Wednesday, June 15, 2016

Calculate Exadata X6 Storage space available for ASM Normal and High Redundancy

When reviewing the datasheet for the Exadata X6 the latest release you may want to understand how much usable space will be available for you to use for Normal and High Redundancy in ASM for your diskgroups.

Each Exadata X6 storage cell comes with 12 x 8TB SAS 7200 RPM disks with a total Raw capacity of 96TB for each cell. Now if you want to see how to calculate the Normal or High redundancy for example of a Quarter Rack it is not simply taking the raw storage amount divided by 2 for Normal redundancy and 3 for High redundancy. The formula is below and I confirmed this with Oracle Support as well.

Please note: The amount of storage that needs to be reserved to recover from the failure of a disk is 15% of the cell disk capacity.

# cellcli -e list celldisk attributes name,size
CD_00_internal 7.1192474365234375T


The calculation is as follows -
Size of one disk : 7.1192474365234375TB
Disks per cell node : 12
No of cell nodes : 3

Total size of Disk : 7.119TB  * 12 * 3 = 256.284TB

Normal Redundancy  = [ Total size of Disk / 2 ] * .85

= [ 256.284TB / 2 ] * .85

= 128.142TB * .85

=> 108.9207TB or 109TB [ The value in the Exadata X6 data sheet ] 

High Redundancy  = [ Total size of Disk / 3 ] * .85
= [ 256.284TB / 3 ] * .85

= 85.428TB * .85

=> 72.6138TB

A table below also shows this information to easily see the values on the different size configurations.


  1. Hi Nabil,

    I'm sorry but I have to disagree. This is the way you calculate the usable space:
    Disk Usable File MB = (FREE_MB - Disk Required Mirror Free MB) / 2 (or divide by 3 for high redundancy)

    Where FREE_MB is the raw free space in the disk group in MB.

    For reference:
    Understanding ASM Capacity and Reservation of Free Space in Exadata (Doc ID 1551288.1)

    1. Hi Sve,

      I am aware of the note you referenced and that is correct.

      However I actually provided my content in this blog post based on feedback directly from Oracle Support and Oracle development. I asked them "Can you please provide a formula or an detailed explanation on how I can easily calculate the normal and high redundancy amounts given the total RAW storage from the datasheets?"

      Basically I wanted to know the formula used to derive the amount of storage available in normal and high redundancy published in the datasheets.