by Jim Lester, Product Specialist, Dell Compellent — June 29, 2011

With the release of Red Hat Enterprise Linux 6, Dell Compellent’s already great thin provisioning just got that much better. Thanks to the adaptation of new SCSI commands defined under the SBC-3 specification by the T10 Technical Committee, Dell Compellent Storage Center can now reclaim freed space from Red Hat systems. This is possible without any agent that has to be maintained, versioned and tested on each server.

Red Hat Enterprise Linux 6 introduced the SCSI UNMAP command to the ext4 file systems to support releasing space on SAN platforms that also implemented the UNMAP command. Compellent began working on implementing the UNMAP command early, recognizing the great value it could have with our thin provisioning. Starting with Storage Center 5.4 all the necessary pieces were in place to provide free space recovery to platforms like Red Hat 6.

Since this is all built in natively to the system, users don’t have to maintain a proprietary plug-in on every system deployed. All that is needed are the built-in tools.

Step-by-Step Guide on How to Use It

1. Make a new ext4 file system and mount it using the new ‘discard’ option. This is the piece that tells Red Hat to send the SCSI UNMAP command to Storage Center when it is done with blocks of storage.
   
   
   
   
   
   
2. Throw some files onto the new file system. Below is the screen shot from the Storage Center manager showing how much space has been allocated, about 24 GB.
   
   
   
3. Compare that with the output from df. We can see that they are relatively close.
   
   
   
4. Now for the magic, delete some data off the volume and look at the file system.
   
   
   
5. We can see that the Storage Center has released the space back into the pool and is now only reserving about 15 GB of space.

This technology will allow customers to get even better efficiency out of their storage. Stop having stranded blocks allocated and not doing anything. This could also help increase the overall performance of the system as Dell Compellent’s other features like Fast Track and Data Progression can do an even better job of ensuring that the best performing space is utilized by real active data. All of this is achieved through native standards that don’t require the administrator to maintain any custom utilities on their servers. It just works, and that’s the way it should be.

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