Making RAID Work into the Future Page 2 - Page 2


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NetApp's Dynamic Disk Pools

NetApp has developed a concept called Dynamic Disk Pools (DDP) as a way to improve on all of the problems that conventional RAID suffers. The concept is pretty simple - continue using RAID concepts but on a more granular level as opposed to the whole disk. In the latest version of NetApp's SANtricity software, the company incorporated DDP which uses RAID-6 concepts on a granular level. DDP allows you to combine all of the drives behind a controller to create a pool of storage. It's just like the classic RAID-5 or RAID-6, but you can use all of the drives in a single pool rather than breaking them into RAID-5 or RAID-6 groups and combining them with lvm (logical volume manager) or some other technique. To better understand how this works we need to dive deeper into the DDP layout.

The lowest level of DDP is called a D-Piece. A D-Piece is a contiguous 512MB section within a disk that is constructed from 4,096 128KB segments. The next level up, called a D-Stripe, is built from ten (10) D-Pieces. Therefore a D-Stripe has a total of 5,120 MB of raw space. The D-Pieces are selected from various drives in the pool using a pseudo-random algorithm. Each D-Stripe is basically a RAID-6 group that with 8+2 configuration (8 data and 2 parity chunks). This means that we have 8*512MB = 4,096 MB of useable space in a D-Stripe. Finally, a volume is constructed from the D-Stripes to meet the desired capacity.

In DDP, we're still using RAID concepts, in this case, RAID-6. But instead of using the entire drive as one of the chunks for RAID-6, DDP breaks the drives into pieces and creates RAID-6 stripes across all of the disks.

To better see this, Figure 1, provided courtesy of NetApp, illustrates how a DDP is built from 12 drives.

Figure 1: Illustration of an example Dynamic Disk Pool showing the various components (copyright NetApp and used with permission)

In Figure 1, each block is a D-Piece (512MB). The same color D-Pieces constitute a D-Stripe. For example, all of the pink D-Pieces form one D-Stripe, all of the light blue D-Pieces form another D-stripe, all of the yellow D-Pieces form a third D-Stripe, and the darker red D-Pieces form another D-Stripe and so on.

The D-Pieces in a particular D-Stripe are selected in a somewhat random fashion so that a particular layout is not repeated. The selection process tries to balance capacity across the drives while maintaining the pseudo-random layout. This insures that the pure random case where all of the data on two D-Stripes is on the same ten drives never happens.

In a production system, a Dynamic Disk Pool is created by first defining how many drives are in the pool. Then you create a volume by just defining the capacity you need. The software creates all of the D-Pieces followed by the D-Stripes across the drives in the pool. The volume is built from a number of D-Stripes to match the desired capacity as closely as possible using integer units of D-Stripes. You can create a number of volumes in the same DDP as long as you have enough capacity in the DDP.

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