We recently bought 2 filers with 12 disks each : FAS2552 - NetApp Release 8.2.3P3 7-Mode
Since the aggr already took 6 disks (3 for each controller), I am thinking of simply adding the remaining 6 disks to it and create my data volume there (alongside the root volume).
Is this recommended ? if not, are there any issues with regard to system stabliity and performance that I need to be aware of ?
It has been not recommended to place data on the root aggregate by security reasons. If You need to save the data space, You may going to Clustered OnTAB 8.3 (now availiable 8.3.2P4) and use advanced data partition feature. Keep in mind that this transition needs to data evacuation.
If going to cluster mode is an option, then reinitializing the system on 8.3 or later with advanced drive partitioning would be ideal.
If you need to stay on 7mode, which may be the case since these are for functional testing and likely need to match the client configuration/ONTAP release as much as possible, then you've got a few choices:
Active/Idle with shared root aggregate
Active/Idle with dedicated root aggregate
Active/Active with shared root aggregate
Active/Active with dedicated root aggregate
Shared root aggregates are not ideal, but it was a common compromise on entry level 7mode platforms.
Active/Idle configurations give you a single larger pool of capacity and performance.
It comes down to what your priorities are and how you intend to use the systems.
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As I stated earlier, with 7-mode, there is no issue on a smaller system with adding disks to root aggr. It's been a common practice for many years... For that people that just know cDOT, you cant do it in cdot.
These systems are dev use only. We can take the risk of encountering issues since it only store test data. For production customers, the systems have plenty of disks to create new aggregates and leave the root aggregate alone.