RAID Groups and Aggregates
Aug 19th 2008Unitek NetApp BlogUncategorized
In the course of teaching Netapp’s Data ONTAP Fundamentals course I have noticed that one of the areas that student’s sometimes struggle with are RAID groups as they exist in Data ONTAP.
To begin with, Netapp uses dedicated parity drives, unlike many other storage vendors. Parity information is constructed for a horizontal stripe of WAFL blocks in a RAID group within an aggregate and then written to disk at the same time the data disks are updated. The width of the RAID group – the number of data disks – is independent of the parity disk or disks. Take a look at this print screen from Filerview:
Notice that the RAID group size is 16. This is the default RAID group size for RAID-DP with Fibre Channel disks. Notice also that the number of disk in Aggr1 is actually 5.
When I created aggr1 I used the command:
aggr create aggr1 5
This caused Data ONTAP to create an aggregate named aggr1 with five disks in it. Let’s take a look at this with sysconfig –r:
If you notice aggr1, you can see that it contains 5 disks. Three disks are data disks and there are two parity disks, parity and dparity. The RAID group was created automatically to support the aggregate. I have a partial RAID group in the sense that the RAID group size is 16 (look at the Filerview screen shot). I only asked for an aggregate with 5 disks, so aggr1 has an aggregate with one RAID group and 5 disk drives in it.
It is fully usable in this state. I can create volumes for NAS or SAN use and they are fully functional. If I need more space, I can add disks to the aggregate and they will be inserted into the existing RAID group within the aggregate. I can add 3 disks with the following command
aggr add aggr1 3
Look at the following output:
Notice that I have added three more data disks to /aggr1/plex0/rg0.
The same parity disks are protecting the RAID group.
Data ONTAP is able to add disks from the spare pool to the RAID group quickly if the spare disks are pre-zeroed. Before the disks can be added, they must be zeroed. If they are not already zeroed, then Data ONTAP will zero them first. This may take a significant amount of time. Spares as shipped by Netapp are pre-zeroed, but drives that join the spare pool after you destroy and aggregate are not.
The inserted disks are protected by the same parity calculation that existed on the parity drives before they were inserted. This works because the new WAFL blocks that align with the previous WAFL blocks in a parity stripe contain only zeroes. They new (zeroed) disks have no affect on the parity drives.
Once the drives are part of the RAID groups within the aggregate, that space can be made available to volumes and used by applications.
An aggregate can contain multiple RAID groups. If I had created an aggregate with 24 disks, then Data ONTAP would have created two RAID groups. The first RAID group would be fully populated with 16 disks (14 data disks and two parity disks) and the second RAID group would have contained 8 disks (6 data disks and two parity disks). This is a perfectly normal situation.
For the most part, it is safe to ignore RAID groups and simply let Data ONTAP take care of things. The one situation you should avoid however is creating a partial RAID group with only one or two data disks. (Using a dedicated aggregate to support the root volume would be an exception to this rule.) Try to have at least three data disks in a RAID group for better performance.
There is a hierarchy to the way storage is implemented with Data ONTAP. At the base of the hierarchy is the aggregate, which is made up of RAID groups. The aggregate provides the physical space for the flexible volumes (flexvols) that applications see. Applications, whether SAN or NAS, pull space that has been assigned to the volume from the aggregate and are not aware of the underlying physical structure provided by the aggregate.
This is why we say that the aggregate represents the physical storage and the volumes provide the logical storage.
