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I'm looking at buying a new laptop. My current system has an SSD for the OS and magnetic hard drive for media. All the new ones I'm looking at have a hard drive plus mSATA-based SRT cache (ex: Lenovo Y500). It looks like this won't be quite as good as a "proper" SSD, but much better than a magnetic drive alone.

Most of the systems I've seen ship with a 16GB mSATA SSD. I see options for 20, 32, even 64GB. The way I understand the technology, it somehow computes what you use most often and mirrors / "stripes" those blocks on the SSD, at the BIOS level. The thing I don't know is, how to calculate when you hit the point of diminishing returns. Is there some kind of instrumentation / diagnostic that can tell me what percentage of reads are hitting cache, and how often it needs to spin up the HDD?

I figure that in a couple of years, increasing the cache size will probably be a sound investment, as sizes increase and costs come down. I'd rather have actual data to base this decision on, if possible, though.

ETA: Two important things I found specific to the Lenovo machine I mentioned: a question about "ExpressCache", which is not quite the same as SRT, and a page from Lenovo about the ExpressCache software that actually does the caching, including instructions for finding out how full the cache is and how often reads and writes hit the cache. That last part is essentially what I was looking for!

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closed as not constructive by Breakthrough, TFM, Ƭᴇcʜιᴇ007, Tog, Dave M Mar 3 '13 at 18:36

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Ewww hybrid drive. Burn it. – Griffin Mar 3 '13 at 15:13
That laptop just uses Intel RST (Rapid-Store Technology), which is built-in to the motherboard, for caching. You don't need a large cache drive at all, and I believe the maximum is only around 30-40 GB to begin with. – Breakthrough Mar 3 '13 at 15:14
Hybrid drives will always use the HDD they very rarely stop using it. The only advantage to hybrid drives is that they give you a small amount of SS for your OS. Some drives do what you talked about with determining what you use most but from my experience they pretty much put a flash drive in your hard drive. BTW in a couple of years you'll have to replace the laptop so that isn't really a problem. – Griffin Mar 3 '13 at 15:15
@Griffin that laptop does not have a hybrid drive, it uses Intel SRT... Although the question does apply for both technologies, as they accomplish the same thing. – Breakthrough Mar 3 '13 at 15:20
SRT != hybrid, as @Breakthrough says. I like that the drive and cache are independently upgradeable, and that I don't need a second bay (as my current, monster 17" laptop has) to get both lots of space and great performance. – James B Mar 3 '13 at 15:21
up vote 2 down vote accepted

That laptop just uses Intel RST (Rapid-Store Technology), which is built-in to the motherboard, for caching. You don't need a large cache drive at all, and I believe the maximum is only around 30-40 GB to begin with. This is even the case for hybrid (i.e. combined flash/mechanical drives). Why?

All a cache is required to do is provide some of the file you're accessing, presumably while the drive heads in your combined mechanical hard disk is seeking to the file. Once the cache is exhausted, the reading can seamlessly continue from the hard drive.

To actually compute the (theoretical) amount you need to cache per file, you can multiply your drive's seek time by the SSD's burst transfer speed (e.g. 10ms * 350 MiB/s = 3.5 MiB). This would be the average amount of data you could read from the SSD while "waiting" for your HDD to get into place.

With lowering storage costs and the cost per gigabyte of SSDs set to fall below conventional mechanical hard drives in a few years' time, I'd expect using flash memory for "cache" drives will become obsolete as we move to directly use flash memory as primary storage (keeping HDDs for long-term archival).

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So, bottom line, don't worry about it because 16GB will probably be "enough", and I wouldn't notice an upgrade to a 32 or 64GB module instead? – James B Mar 3 '13 at 15:20
You might, but at the current point in time, it won't be worth it. You should note that Intel RST has a max. cache size of 64GiB anyways. It really depends what you end up caching on the SSD, and the cache replacement policies you have in use. Put another way: already having a cache drive, simply upgrading it will only lead you to diminishing returns. There's not a lot of stuff a cache drive has to nor needs to store, and you'd only notice a speed improvement if the existing cache happened to be "full". – Breakthrough Mar 3 '13 at 15:22
Another thought to consider, in the future when larger SSDs are cheaper, you may want to see if you can install your OS on the mSATA SSD. This would yield the maximum possible speed benefit while also retaining your 1TB HDD to store static files. – Breakthrough Mar 3 '13 at 15:24
OK, basically your comment (you'd only notice a speed improvement if the existing cache happened to be "full") was the meat of my question -- how can I tell if it's getting "full" / overtaxed? I guess that's not constructive... (funny how Joel said "not constructive" was a bad closure reason on the podcast... :( ) – James B Mar 4 '13 at 23:06
Check the Intel documentation and see if there are any tools you can use to see how much of a benefit you would experience. It's technically up to Intel's implementation. – Breakthrough Mar 4 '13 at 23:51

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