Raid: Who What Where When and Why. 3/23 Draft V1 David Arts 4/3 Draft V2 David Arts 4/10 First Release David Arts

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1 Raid: Who What Where When and Why 3/23 Draft V1 David Arts 4/3 Draft V2 David Arts 4/10 First Release David Arts 1

2 Table of Contents General Concepts and Definitions... 3 What is Raid... 3 Origins of RAID... 3 Why do we need to Discuss Raid?... 4 What is parity?... 4 Degraded Array... 5 Rebuilding a failed drive... 5 Hot Spare... 5 Cold Spare... 6 Concept of performance scaling... 6 Raid Levels... 6 Raid Raid Raid 4 (Not Supported by ViSX)... 8 Raid Raid Raid What Raid level is right for ViSX

3 General Concepts and Definitions What is Raid RAID is a data storage technology that combines multiple disk drive components into a logical unit for the purposes of data redundancy and performance improvement. Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the specific level of redundancy and performance required. The term "RAID" was first defined as a Redundant Array of Inexpensive Disks. Industry RAID manufacturers later tended to interpret the acronym as standing for Redundant Array of Independent Disks RAID is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple physical drives: RAID is an example of storage virtualization and the array can be accessed by the operating system as one single drive. The different schemes or architectures are named by the word RAID followed by a number (e.g. RAID 0, RAID 1). Each scheme provides a different balance between the key goals: reliability and availability, performance and capacity. RAID levels greater than RAID 0 provide protection against unrecoverable (sector) read errors, as well as whole disk failure. Origins of RAID Raid technology came about because of some major deficiencies in server architecture in the past. Before RAID users that needed to expand space beyond the physical limits of the drive capacities of the day, had to add another hard drive to a server and assign another drive letter. There was no way to virtualize both hard drives into the same free space. Companies that had applications with large datasets had to strategically separate the data into discrete units that would fit on the relatively small drives of the time. It was not unheard of to see a server with 15 or more drive letters. Managing this data sprawl and keeping the application stable with 15 drive letters was very daunting. The industry came up with a way to virtualize many drives into one free space pool for a server. This allowed application datasets to grow beyond the limitations of discrete drives without having to maintain many mount points or drive letters in the operating system. This created flexibility and stability to the environment. The solution did however inject some risk. Spanning data across multiple hard drives looking like one increased the risk of data loss. If any drive participating in the virtualization failed, all of the data was lost even if located on a drive that was still operational. Drive virtualization added technology that would protect against drive failures so that one or more drives could fail leaving the access to data intact. This added ability was known as RAID. 3

4 Why do we need to Discuss Raid? We need to discuss RAID because of the many different types of raid, specific applications and user requirements will determine what raid type is used. For example if performance is the driver, Raid 10 is the best. If protection is the driver Raid 6 would be selected. If the need is more efficiency of usable space, Raid 5 would be selected. In our discussion with FLAS technology, many of these needs derived from the spinning hard drive world are moot. Arming you with this information will allow you to have discussions with your customer on how to more efficiently and cost effectively implement FLASH. What is parity? Different raid levels as mentioned above have different characteristics for capacity, performance, reliability or protection, and reliability. Performance is based on two factors; how many drives are available for read and write, and how the protection is generated (parity or copies) Parity is a protection scheme based on math. The basic form of parity conforms with the definition of the term; even or odd. Parity is calculated on a stripe of data on the drives and a parity bit is generated. A 1 if the stripe adds up to odd value and a 0 if the stripe is even. On the example below you can see 4 hard drives in a raid level with parity. Disk 1 Disk 2 Disk 3 Disk 4 (Parity) Stripe Stripe Stripe From this example adding Disks 1-3 up and determining if even or odd calculated the parity drive or bit. Stripe 1 adds up to 2 which is even so the parity bit is 0 or even. Stripe 2 adds up to 1 or odd so the parity bit is 1 and so on. 4

5 Degraded Array If one of the drives in the above example had failed say disk 1. The array would be in a degraded state but the array is still able to remain online. The system would be able to remain online because it can calculate what should be on disk 1 through reversing the parity calculation. If disk 1 was failed the system should add up disk two and three of stripe 1 and come up with 1 which is odd. The parity bit says it should be even so we know that Disk 1 Stripe 1 should be a 1 because that will make the stripe even. The system would do this for every read or write to a stripe of data by the application. The degraded array will not perform as fast in a degraded state because the parity calculation will add latency. Rebuilding a failed drive If a drive has failed and the array is in a degraded state, the only resolution is to replace the failed drive. Once you replace the failed drive and the drive is a data drive (Disk 1-3), the system will use the parity information and immediately start rebuilding the data on the failed drive. This process runs in the background and will take some time to complete. The time to completion depends on the amount of resources allotted to the rebuild process, the workload currently running on the controllers, and the amount of data to be rebuilt/drive size. It is not unheard of for a 3TB NLSAS drive taking days to rebuild. This parity rebuild would occur on Raid5, and 6. At current publish date of this article, all drives supported by ViSX rebuild in less than 2 hours. If the failed drive is party of a non- parity raid set that uses copies for protection, the rebuild is just a copy process. Fewer calculations are involved so the rebuild is significantly quicker. This would be in Raid1 or 10 Hot Spare A hot spare is a drive in the physical drive that is there in case of a failed drive. There is no data on the HS or Hot Spare so it cannot be calculated in the usable capacity of the solution. The HS drive allows the rebuild process to begin as soon as the drive fails thus not having to wait for the drive to be shipped to the site and physically replaced. See diagram below: Disk 4 Disk 1 Disk 2 Disk 3 (Parity) HS Stripe Stripe Stripe A HS drive will reduce the overall efficiency of the array from a raw versus usable capacity perspective and will not increase the performance of the raid set as there is no data on it. Because you do not have to wait for the failure to be recognized, 5

6 shipment of a new drive, and replacement of the drive, this is the lowest risk. Rebuild starts as soon as the drive fails with no user interaction. Cold Spare This is almost the same concept as Hot Spare (HS) but the drive is not installed into a drive slot but sitting on a shelf at the customer site. The rebuild process will begin as sun as the user swaps the failed drive with the Cold Spare. This has more risk than a HS scenario as the array will be in a degraded state longer. This has less risk than no spare as the shipping time is eliminated before rebuild can start. Concept of performance scaling Just like with most tasks and individual can only do one thing at a time and a group of individuals can get a job done faster than one. This concept is the same for drives in an array. Two drives can get the job done twice as fast as one, three drives can do the job 3 times as fast as one. NOTE Parity drives, Hot Spares (HS), and Cold Spares (CS) do not count in this effect. Adding a Hot Spare will not increase performance. Only active drives count to performance. Raid Levels Raid 0 Minimum Drive Count: 2 Maximum Drive Count: none Performance: Highest Protection: None Efficiency: 100% 6

7 Raid zero is the first of the raid levels supported by ViSX. The characteristics are more in favor of performance and not reliability. Raid 0 has no protection of drive failure, has no option for a Hot Spare (HS), and no parity is calculated. If you had 10 drives in a raid 0 you would get 10 drives of performance as described in the concepts section above. The big caveat is that there is no redundancy in this raid type. You can have 100 hard drives in a raid zero for really fast performance but if one of those hard drives were to fail, you will need to restore your data from the last backup. For this reason Raid 0 is reserved for data or applications that use scratch space for really fast calculations but once the end result is achieved, the data is moved to a more secure location. A great example for this would be in genomic mapping. Once the DNA is scanned, the application will perform billions of operations on the dataset to get the ending Genomic map. The data is copied to a raid 0 set for the calculations but the initial DNA sequence and the end map are on raid sets that offer less performance but better redundancy. If the Raid 0 were to fail you still have the initial dataset to start the calculations over from. Raid 1 Minimum Drive Count: 2 Maximum Drive Count: 2 Performance: Good Protection: Good - Single drive failure Efficiency: 50% 7

Raid 1 is often referred to as a Mirror. Only 2 drives can be used in this raid level and the data is on both drives. Reads can be read from either drive or both drives at once.

A1 from Disk 1 and A2 from disk 2 in the first read cycle and A3 from Disk 1 and A4 from disk 2 in the second read cycle.

8 Raid 1 is often referred to as a Mirror. Only 2 drives can be used in this raid level and the data is on both drives. Reads can be read from either drive or both drives at once. In the above example you would only need 2 read cycles to get all 4 blocks of data. A1 from Disk 1 and A2 from disk 2 in the first read cycle and A3 from Disk 1 and A4 from disk 2 in the second read cycle. Writes see no change as you would have to write twice but the two drives can do this in unison. Raid 4 (Not Supported by ViSX) Minimum Drive Count: 3 Maximum Drive Count: None Performance: OK Protection: Good - Single drive failure Efficiency: Varies Number of drives - 1 The only reason to discuss Raid 4 is because you will need the concepts before we go to raid 5. Raid 4 is the concept described above in the Parity definition. You have data drives and a parity drive, a failed drive is compensated for by calculating whet is on the failed drive from the remaining data drives and the parity drive. The major setback for this raid level is that all of the parity is on one drive, which can lead to contention. Contention is when a resource that can only do one thing at a time has multiple requests of it. This will cause the requests to queue up and hinder performance. In the above example if I just wanted to write A1, the controller would write A1 and its associated parity Ap at the same time. If the application wanted to write A1 and B2 if should be able to write these at the same time because they are on different drives. It cant however because A1 needs to write its parity Ap and B2 needs to write its parity Bp at the same time and it can t because the parity drive is already in use writing Ap. Therefore the raid set will slow down. 8

9 Raid 5 Minimum Drive Count: 3 Maximum Drive Count: None Performance: Good Protection: Good - Single drive failure Efficiency: Number of drives - 1 A raid 5 is the same concept as the Raid 4 described above. It gets a better performance rating because the parity is distributed among the drives. There is still only one parity bit per stripe but the distribution of that parity reduces the chances of contention of writing parity. Using the example in Raid 4, I can write A1 and B2 at the same time because the Ap and Bp parity bits are on different drives. There is a chance of contention as the example of writing C2 and B2 however it is much less than in Raid 4. 9

Raid 6 Minimum Drive Count: 4 Maximum Drive Count: None Performance: OK Protection: Very Good - Double drive failure Efficiency: Number of drives 2 Raid 6 like raid 5 uses parity for protection.

10 Raid 6 Minimum Drive Count: 4 Maximum Drive Count: None Performance: OK Protection: Very Good - Double drive failure Efficiency: Number of drives 2 Raid 6 like raid 5 uses parity for protection. And while the first set of parity is rather simple, to get the second is a little more complicated. Needless to say it can generate what is supposed to be on two missing drives using the remaining data drives and the remaining parity drive. The formula looks something like this. Explaining this is outside of the scope of this paper. If more in depth information is needed, consult Wikipedia at the following link. The performance is the same as raid 5 given the same number of active drives but writes will be slower as the controller will have to do two sets of parity calculation for each write. Raid 6 is often used for larger hard drive capacities. Because it is not unheard of for 3TB and larger drives taking days to rebuild, the chance of a second drive failure during a rebuild sequence is too high to maintain the % uptime hurdle. Therefore raid6 and its ability to survive a dual drive failure is needed to maintain % uptime. 10

Raid 10 Minimum Drive Count: 4 Maximum Drive Count: None Performance: Very Good Protection: Very Good - Double drive failure +* Efficiency: 50% Raid 10 is the first of the raid levels that is a

11 Raid 10 Minimum Drive Count: 4 Maximum Drive Count: None Performance: Very Good Protection: Very Good - Double drive failure +* Efficiency: 50% Raid 10 is the first of the raid levels that is a combination of two raid levels together. This is called nested raid and it is the only nested raid supported by ViSX at the current time. Raid 10 gives the protection of raid 1 and the expandability of raid 0. You can see from the above figure that the we are using the striping of raid 0 over an endless set of raid 1 mirrors. Database administrators like this raid level because there is no raid calculation penalty, you can increase performance by just adding drives, and you can get over dual drive failure protection. Since this raid level is a stripe of endless mirror sets you can lose any number of drives and still maintain the data online. This is true as long as you don t lose more than one drive in a mirror set. If you lose more than one drive in a Raid 1 set the overlying Raid 0 will be failed and you will need to restore from the latest backup. 11

12 What Raid level is right for ViSX Culture change??? Now comes the question what raid level is right for ViSX. To get to this answer we will need to know some characteristics of the ViSX All Flash appliance. Consider these facts: The advertised IOP numbers of ViSX are CONTROLLER limitations. This means once we hit 6 active drives behind the controllers, adding more drives will not increase performance. Failed drives regardless of size rebuild in less than two hours. A Raid set cannot span appliances Flash drives are much less likely to fail than spinning hard drives. The answer is obvious: Raid 5 is the best choice. Correct balance of space used for protection versus usable space and great scale for capacity. Reasons Why: Raid 0 You will hit the controller limitation very quickly and you will want some protection against drive failure. This is still an option if you need very fast calculation scratch space with minimal waste of capacity for un- needed protection. Raid 1 too much of an efficiency cost. The robust nature of Flash negates the need to have two copies of data. Raid 6 The robust nature of flash and fast rebuild time negates the need for dual drive failure and the additional use of capacity for protection. Raid 10 Again the low efficiency and controller limitation on performance will negate the need for this level of protection and performance scale. Sources and more reading Wikipedia

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