Exadata Storage Space Calculation

Exadata the Oracle engineered database machine has taken the OLTP database database performance to the next level. It has a number of power pack features flash cache, Smart scans and storage indexes that really provides the great returns on the investment.

How much usable storage a cutomer gets with every configurtion of the Exadata (quarter, Half or full Rack) is a little tricky for the customers guage.


Now below diagram shows the Exadata disk architecture for every disk. The physical is virtualized as a cell disk and then a grid disk to be presented to the Disk groups. (i'll keep the disk architecture discussion for some later post)

now for all the cells the structure would be something like

Lets take the example and oracle Exadata quarter rack example ang calculate what exactly the customer gets in terms of storage space.

The Quarter Rack Consists of

- 2 DB nodes
- 3 Storage Cells


Now the 3 Storage cells ( with high performance disks) has the following configuration


Step 1: Calculate the RAW Storage 

total Raw capacity => 12 disk on each cell * 600 GB each disk * 3 cells
                   => 21600GB or 21.09TB


now 30 GB on the first two disks of every cell is reserved for Linux OS. So by default to a disk group is created with 30 GB of disk space from remaining 10 disks from every cell.

so we have a Disk Group name DBFS => 10 disks from each cell * 30 GB from each disk *3 cells
                                                   DBFS => 900 GB

so We are left with 570GB on each disk i-e a total of "20520 GB" or  20.03TB 



Step 2: Now consider the mirroring requirement

 After NORMAL Redundancy (i-e double mirroring) Avialable RAW CAPACITY 20.03/2 =  10.01TB.

Step 3: Reserve free space for fault tolerance. 

with Disk failure coverage (DFC) one disk of 600GB is required to protect against the failure of one disks in a cell.

NOTE: that is a very important part. You need to plan for a Disk failure coverage or for complete cell node failure coverge. The ORACLE support document "Understanding ASM Capacity and Reservation of Free Space in Exadata (Doc ID 1551288.1)" clearly describes the different strategies and their pros and cos.

NOTE: After the application of BPS10 (11.2.0.4.10) the default is now the Disk Failure Coverage rather than the Cell Failure Coverage.


 This means that available is 10.01 TB - 0.57TB (i-e 600GB) =>  9.4 TB

Step 4: Decide the proportion of the Recovery Area. For example 60/40 (40% for recovery area)


 So the Recovery area size (RECODG) would be => 3.76 TB
and the DATADG would be => 5.64TB