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HP’s RAID 6 (ADG – Advanced Data Guarding)

Continuing my RAID 6 posts, first  HDS’s RAID 6, then  NetApp’s RAID-DP and now this time around it’s HP’s RAID-6  ADG.  Will try to keep this post short in terms of overall RAID 6 concepts, rather jump directly into the technical aspects of HP’s RAID-6  ADG  (Advanced Data Guarding).    

Some upcoming post would include RAID 6 technology and its implementation by Sun, IBM and  EMC. The final post should be about a comparison of all  OEM  products and the usage of RAID 6.

Here are the links to previous post related to RAID 6 and data protection

NetApp’s RAID—DP

Hitachi’s (HDS) RAID 6

Different RAID Technologies (Detailed)

Different RAID Types

HP’s Business Case with RAID-6 Advanced Data Guarding (ADG)

So Advanced Data Guarding….the name is just perfect…. HP’s pitch to their potential storage customers would include a slide on ADG  (I am assuming that is the case).    This cost effective and fault tolerant technology is proprietary to HP and its patented, just cannot find a reference about it on the US PTO’s website.

RAID-6  ADG  is supported on the  MSA  (Modular Smart Arrays) SAN platform.

I believe it’s not supported on any EVA (Enterprise Virtual Array) platforms. No RAID 6 support available on  LeftHand  Network SAN’s.

With HP  XP-24000, HP  XP-20000, HP  XP-12000 and HP-XP  10000 there is no support for RAID-6  ADG, but there is native support for RAID 6 (dual parity).

HP Storage Products traditionally have support for RAID 0, RAID 1, RAID 1+0, RAID 5 and now RAID-6  ADG. Some  LeftHand  Network SAN’s support RAID 3 and RAID 4.

The argument from HP is pretty similar to the ones we already discussed with  HDS  and  NetApp  in the previous post. The push for RAID 6 at HP comes due to the existence of larger disk size and requirements for fault tolerance to run 24 x 7 x 365 applications.

Since there is an additional parity calculation associated with RAID-6, HP’s recommendation is to use RAID-6  ADG  with lower writes and high reads only. If you have an application performing random writes, RAID 6 (ADG) might not be an option for you.

HP’s RAID-6 Advanced Data Guarding (ADG) Technology

Here is a snapshot of how this technology operates.

In the case here, we have 6 disk drives attached on a fiber loop or SCSI bus / controller. Data is striped on Disk 1, Disk 2, Disk 3, Disk 4 and then Parity (P1) and (Q1) are generated and written on Disk 5 and Disk 6. You can assume each data block is 4kb  or 8kb  in size.

Similarly, as the process continues, the next set of data strips start at Disk 1 then go to Disk 2, Disk 3 and Disk 6, while the parity is written on Disk 4 (P) and Disk 5 (Q).  ADG  is based on P + Q algorithm to calculate two independent parity sets. Exclusive OR (XOR) is used to calculate the P and Q Parity. The P Parity is exactly like it would be for RAID 5 and Q is calculated based on Error Correcting Code. The Q is then striped across all the disk within the RAID Group.  

If a single drive is lost in the Raid Group, data is rebuild using ordinary XOR P (Parity). Also the P and Q are both recalculated for each rebuild block. If a second drive fails during this time, the rebuild takes place using the Q Parity. During these times data is still completely available with a little degradation.  

If you do add a spare drive to this configuration, now your raid group can pretty much withstand 3 drive failures before data loss.

This technology can be implemented with a minimum of 4 drives. The overhead with use of 4 drives in a single RAID Group is 50%. If you run 60 drives in a single RAID group, your overhead might be close to 4% {100 – [100 x 2 (parity) / 60 (drives)]}.

The formula to calculate your usable space will be C * (n — 2), where C is the Size of your smallest drive in the RAID Group and n = number of drives. It is highly recommended all your disk drive sizes are similar.

If you are running more than 14 physical drives in a single RAID Group, HP’s recommendation is to use RAID-6  ADG. With 6 drives in a RAID Group, the failure probability is 1E-10. With 60 drives in a RAID Group, the failure probability is 1E-7.

Again HP’s big pitch with RAID-6  ADG  is  Cost Effectiveness with Fault Tolerance,  not really performance.  

About the author

Devang Panchigar

With more than 7 Years of IT experience, Devang is currently the Director of Technology Solutions and IT Operations at Computer Data Source, Inc. Devang has held several positions in the past including Sr. Systems Engineer, Sr. Network Engineer, Technical Support Manager, Director of Storage Support & Operations. He has been responsible for creating and managing worldwide technical support teams, technology solutions team, operations management, service delivery, pre and post sales support, marketing and business planning. In his current role Devang oversees multiple aspects of the Technology Solutions Group that works with various Multinational and Fortune 500 companies providing them infrastructure services. Along with various industry certifications, Devang holds a Bachelor of Science from South Gujarat University, India and a Master of Science in Computer Science from North Carolina A&T State University.

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