Ask the Expert

The current state of tape SAN failover

What are your thoughts on tape drive failover in a SAN (open system HBAs, HBA drivers, operating systems and application failover a tape writing process to an alternate HBA and SAN path to keep that process running)?

Co-workers and customers are always looking for disk SAN failover capability in a tape SAN but I do not think such a capability exists without system administrator intervention and environment reconfiguration). What is the current state of tape SAN failover? Thank you for your help.

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Yeah, this has been an issue for a while now. Writing to tape is a bit different than writing to disk. The data flow is more of a stream rather than bursty blocks. If the data stream to the tape is broken, the tape usually has to reposition itself back over the tape head to continue. If a path fails, the session between the tape drive and the writing host is lost and you have to restart the job. That's why most backup engines show status of all jobs going to tape in near real-time.

I know most of the enterprise class backup software solutions will allow you to automatically failover a job to another media server if your primary media server goes away but the job is usually restarted from scratch. Although a solution may now exist that allows path failover of existing backup jobs, I am not personally familiar with one. Perhaps W. Curtis Preston, the backup expert on this site, can shed more light on this subject!


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This was first published in March 2003

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