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Considerations for solid-state drives (SSDs)

It is important to understand controller functions when using solid-state drives (SSDs).

Hard-disk drives (HDDs) use a spinning magnetic platter to store nonvolatile data in magnetic fields. SSDs are storage devices that use nonvolatile solid-state memory (usually flash memory) to emulate HDDs. HDDs have an inherent latency and access time caused by mechanical delays in the spinning of the platter and movement of the head. SSDs greatly reduce the time to access the stored data. The nature of solid-state memory is such that read operations can be performed faster than write operations and that write cycles are limited. Using techniques such as wear leveling and overprovisioning, enterprise-class SSDs are designed to withstand many years of continuous use.

SSD usage specifications

Intermixing of SSDs and HDDs within the same disk array is not allowed. A disk array must contain all SSDs or all HDDs.
It is important to plan for hot-spare devices when using arrays of SSDs. An SSD hot-spare device is used to replace a failed device in an SSD disk array and an HDD hot-spare device is used for an HDD disk array
Although SSDs can be used in a RAID 0 disk array, it is preferred that SSDs to be protected by RAID levels 5, 6, or 10.
See Installing and configuring Solid-state drives to identify specific configuration and placement requirements related to the SSD devices.
Some adapters, known as RAID and SSD adapters, contain SSDs, which are integrated on the adapter. See the PCIe SAS RAID card comparison table for features and additional information for your specific adapter type.

RAID 0 Auto-created Disk Array

SSD devices are included in 528 bytes per sector Array Candidate pdisk format. During the controller's boot process, any SSD Array Candidate pdisk that is not already part of a disk array is automatically created as a single drive RAID 0 disk array. There are two options to change the RAID 0 disk array to a protected RAID level (5, 6, or 10):

The RAID 0 disk array can be migrated to a RAID 10 disk array using the technique described in Migrating an existing disk array to a new RAID level.
The auto-created RAID 0 disk array may be deleted (see Deleting a disk array) and a new SSD disk array created with a different level of RAID protection (see Creating a disk array).

Adapter Cache Control

Adapter caching improves overall performance by using disk drives. In some configurations, adapter caching might not improve performance when you use SSD disk arrays. In these situations adapter caching might be disabled by using the Change/Show SAS Controller display.

To disable adapter caching, complete the following steps:

Navigate to the IBM® SAS Disk Array Manager by completing the steps in Using the Disk Array Manager.
Select Diagnostics and Recovery Options.
Select Change/Show SAS RAID Controller.
Select the IBM SAS RAID Controller on which to disable caching.
Select Adapter Cache and change the value to Disabled. The display looks similar to the following example.

+--------------------------------------------------------------------------------+

|                           Change/Show SAS Controller                           |

|                                                                                |

|Type or select values in entry fields.                                          |

|Press Enter AFTER making all desired changes.                                   |

|                                                        [Entry Fields]          |

|  SAS adapter                                         sissas0                   |

|  Description                                         PCIe2 1.8GB Cache RAID>   |

|  Status                                              Available                 |

|  Location                                            0C-08                     |

|  Maximum Number of Attached Devices                  512                       |

|  Maximum number of COMMANDS to queue to the adapter  100,300,0                 |

|  Maximum Data Transfer Window                        0x1000000,0x5000000,0x>   |

|  Operating mode                                      Primary Adapter           |

|  Adapter cache                                       Disabled                 +|

|  Preferred HA Dual Initiator Operating mode	          No Preference            +|

|  Dual HA Access State Setting                        Preserve                 +|

|  Dual Initiator Configuration                        Default                  +|

|  Serial Number                                       YL3229016F9F              |

|  World Wide ID                                       5005076c07445200          |

|  Remote HA Link Operational                          No                        |

|  Remote HA Serial Number                                 						            |

|  Remote HA World Wide ID                             									          |

|  Attached AWC Link Operational                                                 |

|  Attached AWC Serial Number                                                    |

|  Attached AWC World Wide ID                                                    |

|                                                                                |

|F1=Help             F2=Refresh          F3=Cancel           F4=List             |

|F5=Reset            F6=Command          F7=Edit             F8=Image            |

|F9=Shell            F10=Exit            Enter=Do                                |

+--------------------------------------------------------------------------------+

JBOD Workload Optimization: Additional optimization parameters are available through the sissasraidmgr utility for the PCIe RAID and SSD SAS adapter 3 Gb x8 (FC 2053/2054/2055; CCIN 57CD) when using SSD module CCIN 58B2. The sissasraidmgr provides a command line function which may enhance I/O performance for some types of I/O workloads. See table 1 in AIX® command line interface to see the complete command string. The command for workload optimization is used to prepare JBOD-only formatted SSDs (512 byte) for this performance tuning. The I/O response time optimization may provide improved I/O performance for response-time sensitive workloads. The default I/O per second optimization is generally preferred for higher command throughput (I/O per second). The JBOD workload optimization setting can be returned to its default value by either executing an I/O per second workload optimization command, recycling the power on the PCIe RAID and SSD SAS adapter, or downloading firmware to the SSD. It is recommended that a script be established to ensure the desired JBOD workload optimization parameters are activated whenever one of these actions occurs.

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Last updated: Thu, October 10, 2013