Preserve your cached data with BBU or SuperCapacitor

Internal backup for RAID controller

RAID controllers form a subsystem within a computer that is exclusively responsible for managing the connected drives – including its own processor and even its own memory. Depending on the RAID mode used, the mini computers must process the data to be stored (e.g. using XOR operations for parities under RAID5 and higher) before it is written to the data carriers.

Areca RAID Controller for BBU or SuperCap use

Write jam in the controller

Ideally, the writing should happen in real time, but this is not always entirely successful. Reasons could be:

  • Commercially available drives such as hard disks and SSDs do not store their data bit by bit, but in blocks. So the controller must successively collect the data before writing.
  • Furthermore, the cooperation between the operating system and the RAID controller plays a role in the cache. Synchronous writes are already acknowledged by the RAID controller to the operating system as written, although they are actually still in the cache at this point.
  • In addition, under heavy load, reading and writing from multiple clients is not always instantaneous. Then the controller must work through the tasks in order. For this purpose, the data is in turn stored in the memory or cache of the controller, sorted, processed and finally written.

 

CacheVault Broadcom

Murphy’s law comes true

And this is where a problem area opens up: What happens to the data in the write caches, which are often several gigabytes in size, if the power fails? Will the data be lost if not written to the attached drives?

Answer: Yes

If you do not secure the memory, you will lose the data that was waiting to be written. In the worst case – for example with large databases – this results in an inconsistency between the logged writes and the contents of the active database.

BBU SuperCap

Battery backup unit or supercapacitor

Solution: A BBU or a SuperCapacitor

  • A BBU has a docked battery that powers the volatile cache memory for up to 72 hours. Like all Li-ion batteries, they will age and need to be replaced in a maintenance slot after about three to five years.
  • A SuperCapacitor works differently, but also provides higher security: With the energy stored in the capacitor, the data is quickly shifted into a non-volatile memory and is thus ready for the next start.

When the server/storage system is restarted, the RAID controller completes its unfinished write operations via the cache saved in this way or informs the operating system that they must be repeated.

Note: If the backup via BBU or SuperCap is switched on, the drive cache should be switched off. Otherwise, in the event of a power failure, the data in the drive cache may be lost even though the controller cache has been backed up. Only SSDs with Power-Loss Protection can leave the drive cache enabled.

Brand names of our BBUs or SuperCaps!

Areca: BBM/FBM (Battery Back-up Modules/Flash-based Back-up Modules)

ATTO: CacheAssure™ Supercap Technology

Broadcom/LSI: CacheVault® Flash Cache Protection

USV Cyberpower

Another protection in case of power failure

All-round protection via UPS

A UPS, on the other hand, protects not only the RAID controller cache but also all connected servers and storage systems from power failure. Built-in batteries ensure that the IT systems continue to operate for a certain period of time. Depending on the capacity of the batteries, this can be a few minutes or several hours.

The size depends on your needs: Do you want the server to only complete its writes and shut down securely, or do you want it to continue functioning for as long as possible. By the way, the UPS permanently signals the server whether there is still power on the input side. Thus, the server, for its part, immediately initiates the shutdown procedure if the power suddenly runs out.

Have we aroused your interest in our solutions? – We look forward to talking with you!

Starline contact

Any questions? Please contact us.

Since 2017, the former technology journalist has been providing new content about Starline products and services in press releases, newsletters, catalogues and on the web. To do this, he had to acquire a few gigabytes of technical data about this new profession for him, in order to familiarise himself with the professional world. Currently he has extended his working environment to include marketing and helps with tradefair preparations, presentation materials and business equipment. After work, he devotes himself entirely to his craft and enjoys working with wood – as an analogue 3D printer, so to speak.