28 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
2.5.11  Read Intensive Flash Drives
Generally, two types of SSDs for Enterprise Storage are available: The Multi-level cell (MLC) 
and Single-level cell (SLC).
The most common SSD technology is MLC. They are found in consumer products, such as 
portable electronic devices. However, they are also present in some enterprise storage 
products. Enterprise class SSDs are built on mid- to high-endurance multi-level cell flash 
technology, known as mainstream endurance SSD.
MLC SSDs uses multi-cell to store data and feature the Wear Leveling method, which is the 
process to evenly spread the data across all memory cells on the SSD. This method helps to 
eliminate potential hotspots caused by repetitive Write-Erase cycles. 
SLC SSDs use a single cell to store one bit of data, which makes them faster.
To support particular business demands, IBM Spectrum Virtualize qualified the use of Read 
Intensive (RI) Flash Drives (based on SSD technology) with applications where the read 
operations are significantly high.
RI Flash Drives are available as an optional purchase product to IBM SAN Volume Controller 
and IBM Storwize Family.
2.5.12  Managed disks
A managed disk (MDisk) is the unit of storage that IBM Spectrum Virtualize virtualizes. This 
unit might be a logical volume on an external storage array that is presented to the IBM 
Storwize V7000, or a RAID consisting of internal drives. IBM Spectrum Virtualize can then 
allocate these MDisks into various storage pools. An MDisk is not visible to a host system on 
the storage area network because it is internal or zoned only to the IBM Storwize V7000 
system.
An MDisk has the following modes:
/SM590000Array
Array mode MDisks are constructed from drives by using the RAID function. Array MDisks 
are always associated with storage pools.
/SM590000Unmanaged
Unmanaged MDisks are not being used by the system. This situation might occur when an 
MDisk is first imported into the system, for example.
/SM590000Managed
Managed MDisks are assigned to a storage pool and provide extents so that volumes can 
use it.
/SM590000Image
Image MDisks are assigned directly to a volume with a one-to-one mapping of extents 
between the MDisk and the volume. This situation is normally used when importing logical 
volumes into the clustered system that already have data on them, which ensures that the 
data is preserved as it is imported into the clustered system.