Dell Compellent Free Space and Allocated Space

Hi Alex,

i will try to explain it on one of my systems. The SC have 97.37TB available space in total.  I have 2 storage types but we only focus on tier3 which are 10K HDDs.

 

To hold 37.87TB of data(including raid protection) in tier 3 the SC have allocated up to 47.81TB right now. The SC needs to be prepared for incoming data and also for moving data around when raid tiering compression,dedup and data progression is going on.  This allocated space is a high water mark value for the current state. The SC will dynamically adjust this value if needed.

Within the allocated space there is 9.94TB free space so that data can be saved and for SC doing its job. On my 10K there is 30.78TB non-allocated which means "virgin" space.

The used space contains the Hosts data included raid protection.  The  free and un-allocated space is without raid protection.

 

I agree that this is not easy to understand. Also the SC use the "free space" for different meaning but to be fair they used it based on what kind of element youre looking into.

The GUI can only give a answer how much space is left and never answered to somehing like:

How much can i expland a value? - Reason: SC is improvisation by default doesnt need any space to present an empty volume regards to its size

Can i store 2TB of more data? - Reason: It depends on the selected storage profile (RAID 10,DM10,5,6) and if the new data coming into between 2 periods of on demand dataprogression or not. In worst case you need for your 2TB up to +4TB(RAID10) or +6TB(DM10).

 

Regards,
Joerg

 

 

Hi Joerg

Thanks for you help, so the free space show in the storage summary is the free space in the storage + unallocated space?  and in your case it should be 9.94 + 30.78 = 40.72?

Then SC will change unallocated space to allocated if free space got low dynamically?

So all space here (no matter in Storage Summary or in Storage Types) are non RAID free space, and I use traditional RAID calculation for the assumption. Just like your example, I need to expand 2T space, then after RAID will occupied around 2.3T for Raid5-9 and 4T for RAID-10.

 

Best regards

 

Alex Tsang

 

Hi @Alex-Technergy 

If we look at the storage type summary that this is correct. 40.72TB of "available".

Yes, the SC will dynamically adjust the values and allocate space when needed. Without real preasure the other way around will not taken so often. Sometimes i ask Dell Support during SCOS upgrades if they can shift the "free" space back into un-allocated when there is huge free space within the allocated.

All free space and un-allocated is non raid space. The used space always contains the raid protection.

 

About the 2.3T for Raid5-9 and 4T for RAID-10. Yes, but 2.3T is the theoretically minimum and 4TB the maximum(without snapshots). The reality is something between and depends on:

• My redundancy level (Single redundancy is RAID5/10 and dual redundancy is RAID6/DM10
• Number of drives because a  "stripset" goes over 5 or 9 (RAID5) or 6 or 10 (RAID6) disk, which ends in 89% or 80% space effiency. The RAID 10 have only 50% and the DM10 only 33%
• My selected storage profile
• Enabled space optimization (compression and/or dedup)
• If you use the "Balanced" or "Recommended" storage profile the data is shifted around because of RAID tiering and spread over tiers because of data progression.
• Use of snapshots

If you took a look to Hardware->Select a Volume->Statistics you get information how much space currently is use. Keep in mind that this can changed during runtime because cold/hot data is migrated through the tiers, so the amount of needed space to hold 2TB data dynamically.

Example of one my volumes:

 

6.13TB needed to store and protect 3.95TB of data.

 

The "historical usage" will show the changes over time.

The blue line is the 6.13TB.

 

Regards,
Joerg