Today browsing one of my favorite 3PAR related websites (3parug.com) I came across topic asking for a “real” free space. I assume someone is trying to find out how more of the actual data he/she can fit before running out of space.
Before we answer this question lets take a look at different “layers” of free space.
1. Physical Drive space
Let’s take for example 900GB FC drive. Inside 3PAR MC it will report as Total Capacity of 819GB. On the other hand 900GB SSD will report Total Capacity 852GB.
Note: I don’t have information (formula) on how Total Capacity derived from capacity reported by HD manufactures.
Now let’s take a look what is used within Total Capacity You can view it by issuing showpd -space command.
– Size – total size described above
– Volume – how much space is actually used by Volumes
– Spare – space used by spare chunklets
– Free – space available for Volumes
Now let’s look at MC:
Total Capacity = Size
Free Capacity = Free
Allocated Capacity = Volume + Spare
2. CPG space
In order to “use” PD space described above you need to assign drive to one CPG. CPG creates underlaying RAID from chunklets (1GB in size). So for example CPG with 5 – Data 1 – Parity will consume 6 GB of Free Capacity on physical drives for each 5GB of data.
Estimated Free System Space should give a good indication on how much “real” free space (after RAID parity) remains on your 3PAR for a given CPG.
Please remember with Thin Volumes you can over-provision space as 3PAR’s ASIC removes all “zeros” on the fly.