Thin device Space reclaim

1) I have no idea what a free w.r.t.a command is.  The internal space reclaim in VMAX looks for 768k VP extents that have all zeros in them and returns those to pool as free.  If any part of the extent is not zero, it is not freed.

2) No impact on cache at all.

Hi Quincy,

Thanks for the response, however, I've a few other questions regarding the thin provisioning:

1. If a TDEV is persistently allocated, the EMC documentation says that the space reclaim is not possible using start free command as this command does not have the allocate_type=persistent tag.

2. So, I tried using start reclaim on tdev command (with allocate_type=persistent). However, the command returns a success on execution, but space does not get reclaimed in practice, so is there something that I'm missing out here.

Please advise....

There are few caveats on how reclaim works with diff apps, perhaps the tracks that you believe should be reclaim are not Zeroed?

Check this doc

https://support.emc.com/docu32785_White_Paper:_EMC_Symmetrix_VMAX_Virtual_Provisioning_Space_Reclamation_and_Application_Considerations.pdf?language=en_US

Please ignore if you have read it already

regards,

Saurabh

According to this discussion (https://community.emc.com/message/698976#698976), there is no difference in the free and reclaim commands assuming you used a start_cyl=0 and end_cyl=last_cyl on the free command.  The "start free" command is from SE 7.2 while "start reclaim" was added in SE 7.3.

If you are not seeing the space reclaimed that you expected trying running a tool to zero the free space on the drive before running the reclaim:

• Windows - sdelete
• UNIX/Linux - dd if=/dev/zero of=delete_me bs=1024 count=1024 (1 GB file of zeros)
• ESXi - vmkfstools -y 60 (fill 60% of free space with zeros)

Lastly, try running symcfg list -tdev -detail -gb to identify TDEVs with high allocation but low written GBs.  Once you find a TDEV you want to reclaim run symcfg list -tdev -detail -gb -i 10 and watch the allocated GBs drop.

The ESXi tool "vmkfstools -y" actually reclaims the space--not just writes zeros. It will issue UNMAP and reclaim the space, so zero reclaim after the fact is not required.

You are correct - good catch.  Thanks for correcting it.

Just a short addition here windows 2012 also supports the unmap, or TRIM as it's referred to.  Through the optimize drives interface you can kick this off, you can do an analyse to see what will get returned to the pool, then optimize to execute.