Dell PowerVault MD3820f - Manual (Page 127)

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Using SSD Cache
The SSD Cache feature utilizes solid-state disk (SSD) physical disks to improve read-only performance in
your storage array. SSD physical disks are logically grouped together to provide secondary cache for use
with the primary cache in the RAID controller module memory.
Using SSD Cache improves application throughput and response times and delivers sustained
performance improvement across diverse workloads, especially high-IOP workloads.
How SSD Cache Works
Following a host read, data is stored in DRAM and is copied from user-specified base virtual disks and
stored on two internal RAID virtual disks (one per RAID controller module). These virtual disks are
automatically created when you initially set up an SSD cache. Neither virtual disks is accessible for read-
write operations and cannot be displayed or managed in the MD Storage Manager interface.
Simple virtual disk I/O mechanisms are used to move data to and from the SSD cache.
Storing the data on the SSD cache eliminates the need for repeated access to the base virtual disk.
However, both SSD cache virtual disks count against the number of virtual disks supported on the storage
array.
Benefits Of SSD Cache
Benefits of using the SSD Cache feature varies depending on your system configuration and network
environment. However, workloads that typically benefit the most from using high-performance SSD
cache include:
• Workloads where performance is limited by physical disk input/output processes (IOPs).
• Applications that generate a much higher percentage of physical disk reads versus physical disk writes.
• Repeated reads to the same and/or adjacent areas of the physical disk.
• Overall data accessed by an application is routinely less than potential SSD cache capacity. To
determine whether this is the case, reviewing the number of virtual disks and sizes that are cached
often yields a reliable estimate. The more virtual disks that are cached, the more likely it is that your
application accesses more data capacity than can be configured in SSD cache.
Choosing SSD Cache Parameters
When you create an SSD cache, you can choose which I/O type best matches your applications:
• file system
• database
• web server
You also have the following options:
• capacity of the SSD cache from a list of possible candidates consisting of different counts of SSD
physical disks.
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