Virtual Strategy Mag

Benefits of Recycling Dead SAN Space: Interview with Virtual Strategy Magazine

Greg Hayes, Raxco’s Product Manager, talks to Virtual Strategy Magazine about the importance of recycling expensive SAN space and how it can improve your organization’s bottom line.


VSM: What exactly is SAN space reclamation?

GH: Everyone is familiar with the fact that deleted data can still be read on a PC or server disk and the same is true in a SAN. When a user deletes a file on a guest system Windows deletes the pointers to that file in the index file on the appropriate volume. With Windows 2003/2008 Server nothing tells the SAN that the blocks the file occupies on-disk are no longer needed. The space consumed by the deleted file is now “dead” space since it no longer has a valid file pointer. This space cannot be reused by the storage controller until it is reclaimed to the storage pool.

VSM: Don’t the storage vendors provide a mechanism to reclaim this space?

GH: Yes and no. Most modern SANs support one of two mechanisms for reclaiming space: Zero Detection or UNMAP recognition. Zero Detection is built into the storage hardware and reclaims space when zeros are written to the dead clusters. UNMAP is a command the storage recognizes and frees up the space. However, before any space can be reclaimed on the storage the candidate space needs to be identified in the guest systems. The storage vendors do not provide any tools that run on the guest to identify the reclaimable space; this is the gap PerfectStorage fills.

Read more at Virtual Strategy Magazine: Automate Space Reclamation on the SAN: Interview with Greg Hayes, Product Manager for Raxco Software

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