Making RAID Work into the Future Page 4 - Page 4


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There are several technologies that allow us to extend or replace the benefits of RAID into the future. The example examined here, Dynamic Disk Pools from NetApp, give us a number of benefits. First, DDP is very easy to understand because it is still RAID. However, it isn't fixed to a specific set of drives in creating RAID groups. Rather, it spreads the underlying RAID "chunks" making up the RAID-6 8+2 group from across a random set of drives in the pool.

Second, in the event of a drive failure, we can get a much larger number of drives involved in the rebuild with DDP. And they all do both read and write, eliminating the single-drive write performance limitation. This can greatly reduce recovery times.

Third, during a reconstruction, only the missing D-Pieces are regenerated. Remember, in a classic RAID-6 you have to read the entire contents of all of the remaining drives even if all of the contents are not being used, perhaps getting us dangerously close to the URE limit.

Moreover, in classic RAID-6 you don't regain the full RAID-6 protection until the reconstruction is finished (i.e. it's not 100% until everything is done). In the case of DDP, you quickly regain full RAID-6 protection on some of the D-Stripes. As recovery proceeds, a larger percentage of the total pool has full RAID-6 protection. So it's not an "all or nothing" proposition.

Additionally, in the case of DDP it is very easy to create a volume with the needed capacity, leaving some spare capacity in the pool. Then when you need the capacity, you just add it to the pool and DDP dynamically takes care of the underlying storage for you (but naturally you have to extend the file system). With classic RAID we would need to create a new volume, add it to the existing pool using something like lvm and then grow the file system.

And finally, the use of DDP means that you can lose more than two drives in a pool without data loss. When classic RAID-6 loses more than two drives in a LUN, then the LUN cannot be rebuilt, and the data has to be restored from a backup or copy. In the case of DDP, several drives can be lost in a single pool without losing data.

But everything is not as perfect as it would seem (nothing ever is). When a drive is lost in a DDP, a larger number of drives participates in the reconstruction, which can adversely impact some performance aspects. In NetApp's Dynamic Disk Pools, you can turn down the "priority" of the reconstruction so the impact of performance is less. Or you can turn up the priority to finish the reconstruction as quickly as possible. It's really up to you which approach is more important to you.


RAID, as it is currently works, is not long for this world. With drives getting larger at a rapid clip while the drive speed and the URE rate stay pretty stagnant, classic RAID levels are not going to last much longer. There have been a number of approaches for "fixing" RAID that give us similar features without the problems. Some of these approaches can be very complicated and expensive, which can possibly explain why they haven't gotten widespread adoption.

In this article, I examined one approach that applies RAID concepts but on a more granular level). NetApp's Dynamic Disk Pools use the concept of RAID to create a solution that allows you to easily combine all of the drives into a single pool. It also provides protection for losing more than two drives (which causes a RAID-6 group to fail). The nice thing about DDP is that it is very easy to understand, so we don't have to dump all of our RAID knowledge built over the years.

Approaches such as DDP will become increasingly important as drives get larger and the amount of data we generate increases almost exponentially. Classic RAID will have to give way to other concepts, such as DDP, or we run the risk of losing our data.

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