HPE Storage Users Group

Hello all,

can someone explain to me why my 3PAR shows "Reserved User Size" shows 303GB of Data of a 1TB Virtual Volume, but when i go to the VMware DataStore of the same Virtual Volume it shows only 363GB of free disk space.

Can Someone please explain why this happens?

Regards

Boris

Dedupe or zero detect is saving you a bunch of space.

Zero Detect

I know that Zero Detect saves me space but why cant i then see the space as empty in VM and use it ?

the zero Detect feature as I understand it is suppose to reduce the used disk space and give it back to us so that we will be able to use it, if this is not the case why do i need this feature?

Regards

Boris

Because VMware has no concept of the space saving, it's invisible to the host. It saves you space on the backend (3PAR) which means you can safely over provision your datastore size on the front end (VMware).

For VMware to have any knowledge of this would mean that VMware had to do the zero detect work itself, but it doesn't it simply passes all data to 3PAR and the array strips zero's but returns confirmation of all writes to VMware. Not to do so would lead to corruption in the datastore.

So you are saving space within the array, just not within the boundaries of the filesystem.

Thank you JohnMH for the response.

so just to make this clear for me the space is actually being Saved on the 3PAR but im still unable to Utilize it via vmware without over provisioning on the VMware side?

Thank you

Yes it's all about layers of abstraction and how they map onto each other.

VMFS records all writes from specific VM's as used space within it's file system, since it isn't doing zero detect this is correct from VMFS's view point. But ultimately these are then passed through to the array which examines the data stream and will discard 16KB contiguous pages of zeroes, only writing the remainder to the 3PAR disks.

As a very simple example suppose you have two 50GB VM's, these would consume 100GB's in VMFS but depending on the number of 16KB pages of zero's they write may only consume 60-70GB's on the 3PAR.

There are different ways you can deploy such as thick on thin / thin on thin etc depending on how you want to account for the saving (I'm not a VMware expert), but the array will always avoid writing these pages to the backend disk.

Hope this makes sense.

Hello again JohnMH,

Thank you for the great explanation and it dose make sense to me now.

Again Thank you for the explanation.