything about the “7+5 drive” configuration ? Almost exactly 2TB less than if you would have purchased all 12 drives to start with.
I assume this is because initially the Data Domain is configured with 1 hot spare drive, and 6 drives in a RAID6 configuration… giving you about 3.3TB usable after formatting, and then when you add the 5 new drives the unit will configure them as a new RAID 6 array giving you a 3+2 drive config… which is about 2.5TB usable after formatting. So total you end up with about 5.8-5.9TB usable for storing your backups. Basically instead of only losing 2 drives for parity like the factor unit does, your going to lose 4 drives total for parity after you add the new drives.
Since I like to get the most for my money, I opened a ticket with Data Domain to see if it was possible to bypass this upgrade penalty by reimaging the DD Operating System after installing the new drives. They confirmed that reimaging the system would be the only way to forgo giving up that extra 2TB of usable space. So if you are in the market for upgrading your Data Domain DD620 or DD160 I would make sure you contact support and have them get you access to the reimaging USB software.
So now your probably thinking great…. all i have to do is reimage the box and kill all my backups to get that extra 2TB… and you would be right, but if you have a pair of data domain’s you can probably get around losing the data.
First upgrade your primary DD appliance with the new drives and do the reimage process.
Next re-establish the replication pairs but in reverse order so that your DR site box sends all of the data back to the primary box. You will probably want to bring your DR box back to HQ for this.
Then after replication is complete you can unpause your backup jobs and start the upgrade on the DR box
Once that box has been upgraded with the new drives, re-enable replication in the proper direction and let the data re-replicate back to the DR box.
At this point both boxes should see all 7.8TB of space and you didnt lose your backup data!
Note: I have not had to do this yet… but if I ever have to do this in the “real world” I will certainly post my results as an update.