Dell PowerVault MD3000i Dell PowerVault MD3000/MD3000i Array Tuning Best Pract - Page 12

Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices 4.6 Cache Settings Read-ahead cache can be configured in the MDSM and through the CLI. The MDSM has only the defaults to work with, while the CLI can fully configure the read-ahead cache. Additionally, the global cache block size for read and write cache can be adjusted through the CLI. Please refer to the Dell™ PowerVault™ Modular Disk Storage Manager CLI Guide on the Dell™ technical support site (http://support.dell.com/manuals) for a complete list of supported commands including the following cache specific commands. 4.6.1 Setting the Virtual Disk-Specific Write Cache and Write Cache Mirroring Configured through the CLI - These commands are available at the virtual disk level. Write Cache - Disabling write cache puts the controllers into a write-through mode, adding additional latency while data is flushed to the disk. Except for specific read-only environments, it is recommended that this setting stay enabled. Write cache is automatically disabled in the case of cache battery failure or a cache battery learn cycle. Write Cache Mirroring - Write Cache Mirroring provides an additional level of redundancy and fault tolerance in the MD3000 and MD3000i. As a side effect, it reduces available physical memory and intra-controller bandwidth to perform this operation. In select, non data-critical cases, it can be beneficial to adjust this parameter. For normal use, Dell™ always recommends Enabling Cache Mirroring. Cache Mirroring is automatically disabled in the event of controller failure or when write caching is disabled. WARNING: Data loss can occur if a RAID controller module fails while writecaching without cache mirroring is enabled on a virtual disk. 4.6.2 Setting the Virtual Disk-Specific Read Cache Pre-fetch Configured through the CLI - This command is available at the virtual disk level. Read Cache Pre-fetch - The Read Cache setting can be toggled on a virtual disk level. Disabling read pre-fetch is primarily useful in primarily small transfer size random read environments, where pre-fetch of random data would not provide sufficient value. However, the normal observed overhead of read pre-fetch is negligible. For most environments, Dell™ always recommends Enabling Read Pre-fetch. December 2008 - Revision A01 Page 12