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About UNMAP feature in ABS

  • 18 July 2017
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Hello Nutanix support,

I have some questions related to ABS Best Practice Guide mentioning about UNMAP

As I am not quite familiar with UNMAP, would you please make clear to me about this support in ABS?

1. Follow my understanding about UNMAP, the client will reclaim the space after a file is deleted in mounted vDisk automatically in both Windows and Linux clients? OR do I need to execute any command manually to do that?
2. The reclaimed space will be the free space in mounted vDisk? I really confused as this step because UNMAP leads me to an HDDs behavior in which the disk actually not deletes the deleted files and this UNMAP command is used to reclaim those deleted files space in HDD tiers?
3. The free space in mounted vDisk is same between before and after executing UNMAP command, isn't it?

Really need your explanation on this
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Best answer by mmcghee 18 July 2017, 14:47

Hi hienle

For 1: With Windows 2012 or higher the unmap on deletion should occur automatically. There's also a scheduled task (optimize drives) which runs on a regular basis that will issue unmaps as well. For Linux I think it depends on the distribution. In some cases it's not enabled by default and in others there's a scheduled job which runs and will perform the fstrim (check your cron jobs). To make it automatic on delete with Linux you'll need to mount the filesystem with the discard option set.

For 2: Successfull unmap commands will free space within the Nutanix storage pool. Whether that lives on SSD or HDD does not matter.

For 3: The free space from the perspective of the host (free space in the file system of the mounted disk) will not change. Logically the file delete will already show the free space. The unmap is to allow the underlying storage to also free the space associated with the free space in the file system. Hopefully that answers the question, if not please let me know.

Thanks,
Mike
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Userlevel 4
Badge +18
Hi hienle

For 1: With Windows 2012 or higher the unmap on deletion should occur automatically. There's also a scheduled task (optimize drives) which runs on a regular basis that will issue unmaps as well. For Linux I think it depends on the distribution. In some cases it's not enabled by default and in others there's a scheduled job which runs and will perform the fstrim (check your cron jobs). To make it automatic on delete with Linux you'll need to mount the filesystem with the discard option set.

For 2: Successfull unmap commands will free space within the Nutanix storage pool. Whether that lives on SSD or HDD does not matter.

For 3: The free space from the perspective of the host (free space in the file system of the mounted disk) will not change. Logically the file delete will already show the free space. The unmap is to allow the underlying storage to also free the space associated with the free space in the file system. Hopefully that answers the question, if not please let me know.

Thanks,
Mike

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