iSCSI Disk Management and Usage
42
Snap Server Administrator Guide
Isolate iSCSI Disks from Other Resources for Backup Purposes
It is important to isolate iSCSI Disks from other resources on the Snap Server for
two reasons: (1) the file system of an iSCSI Disk differs fundamentally from the
Snap Server’s native file system; and (2) iSCSI Disks are managed from client
software rather than the Snap Server’s Administration Tool. For ease of
management and particularly for data integrity and backup purposes, either
dedicate the entire Snap Server to iSCSI Disks, or if the server is to be used with
other shared resources, place the iSCSI Disk and the other shared resources on
separate volumes.
• Backup an iSCSI Disk from the Client, not the Snap Server —
An iSCSI Disk is not
accessible from a share and thus cannot be backed up from the Snap Server. The
disk can, however, be backed up from the client machine from which the iSCSI
Disk is managed.
Tip
While some third-party, agent-based backup packages could
technically
back
up an iSCSI Disk on the Snap Server, the result would be inconsistent or
corrupted backup data if any clients are connected during the operation. Only the
client can maintain the file system embedded on the iSCSI disk in the consistent
state that is required for data integrity.
• Do not use snapshots on a volume containing an iSCSI Disk —
Running a
snapshot on a volume containing an iSCSI Disk will abruptly disconnect any
clients attempting to write to the server’s iSCSI Disk and the resulting snapshot
may contain inconsistent data.
iSCSI Multi-Initiator Support
The Support Multi-Initiator checkbox allows two or more initiators to
simultaneously access a single iSCSI target. Multi-Initiator Support is designed for
use with applications or environments in which clients coordinate with one another
to properly write and store data on the target disk. An environment such as
clustering avoids potential data integrity issues. Data corruption becomes possible
when multiple initiators write to the same disk in an uncontrolled fashion.
The warning message “Uncontrolled simultaneous access of multiple initiators to
the same iSCSI target can result in data corruption. Only enable multi-initiator
support if your environment or application supports it.” occurs when the checkbox
for Support Muli-Initiator is selected. It functions as a reminder that data corruption
is possible if this option is used when creating an iSCSI disk.
Write-Cache Options with iSCSI Disks
To ensure the fastest possible write performance, Snap Servers can buffer up to 1GB
of data to efficiently handle data being transmitted to a Snap Server. As every
administrator knows, this widely accepted method of improving performance is not
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