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, 10, 5T2, 6T2, or 10T2.
- 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.
- SSDs are supported only when formatted to a RAID block size and used as part of a RAID array.
- RAID 0 Auto-created Disk Array on a PCIe or PCIe2 SAS RAID Controller
During the controller's boot process, any 528 bytes per sector (not 4224 bytes per secotor) SSD Array Candidate pdisk attached to a PCIe or PCIe2 SAS RAID Controller, 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).
- Certify Media Diagnostics Task
- The Certify Media Diagnostics Task is not useful when it is run after formatting modern SSD drives. The Diagnostics Format Task might suggest performing a Certify Media Task against the drive to validate whether all the media is readable. However, all IBM enterprise-class SSDs that have been sold after 2011 clear their internal directory on a format and mark all physical data storage as unused. Performing Certify Media after this point will not cause the SSD to actually read any physical data storage due to the directory not pointing to any used storage.
- 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.
- 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.
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| 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 |
+--------------------------------------------------------------------------------+