All About Cache Batteries

RAID Cache Battery Overview

Cache batteries are an important part of the storage ecosystem, preventing data loss and improving performance. They are also one of the most common failures/replacements in the entire system. Because a failed cache battery might not lead to immediate downtime, it might be tempting to procrastinate scheduling a replacement. But trust us, you're better off dealing with it sooner rather than later. We've seen failed cache batteries take down whole systems (admittedly rare, but it can definitely happen!)

But first, it might be useful to understand a little more about how RAID cache batteries operate, their job, what a cache battery learning cycle is, and what happens when a failure happens. Lucky for you, we cover all of that here, so let's get into it!

What is a cache or RAID battery?

Cache batteries power the cache memory that stores data temporarily until it can be written to the disk. They are also sometimes called RAID cache batteries (learn more about RAID groups and disk redundancy).

RAID cache batteries can be connected directly to the memory modules,  plugged into the system board, or part of the rack on larger storage systems. They can be expected to be replaced about every 3 years.

A variety of household batteries

What's the purpose/function of a RAID battery?

The cache battery's job is to keep the data on the memory “alive” while there is no power to the system. When power is restored, the data on the cache can be written to the disk. You could say the RAID cache battery has two main functions:

  • Prevent data loss — In the event of a power outage, cache batteries keep data safe that hadn't had time to be written to the disk yet.
  • Improve performance — Cache memory is typically faster than the system's disk. When cache batteries fail, data is written to the disk instead, slowing the whole system down.
Hand holding handles of cloth grocery bag full of produce

Think of cache batteries and their corresponding cache memory chips like the bags you might use at the grocery store. Sure, you could carry each item one by one from the store to your car and then to your house, but it's definitely not as efficient as using a bag to temporarily carry and protect your items until they are stored in their final location (your shelf).

If the cache chip or module is the grocery bag, the cache battery is like the reinforced handles that ensure the bag doesn't rip (a power outage) and your groceries splatter all over the pavement (think data loss). Much like a grocery bag, cache batteries are often taken for granted, and you don't really think much about them until they break or their absence becomes a real nuisance.

Learning Cycles in Cache Batteries

What is the learning cycle of a cache battery?

Much like you might test your car battery, cache batteries also periodically undergo a test called a learning cycle to ensure that they will have the appropriate capacity to sustain a power outage.

During a learning cycle, the battery purposely discharges and recharges itself. The system measures how long it takes for the battery to discharge and recharge, voltages, and any potential oddities.  After this test, an estimate of how long the battery can keep the cache alive is used to determine if the battery is healthy or failed.

Hand held out with energy sparks above it with zeroes and ones in the energy rendering

When are learning cycles performed?

Most systems run a test or learning cycle when a cache battery is replaced. Additionally, if the battery is installed and working properly, learning cycles will happen periodically throughout its life. Depending on the system, this could be daily, weekly, monthly, or even less often.

How do I know if my cache battery is in a learning cycle or if it has failed?

Occasionally, when a battery is in a learning cycle, it will show as having an error or even send out an alert. Most of the time, these errors resolve themselves when the battery has recharged. Sometimes, especially in older systems, an alert that the battery has failed will show up during a learning cycle and not be resolved even though the battery is fine. It can be tricky to tell if your cache battery needs to be replaced or is in a learning cycle because the test involves completely discharging the battery.

Determining whether a RAID cache battery has truly failed or is in a learning cycle might take a little bit of detective work and some patience. Here's a few things you can do:

  • Start by looking at when the failure alert occurred. Some systems schedule learning cycles monthly, weekly, or daily, so if you see a failure during a regularly scheduled learning cycle time, it doesn't necessarily mean the battery has failed.
  • Check the logs. Some systems will record when a learning state has started.
  • Keep an eye on the learning cycle. If you know a learning cycle has started, it's wise to still keep an eye on the batteries — they might fail their test and still need to be replaced. If a battery is functional, it will recharge when the test is complete.

The length of time it takes to complete a learning cycle will depend on the size of the battery and the type of system. It could be ready in as little as a few minutes, but typically, it doesn't take longer than a day.

RAID Cache Battery Failures

What happens when a cache battery fails?

Woman with hands over her face and one eye peeking out

When a cache battery fails, system software will usually start writing data directly to disks, skipping the cache memory so that no data is lost in the event of a power failure. This is known as write-through mode and will eventually cause performance issues.

Using our grocery bag metaphor, it's like your bag is broken and you have to carry all your groceries in your hands instead. It might not be too difficult at first, especially if you've only got a few items to carry, but it'll be become a giant pain when you have far too many groceries to carry without a bag.

Pro Tips on Cache Batteries from an MGS Engineer

What to do (& not do) when you have a failed RAID cache battery

Don't panic, but don't procrastinate either. Report failures to your provider right away. We will help determine first if it's part of a learning cycle or a true failure and then schedule your replacement. Panicking is never useful (and we wrote a whole article on how to avoid panicking in the midst of an IT disaster), and most of the time, a cache battery failure will not immediately cause a major problem.

However, we've seen some pretty unfortunate issues arise from not dealing with a cache battery failure — everything from performance issues leading to a full system crash to the batteries expanding, leaking chemicals, or even exploding. So the moral of the story is while there probably is no need to freak out when you have a cache battery failure, don't ignore the problem either. Just call us. Actually, scratch that - if you've read our other content, you know that email is more efficient😉

Don't try to swap it before talking to your support provider. The data on the cache might need to be modified, moved, or verified before the battery can be replaced. The battery status also should be confirmed before the replacement happens. Swapping a good battery will likely mean it still has some cache data still on it. Additionally, the system usually has to be powered down to make the swap, which can sometimes cause issues. Following incorrect procedures could result in data loss, and nobody wants that!

Store cache batteries properly

Usually, vendors will send a freshly tested and charged battery when replacement time comes. But just in case you are storing any cache batteries on site, it's important to keep power to them. What a pain to replace a battery with one that was not stored properly and now can't hold a charge!

Check your hardware support contract to see if RAID batteries are covered

It's a good idea to double-check that RAID cache batteries are covered with your support contract. MGS support contracts typically include RAID cache batteries, but that may not always be the case with every provider. Depending on the situation (and the provider), cache batteries can sometimes be covered but can also be considered a consumable/chargeable part. Knowing from the get-go will mean no surprises when it's replacement time.

Let M Global Help

Got a cache battery issue? (Or really any other hardware related problem!) We can help! We're all about turning challenges into successes and obstacles into an opportunity for growth. We love creating solutions for our clients, no matter how difficult the challenge.

We want you to consider us an extension of your team, a trusted resource and an advisor. Fill out the form or give us a call at 855-304-4600 to find out more.

Let's Chat.

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Author Note:

By Angie Stephens with contributions from experts at M Global.