We’ve been talking about the merits of SSD caching vs. tiering for a while now, but at last someone else did it right. Enter Apple’s Fusion drive announced earlier today.
For those needing a technical refresher on caching vs. tiering, see my earlier blog posts SSD Tiering versus Caching: Part 2 and SSD Caching versus Tiering. Unfortunately, confusion still reigns in many of the blog posts we’ve seen today and most people still don’t get what the real difference is. Yes they achieve something very similar, and no, Apple’s approach is not like Intel’s caching or anyone’s caching. It’s an approach like we do at Enmotus with our MicroTiering i.e. we move the data you care about the most onto the faster SSD and make sure the stuff you don’t use a whole lot lives on the slower hard drive. For the technical folks out there, the only difference is that Apple does it at file level and we do it at block level.
For the less technical, the best everyday parallel I can think of is that caching is like staying in a hotel room whereas tiering which is more like living in a home. A hotel serves as a temporary place to hang out for a specific occasion and leave after a relatively short stay. If the hotel is busy, you generally have to move out after your pre-allotted time with no possibility to stay on. If it’s not so busy, you may be able to extend your stay. A tier on the other hand is more like a permanent home you moved to and where you live, have all your belongings with you and you can stay a whole lot longer, often for the rest of your life. Furthermore, your home is a whole lot nearer the everyday places you go most of the time and it will still be there and available to you every day as long as you need it.
So what’s the fuss about? Apple are the first guys to openly admit and support the notion that tiering is better than caching where SSDs are concerned, which I and the team at Enmotus whole heartedly agree with. Why? Here a few reasons we’ve discovered along the way.
- Tiering delivers superior performance as it provides the full bandwidth of the SSD to the user for the active portions of your data
- Almost all of the SSD’s capacity is useable storage and not hidden as with caching. In other words, I paid for this SSD and it visibily adds to my overall capacity and it’s not hidden behind a disk drive.
- SSDs wear out the more you write to them. Caching increases the number of writes to the SSD and wears them out faster. Teiring is far friendlier. We’ve measured extremely low overhead writes to SSDs using tiering at Enmotus of the order of 0.01% to 1% above and beyond what the host wrote over just a few hours of usage.
Why should we care about tiering? Even the professional vendors of PCIe SSDs agree – skip caching and put all the user data on one large SSD if you can. Problem is, this is just too expensive for us everyday users who are not trying to replace large numbers of disk drives. Tiering provides that perfect balance between performance and cost while providing enough capacity to still store those large videos, photos and games.
The future is definately bright for SSD tiering.