Dell PS Series Storage: Choosing a Member RAID Policy This paper describes the supported RAID policies for Dell PS Series arrays and provides information on the RAID policies under normal and failure conditions to help administrators choose the appropriate RAID policy.
Revisions Version Date Description 1.0 2007 Initial Release 4.1 2009 Added RAID 6 4.2 June 2012 Added large disk recommendations and RAID information for 24 disk chassis 4.3 August 2012 Added RAID configuration recommendations and changes reflected in PS firmware v6.0 4.4 April 2013 Added info regarding improved RAID reliability score in SANHQ 4.5 December 2014 Added 1 TB Raid Policy recommendation 4.6 December 2015 Added PS6610, PS4210, large drive RAID 6 enforcement 4.
Table of Contents 1 Introduction to member RAID policies ..........................................................................................................................4 1.1 2 2 RAID policy descriptions .....................................................................................................................................5 RAID policy availability and performance comparisons ...............................................................................................6 2.
Executive summary One of the most important decisions to make when deploying a new storage solution is which RAID type(s) to use. Each RAID type has its advantages and disadvantages in terms of protection, efficiency (usable storage), and performance—both during normal operations and in terms of rebuild time. Regardless of the storage solution, RAID decisions will be based on the same set of criteria.
1 Introduction to member RAID policies PS Series arrays include redundant, hot-swappable components- for example physical disks, control modules, fans, and power supplies - for a no-single-point-of-failure configuration. Along with redundant hardware, PS Series arrays support several different RAID types with each type optimized to maximize performance of the PS SAN architecture. When an array is added as a member to a PS Series group, the RAID policy for that member is chosen.
1.1 RAID policy descriptions The following RAID policies are available using the PS Series Group Manager GUI or the CLI: RAID 6 – One or more RAID 6 sets, with one or two spare disks. RAID 6 delivers multiple parity protection and provides the highest availability of any of the supported RAID policies because it can accommodate two drive failures at any one time. RAID 6 offers reliable performance for workloads consisting mainly of sequential reads and writes, or random reads.
2 RAID policy availability and performance comparisons Although all RAID levels provide good performance and data protection, there are some differences. When choosing a member RAID policy, the performance and availability needs of the workload should be identified to select a RAID policy that meets those needs. If the workload has mixed requirements in terms of performance and availability, consider mixing RAID types in a multi-member group.
12.0 10.0 8.0 6.0 4.0 2.0 0.0 400 GB SSD 800 GB SSD 600 GB 15K 1.2 TB 10K 1 TB 7.2K 3 TB 7.2K 4 TB 7.2K 6 TB 7.2K -2.0 RAID 6 RAID 10 RAID 50 RAID reliability scores for different PS Series disk types NOTE: The y-axis is the relative average reliability score for several PS Series disks. RAID 5 is deliberately left off this graph to reflect current PS Series best practices. Key observation: RAID 6 demonstrates a much greater reliability than other RAID types.
Excellent = 1.19 or greater Fair = 0.x to 1.18 Poor = 0 and below SANHQ representation of array reliability SAN Headquarters RAID Policy Reliability Score The RAID policy reliability score in SAN Headquarters is calculated at the member level from the drive speed, size, disk mean time between failure (MTBF) hours, controller type, RAID policy, RAID geometry (number of disks in each RAID set), workload, and rebuild times. These factors result in a calculated score ranging from approximately –5.
2.2 RAID policy impact on performance A PS Series group provides storage to a broad range of environments and delivers good performance for a variety of workloads and applications, regardless of the RAID level on the member. However, for some applications, choosing the correct RAID level can make a difference in performance, under both normal operating conditions and failure conditions.
Corner testing results Key Observation: As depicted by Figure 3, PS Series had considerable variation in performance amongst the RAID levels for random write I/O, as compared to the other three corners. Generally speaking, RAID 10 is very effective with applications that display small, random write intensive I/O, whereas RAID 6 pays a slight performance penalty for writing out the two parity data to two different disks.
160% 140% 120% 100% 80% 60% 40% 20% 0% RAID 6 RAID 10 RAID 50 RAID 5 50/50 Random 81% 151% 100% 106% 50/50 Seq 82% 99% 100% 92% Relative performance by RAID policy Key Observation: RAID 10 still offers the best random performance. Another factor to consider, when comparing the performance of different RAID levels is the performance impact of an individual RAID set operating in a degraded mode.
2.3 With RAID 10, data reconstruction involves only two disks (one active and one spare) out of the total number of disks in the array, so the overall performance impact is minimal. With RAID 5 and RAID 50, data reconstruction involves multiple disks, so the overall performance impact is significantly higher than other RAID types. For example, reconstruction in a six-disk RAID set involves six disks (five active and one spare).
If a set is degraded: Data is reconstructed on the new disk and after reconstruction, performance returns to normal. 2.4 Examples of data protection with different RAID policies The following table and figures show the results of a 24-disk, PS6210 series array responding to failures in the same RAID set based on the member RAID policy. Note: The figures show a logical disk layout when an array is initialized for the first time.
Surviving disk failures with RAID 6 as the member RAID level Example of RAID 6 surviving a double disk failure in each RAID set 2.4.2 RAID 10 The following figures show how using RAID 10 can increase (but cannot guarantee) the chance that an array can survive multiple disk failures in a 24-disk, PS6210 Series array. RAID 10 can survive only one simultaneous disk failure per RAID 1 mirror but allows multiple disk failures in a single array as long as the failed disks fall in different mirrored sets.
Surviving disk failures with RAID 10 as the member RAID policy 2.4.3 RAID 50 The following figures show how RAID 50 provides more capacity than RAID 10 while continuing to provide protection from disk failure and good performance and availability for a 24-disk, PS6210 Series array. RAID 50 has less availability than RAID 10 due to fewer RAID sets that make up the overall RAID 50 policy.
2.5 Relative capacity utilization by RAID policy Selection of the RAID policy impacts the capacity utilization of the RAID set in the array. Some RAID levels are more efficient and provide greater usable capacity for a given amount of raw capacity. The graph below illustrates the relative capacity utilizations of the various RAID policies supported by PS Series arrays. These metrics are normalized to RAID 50 (which is depicted as 100%).
3 Setting the member RAID policy When a new group is created, or a new member is added to an existing group, the RAID policy must be selected before the space on the member is available. Once the RAID policy is select the disks are automatically configured according to the designated RAID level, with the appropriate number of spare disks. Note: Starting with PS Series firmware v6.0.
4 Displaying the RAID level space distribution To display the RAID level distribution for PS Series group members, open the Group Manager GUI. In the Group Disk Space panel, select the RAID level space distribution button. A pie chart displays the RAID level distribution. PS Series Group RAID level 18 Dell PS Series Storage: Choosing a Member RAID Policy | TR1020 | v 4.
5 PS Series array disk layout To display the disks in a PS Series array, click: Members > member_name > Disks tab The Member Disks window displays a graphical representation of the disks in an array and also the status of each disk. The following figure shows an example of the member Disks window for a PS6610ES array with 14 x 400 GB SSD disks and 70 x 2 TB NLSAS disks, using RAID 6 accelerated, with one spare for each disk type.
6 Converting or migrating from a member RAID policy There are different methods available to convert or migrate from one RAID policy to another with PS Series arrays. These options can include: 6.1 Online conversion Array evacuation Online volume migration Backup and restore Large disk policy for RAID 50 and RAID 10 Beginning with version 7.0.0 firmware the Group Manager GUI and the CLI allow users to configure RAID 50 for members using disks with a capacity of less than 3 TB only.
Migration using Group Manager To convert the member RAID policy, open the Group Manager GUI and click: Members > member_name > Modify RAID configuration The Modify RAID Configuration wizard displays the current RAID policy radio button selected. Note: If a certain conversion option is not allowed, it will appear grayed out.
Selecting RAID policy 6.3 Array evacuation If the current RAID policy does not support an online conversion and the group has other members with sufficient free space, a member can be converted to any other RAID policy by removing the member from the group (which resets the array) and adding it back to the group, then selecting a different RAID policy. The following RAID policies must use an offline migration method: 6.
Once those criteria have been met, the volume can be migrated to another storage pool by choosing Move volume option in the Group Manager GUI and selecting the desired storage pool. Move volume window 6.4.2 RAID preference The other migration option is to set the RAID preference for the volume. This can be done using the Modify Settings option for the volume. Once in Modify Settings, choose the Advanced tab and set the RAID preference for the volume.
Modify volume settings window 6.4.3 PS Series replication PS Series replication can be used to migrate volume data. Volumes can be migrated using either standard asynchronous replication or in firmware version 6.x and higher synchronous replication. Asynchronous replication moves data between PS Series groups while synchronous replication moved data between storage pools within the same PS Series group.
7 Summary Understanding and using proper RAID levels can be beneficial from a performance perspective and crucial for protection against failures in business critical environments. Dell recommends choosing RAID policies based on evolving storage technologies and protecting storage needs of customers. PS Series arrays allow administrators the flexibility to choose and modify RAID configurations that best meet the needs of their data center applications.
A 12-Disk PS Series array RAID configurations PS4210, PS4100 and PS4110 12-disk RAID Set Relationship RAID Policy Spare Disks RAID Set Relationship Best Practice RAID 6 1 Spare Disk (9+2) Yes RAID 10 2 Spare Disks (5+5) No for HDD or SSD greater than 6 TB. RAID 50 2 Spare Disks (4+1)(4+1) For selected configurations1 RAID 5 1 Spare Disk (10+1) Not for business critical data 1 RAID 50 is not recommended for arrays with 1 TB or larger disks.
B 14-Disk PS Series array RAID configurations The following table shows the blade version of the PS Series arrays. PS-M4100 and PS-M4110 14-disk RAID Set Relationship RAID Policy Spare Disks RAID Set Relationship Best Practice RAID 6 1 Spare Disk (11+2) Yes RAID 6 Accelerated 1 Spare HDD (6+2 HDD) (6+2 SSD) N/A RAID 10 2 Spare Disks (6+6) No for HDD or SSD greater than 6 TB.
C 16-Disk PS Series array RAID configurations The following figures shows the response of a 16-disk, PS6000 Series array to failures in the same RAID set, based on the member RAID policy. The figures show a logical disk layout. The actual physical layout of disks can change due to administrative actions. It is not possible to determine which disks are associated with each RAID set.
Surviving disk failures with RAID 50 as the member RAID policy C.2 RAID 5 Figure 18 shows that RAID 5 provides capacity and protection from disk failure but less availability for a 16disk, PS6000 array. RAID 5 provides less availability because there is only one set and one spare. This level of RAID is not recommended for any production PS Series arrays. In the following figure, the member RAID policy is RAID 5 with spares. Spares and RAID protect the data.
Surviving disk failures with RAID 6 as the member RAID level 30 Dell PS Series Storage: Choosing a Member RAID Policy | TR1020 | v 4.
D 48-Disk PS Series array RAID configurations RAID set relationships for a 48 disk PS Series 65xx array RAID Policy Spare Disks RAID Set Relationship Best Practice RAID 6 1 Spare Disk (12+2) (12+2) (12+2) (3+2) Yes (12+2 HDD) (12+2 HDD) (10+2 HDD) (5+2 SSD) N/A1 RAID 6 Accelerated 1 1 Spare HDD2 RAID 10 2 Spare Disks (7+7) (7+7) (7+7) (2+2) No for HDD or SSD greater than 6 TB.
E 42 or 84-disk PS Series RAID configurations Dell Storage PS6610 arrays are offered in 84 or 42 drive configurations. The hybrid version of the PS6610 is only available in the 84 drive configuration. The following tables show the RAID relationships for these configurations. RAID set relationship for an 84 drive PS Series 6610 array. 1 2 RAID Policy Spare Disks RAID Set Relationship Best Practice RAID 10 2 Spare Disk (7+7) (7+7) (7+7) (7+7) (7+7) (6+6) R10- No for HDD or SSD greater than 6 TB.
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