The confusing case of cache protection …

August 11, 2011

Once upon a time we had batteries to protect the cache on a RAID card. Protecting cache on the card is an important issue, especially in enterprise servers. When the OS writes data to the disks it actually goes to the RAID card. If write cache is turned on, then the card takes the data into the cache and reports back to the OS that the data has been written. The card then in turn writes the data to the disk when the disks are available (which is only a short period of time).

The end result of all this is that there is almost always data in the cache that the OS thinks has been written to disk, but is still waiting to be written to the disk. If the power goes out at this point in time then that data will be lost as RAID card cache is DDR (needs power to protect it).

The old-school way of doing this was to put a lithium ion battery on the RAID card which kept power to the DDR. This battery would keep the data alive for as long as the battery had power.

Quite some time ago Adaptec decided there was a better way of handling this. For lots of reasons batteries are a pain in the proverbial. They go flat, they don’t last, they have limited shelf-life, short warranties and long charge times, but they were all we had so everyone told you they were a good thing.

So, we developed what we call ZMCP or “Zero Maintenance Cache Protection”. This basically comes in the form of a small circuit board attached to the card, and a tethered supercapacitor. The daughter board has 4gb nand flash onboard. Basically, when the power goes out the data that is the DDR is copied to the nand flash where it is safe for years (the power to do this is provided by the supercap). When the drives come back up the data is copied from the nand flash to the drives where it was meant to go.

Now this post is not about that technology, but more about how we have marketed and implemented it. This technology was developed when the 5 series controller was our flagship, so we made a new model of card (due to connector restraints) and called it a “Z” card … in other words the 5 series controller has a “Z” on the end (eg 5805Z).

Then along came the 6 series. Adaptec made a conscious decision that ZMCP was a better way to go than batteries for lots of reasons (number one being it has a 3-year warranty compared to 1 year for batteries) and decided that this technology would be the only option we offer for cache protection on our 6 series cards.

Therefore, if all cards use this “ZMCP” technology there is no need to put a “Z” on the end of the card, right? Logically you’d think so. So you look at the 6 series product lineup and you don’t find a “Z” anywhere. If you look on the pricelists you won’t find a battery anywhere either. You’ll find a thing called an AFM600 (stands for Adaptec Flash Module). This is the ZMCP cache protection that fits natively on every 6 series card.

Sounds simple to me (or at least it did to our marketing people), but the world doesn’t get it.

Therefore the bottom line is …

If you have an Adaptec 6 series RAID controller and want to protect the cache, there is no battery option.
The only option you have is to put an AFM600 on the card which is our “Zero Maintenance Cache Protection”.

Confused? If not great, if yes then join the rest of the world :-)

Ciao
Neil

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