Troubleshooting
Problem
Symptom
Environment
Resolving The Problem
The latest generation of IBM Serial-Attached SCSI (SAS) adapters is optimized for disk operations with data aligned on 4096-byte boundaries. This design produces a noticeable degradation of the performance of disk requests with data that is not aligned on such a boundary. Specifically, virtual disk units provided to an IBM i client partition from a VIOS partition that is using SAS hdisks as the backing storage devices experience this performance degradation. In this type of configuration the 4608-byte-aligned I/O requests initiated by the IBM i partition are passed to the SAS adapter with no change in alignment, resulting in less than optimal performance.
HOW TO CONFIRM
RUN AA macro
dumpld -allcfg
If this shows "HAS 512 BYTE SECTORS" TRUE then this is the cause for the POOR PERFORMANCE
HOW TO FIX
VIOS Configuration
Creating RAID Arrays
For an IBM i configuration with VIOS, a virtual disk that supports 520-byte sectors can be created only from a physical volume (“hdisk”, in VIOS terminology) that represents a SAS RAID array. Each eligible hdisk must be created from one or more physical SAS disk devices (“pdisks”) that have been grouped together into a RAID array. Prior to being added to a RAID array, each pdisk must be formatted to use a RAID block size to make it capable of storing the additional 8 bytes of data per sector needed by the IBM i client partition. Virtual target devices backed by other types of disk storage -- such as logical volumes, logical devices from a shared storage pool, or other physical volume types -- do not support 520-byte sectors.
In the simplest configuration, one physical pdisk may be used to create a RAID 0 array which produces a single hdisk that will then be capable of supporting 520-byte sector I/O. Multiple pdisks may also be grouped together to produce an hdisk with higher levels of protection, if desired.
A pdisk that is not added to a RAID array, but is instead formatted as a standalone SAS hdisk, will not support the IBM i 520-byte sector format.
See the InfoCenter topic on SAS RAID Controllers for AIX for more information on managing SAS RAID arrays in the VIOS.
Creating Virtual Disks
Once a SAS RAID array hdisk has been created, a virtual disk can be constructed from that device and assigned to a server vhost device using the mkvdev command. For example, the following command creates a virtual disk named “vtscsi0” from a physical volume named “hdisk0” and attaches the virtual device to virtual adapter “vhost0”:
$ mkvdev –vdev hdisk0 –vadapter vhost
vtscsi0 available
See the InfoCenter topic on VIOS and IVM Commands for more information on the mkvdev command.
Only virtual disk devices created from a RAID array on a supported SAS adapter with the appropriate microcode and software levels will be seen by the IBM i client partition as 520-byte sector capable disk units. All others will remain 512-byte sector disks.
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Document Information
Modified date:
12 October 2024
UID
nas8N1019845