Cache is exposed to the risk or uncommitted data due to power failure. This problem can be addressed in various ways
1. Powering the memory with a battery until AC power is restored
2. Using battery power to write the cache content to the disk.
In this event of extending power failure, using batteries is not a viable option because in intelligent storage system large amount of data may need to be commited to numerous disks & batteries may not provide power for sufficient time to write each piece of data to its instead disk.
Therefore, storage vendors use a set of physical disks to dump the contents of cache during power failure. This is called cache vaulting & the disks are called Vault Drivers.
When power is restored, data from these disks is written back to write cache & then written to the intended disks.
All Clariions have Vault Drives. They are the first five (5) disks in all Clariions. Disks 0_0_0 through 0_0_4. The Vault drives on the Clariion are going to contain some internal information that is pre-configured before you start putting data on the Clariion. The diagram will show what information is stored on the Vault Disks.
The vault is a ‘save area’ across the first five disks to store write cache from the Storage Processors in the event of a Power Failure to the Clariion, or a Storage Processor Failure. The goal here is to place write cache on disk before the Clariion powers off, therefore ensuring that you don’t lose the data that was committed to the Clariion and acknowledged to the host. The Clariions have the Standby Power Supplies that will keep the Storage Processors running as well as the first enclosure of disks in the event of a power failure. If there is a Storage Processor Failure, the Clariion will go into a ‘panic’ mode and fear that it may lose the other Storage Processor. To ensure that it does not lose write cache data, the Clariion will also dump write cache to the Vault Drives.
The Operating System of the Storage Processors is stored to the first five drives of the Clariion.
Now, please understand that this information is NOT in any way shape or form laid out this way across these disks. We are only seeing that this information is built onto these first five drives of the Clariion. This information does take up disk space as well. The amount of disk space that it takes up per drive is going to depend on what Flare Code the Clariion is running. Clariions running Flare Code 19 and lower, will lose approximately 6 GB of space per disk. Clariions running Flare Code 24 and up, will lose approximately 33 GB of space per disk. So, on a 300 GB fibre drive, the actual raw capacity of the drive is 268.4 GB. Also, you would subtract another 33 GB per disk for this Vault/PSM LUN/Flare Database LUN/Operating System Information. That would leave you with about 235 GB per disk on the first five disks.