LaCie 131018 - StorView Snapshot - Mac, User Manual

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C.7.2 Operating System Cache
The second highest level of caching disk data is provided by the operating system (OS). The OS will
dedicate a large percentage of its DRAM space to be used for disk data caching. It is required to ask the
OS to flush its entire disk cache just before the snapshot is issued.
Refer to your operating system user guide for more information.
C.7.3 RAID Controller Cache
The RAID Controller(s) implement large caches for disk data. A large portion of these caches are used
as delayed writeback caches for holding data blocks not yet written to the disk drives. It is very important
to have the RAID Controller(s) perform a flush of its cache before a snapshot is started.
C.8 Virtual LUNs
Virtual LUNs are also known as snapshot LUNs and can be made available for access through a
dynamically created LUN. Reads issued to a virtual LUN will be serviced in a similar way to reads issued
to a normal LUN. The data will be the exact data as it were at the time of the snapshot. This allows an
application, such as disk backup software to perform a backup operation of the Virtual LUN.
A virtual LUN will respond to any type of incoming SCSI command in the same way as the original logical
drive. The exception is with write commands, since the snapshots represent frozen data in time, it is not
allowed for the host to modify this data.
C.8.1 Virtual LUN Numbering
Each Virtual LUN will be presented to the host side as a regular logical drive (LUN). A logical unit number
will be assigned and used by the accessing host. These LUN numbers are automatically assigned.
Up to eight (8) ODAs can be defined within the array. Therefore the ODAs are numbered 0-7. Each ODA
can store up to 24 Snapshots, so the Snapshot number will range between 0-23. The results is the
possible range for the Virtual LUNs are 0-192.
The side effect of LUN numbering is a conflict with a normal LUN is high. Therefore, the virtual LUNs are
not automatically made visible to the host system. This is accomplished using LUN mappings and each
Virtual LUN is manually assigned a LUN number that will then become visible to the host.
Example: a disk array with three normal data LUNs configured as LUN 0-2 and one ODA, will have its
first Virtual LUN numbered 0. To avoid a conflict between the original data LUN 0, the Virtual LUN will
have to be LUN mapped to a non-conflicting LUN number, e.g., LUN 10. All accesses to LUN 10 will then
be translated into Virtual LUN 0 internally by the RAID Controller.