Dell PowerVault ML6000 - Manual (Page 19)

PowerVault ML6000 SCSI Reference Guide 9
Logical Libraries
The underlying physical library is not exposed externally to applications. Rather, logical representations of
one to 18 Media Changer devices are created, and these are presented instead. Through this method the
physical library can be partitioned and concurrently shared in a heterogeneous environment. Storage and
Data Transfer elements cannot be shared across logical libraries; they can only be assigned to one logical
library at a time. The Medium Transport element (the robotic mechanism) is shared across all logical
libraries, and as a result there may be some delays encountered as each logical library waits its turn for this
shared resource.
Import/Export elements can also be shared across logical libraries. This is further discussed in Mailbox
Behavior . All other aspects of the logical Media Changer devices are identical to an independent physical
Media Changer.
Mailbox Behavior
The following characteristics affect Import/Export elements:
• Some Dell libraries are configured with multiple physical mailboxes, each containing their own set
of magazines. Whenever a mailbox is opened, the status for the elements it contains will indicate
that they are not accessible until the mailbox is closed again.
• The mailbox magazine(s) can be assigned to and shared by different logical libraries. The Import/
Export elements they contain are then used on a “first come, first served” basis. When shared
Import/Export elements are in use by one logical library, element status for those elements will
indicate that they are empty, but not accessible for all other logical libraries that share them. The
presence of media and associated volume tag information will only be available to the logical library
using the elements at that time. When media is removed from the shared elements, they become
available for use by the next logical library requesting them, and their element status indicates that
they are accessible.
These characteristics require applications to process the complete element status returned in the element
descriptors (including accessibility and exception conditions) to achieve optimum usage of the Import/
Export elements. Reliance on only full or empty element status may result in failed operations (e.g., an
Export). This might be due to not locating a usable Import/Export element when several may actually be
available, if the search had only taken into account full status rather than accessibility.
Autocleaning
Dell libraries support a user-configurable option to have the library automatically clean drives when
requested by the drives (refer to the specific Dell library documentation for details). This process involves
the library loading a special cleaning cartridge into the requesting drive, allowing the drive to perform the
cleaning operation, and then the library unloading the cartridge and returning it to storage.
If this option is enabled and configured, the library maintains a pool of storage slots that contain the cleaning
cartridges. These slots and cartridges are not associated with or counted towards any logical library, and
as such are not reported to any application.
The library typically checks for cleaning requests from drives after unloading data cartridges. If a cleaning
request is found, the library will select a cleaning cartridge from the pool and perform the clean. While the
cleaning operation is in progress, the logical library containing the drive being cleaned will continue to
accept and perform SCSI commands. If a Move Medium command is received with the drive as a
destination, and it is still being cleaned, the command will be queued until the cleaning operation completes.
Cleaning operations vary by drive and conditions, but can take up to a few minutes to complete.