Administrator Guide

The I/O characteristic types shown below are only presented during the create virtual disk process.
When you choose one of the virtual disk I/O characteristics, the corresponding dynamic cache prefetch setting and segment size that are
typically well suited for expected I/O patterns are populated in the Dynamic cache read prefetch field and the Segment size field.
To change the I/O type:
1. To enable read caching, select Enable read caching.
2. To enable dynamic cache read prefetch, select Enable dynamic cache read prefetch.
3. To enable write caching, select Enable write caching.
4. Select one of the following:
• Enable write caching with replication — Select this option to replicate cached data across two redundant RAID controller
modules that have the same cache size.
• Enable write caching without batteries — Select this option to permit write caching to continue even if the RAID controller
module batteries are discharged completely, not fully charged, or are not present.
NOTE: Cache is automatically flushed if you disable Enable write caching.
5. Click OK.
6. In the confirmation dialog, click Yes.
A progress dialog is displayed, which indicates the number of virtual disks being changed.
Thin virtual disks
When creating virtual disks from a disk pool, you have the option to create thin virtual disks instead of standard virtual disks. Thin virtual
disks are created with physical (or preferred) and virtual capacity, allowing flexibility to meet increasing capacity requirements.
When you create standard virtual disks, you allocate all available storage based on an estimation of how much space you need for
application data and performance. If you want to expand the size of a standard virtual disk in the future, you must add physical disks to
your existing disk groups or disk pools. Thin virtual disks allow you to create large virtual disks with smaller physical storage allocations that
can be increased as required.
NOTE: Thin virtual disks can only be created from an existing disk pool.
Advantages of thin virtual disks
Thin virtual disks, also known as thin provisioning, present a more logical storage view to hosts.
Thin virtual disks allow you to dynamically allocate storage to each virtual disk as data is written. Using thin provisioning helps to eliminate
large amounts of unused physical capacity that often occurs when creating standard virtual disks.
However, in certain cases, standard virtual disks may provide a more suitable alternative compared to thin provisioning, such as in
situations when:
• you anticipate that storage consumption on a virtual disk is highly unpredictable or volatile
• an application relying on a specific virtual disk is exceptionally mission critical
Physical vs virtual capacity an a thin virtual disk
When you configure a thin virtual disk, you can specify the following types of capacity:
• physical (or preferred)
• virtual
Virtual capacity is capacity that is reported to the host, while physical capacity is the amount of actual physical disk space allocated for
data write operations. Generally, physical capacity is much smaller than virtual capacity.
Thin provisioning allows virtual disks to be created with a large virtual capacity but a relatively small physical capacity. This is beneficial for
storage utilization and efficiency because it allows you to increase capacity as application needs change, without disrupting data
throughput. You can also set a utilization warning threshold that causes MD Storage Manager to generate an alert when a specified
percentage of physical capacity is reached.
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Disk groups, standard virtual disks, and thin virtual disks