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First, make a schedule where the rows are SAN links. Assign each target's backup window to a link and the estimated transfer rates for each backup window are combined to show what the instantaneous load is on any link in the system. If the load exceeds 75% of the link speed on any FC link, expect performance degradation due to resource contention. You'll need to add more links, change the link assignment or reschedule the backup windows.
Next make a router port schedule, where the rows of the schedule represent storage router SAN ports. Assign each target's backup window to a storage router port that can be connected to the SAN link identified in the SAN link schedule. If the load exceeds 70MB/s through any router port, consider making adjustments in the hardware configuration or the timing of your backup windows.
Each backup stream will be processed by the storage router and assigned to a SCSI bus. As in the previous schedules, make sure the SCSI bus can make a connection to the storage router SAN port. In general, this won't be a problem, but it's a detail that needs verification.
Traffic maximums on the SCSI bus can be conservatively estimated at 70% of the rated bus speed. If the backup load exceeds that, then you'll probably want to make hardware or schedule adjustments.
The last resource schedule
is for tape drives. The point of this schedule is to see if there are any conflicts over any individual tape drives. Tape drives can be assigned to different backup targets at different times, as long as there's a good way to ensure the proper tapes can be loaded.
It's preferable for different targets on the same server to have backup windows that don't overlap - especially if high-speed tape devices are being used. This prevents the system's I/O capabilities from being a surprise bottleneck. Looking for these types of system bottlenecks doesn't require a separate analysis schedule as they can be determined by analyzing the master operations schedule.
As adjustments are made in any of the resource schedules, they need to be reflected in all the others. Each update to a specific resource schedule should cause a recalculation of the workload for that resource to ensure that the bottleneck is being removed from the system and not merely transferred to another resource.
After working through the detailed resource schedules, you can create a finalized LAN-free backup schedule and SAN design. One of the benefits of working through resource schedules is that it determines how many resources you'll need to incorporate in your LAN-free backup system. You'll also have some reasonable ideas of which resources you are likely to need as your SAN expands.
Of course, the final schedule and SAN design will change over time as the system environment changes. You should monitor daily operations on a regular basis to identify problems that may be occurring.
Beyond hardware and media errors, the place to look for impending problems with your LAN-free backup system is whether or not your backup windows are still longer than the actual backup times. The concept of the headroom in a backup window may be helpful to some readers.
As the amount of data increases, the backup windows will be exceeded, creating overload situations and bottlenecks in the SAN. When that happens, it may trigger another iterative round of schedule juggling or additional resource purchases to allow the backup performance to stay in compliance with your backup goals.
Moves and changes
Resource overloads can occur suddenly if applications are relocated from one server to another. For instance, an application that's moved from one system to another could change the amount of data in the receiving system's file system, creating a sudden change in the workload. This workload change could cause the backup target's actual backup time to exceed the time allowed by the backup window.
Similarly, the installation of new applications can also cause significant changes to the backup system. Generally, new installations won't be as immediately noticeable as a moved application because it normally takes some time for the new application's data to grow.
LAN-free backup can solve numerous backup problems currently in your computing environment. As you plan your LAN-free backup solution, you need to understand the various constraints to backup performance, which can include system I/O performance in addition to tape and network performance.
To make LAN-free backup work, you need to assign realistic backup windows to each backup target and to manage the scheduling of these time containers within a master operations schedule. The master schedule is used to create several derivative resource schedules which can be quickly viewed and analyzed to determine if backup has gone out of compliance and whether or not these changes are likely to result in system overloading and performance degradation.
This was first published in September 2002