RAID controller evolution in Exadata

Some things do not lie on the surface, an interesting moment was found out in the Exadatas X7-2 and X6-2: a RAID controller lost a battery. In previous generations of Exadata the RAID controller had a battery.

On the one hand to live without a battery is even better: you do not need to change them. But on the other hand, is there no more way to have a Write-Back cache on RAID controller?

 I selected from documentation:  https://docs.oracle.com/cd/E80920_01/DBMSO/hardware-components-exadata-db-machine.htm#DBMSO-GUID-7BEBDA9C-61E1-4B4A-9FDE-76AE2215E3BB

1. Cells with High Capacity disks:

Х7-2 Disk controller HBA with 2 GB cache

Х6-2 Disk controller HBA with 1 GB cache

Х5-2 Disk controller HBA with 1 GB supercap-backed write cache

Х4-2 Disk controller HBA with 512 MB battery-backed write cache, and swappable BBU

Х3-2 Disk controller HBA with 512 MB battery-backed write cache

Х2-2 Disk controller HBA with 512 MB battery-backed write cache

2. Database servers lost the battery too:

Х7-2 Disk controller HBA with 2 GB cache (no batteries)

Х6-2 Disk controller HBA with 1 GB cache (no more batteries)

Х5-2 Disk controller HBA with 1 GB supercap-backed write cache

Х4-2 Disk controller HBA with 512 MB battery-backed write cache, and swappable battery backup unit (BBU)

Х3-2 Disk controller HBA with 512 MB battery-backed write cache, and swappable battery backup unit (BBU)

Х2-2 Disk controller HBA with 512 MB battery-backed write cache

Is there no more way to have a Write-Back cache on RAID controller?
No! in the X7 and X6 oracle use the capacitor instead of battery.
So write-back will take place and write IO promises to be excellent.

3.

Is there RAID controller in the Extreme Flash cells ?

NO: Extreme flash cards directly attached to PCI, so there is no any RAID controller in EF-cells.