What is the optimum size for an SSD cache?

[desura]

I have a 120gb ssd and a 2tb hd.

Windows obviously is installed on the ssd.

I'm thinking of slicing off a partition to help accelerate the terabyte HD.

Something small, though. Like, the momentus XT seems to do fine with 4gb of ssd cache, so like 10gb should be a noticable improvement, no?

 

[Zap]

Bigger = better

What do you run on the HDD that you feel needs to be accelerated?

 

[Emulex]

HP thinks 10% .

Depends on your application. If you rock sql server with gobs of ram , the entire database and indexes load into ram and it never asks for the same block again (no point in read-ahead caching!).

Check out Apple fusion drive, they use a portion for o/s, a portion for read cache and a bit for write cache.

Seems smart. I'd go one step further and use SLC -> MLC -> TLC(/SATA) tiering

 

[desura]

Bigger = better

What do you run on the HDD that you feel needs to be accelerated?

Umm, just about everything?

I have a 120 gb ssd in the system + 2tb...I mentioned earlier, but I'm thinking of picking up another SSD to use as an accelerator and then install the system all on the 2tb accelerated.

Makes things easier. Don't have to change install paths everytime I want to install something.

Is there anything that I should look for in searching for SSD's or is it generally just get the cheapest for capacity that you can?

Also, usually with these things there's a diminishing returns. Intel decided like 64gb was the limit. I wonder how the improvement worked out. Like 4gb probably got you a 50% improvement, 8gb a 60%, 16gb 68% and etc etc 64gb probably gets like a 75% improement.

 

[kbp]

If your going to use Intel Rapid Storage your max is 64gb. So if your going to buy, just get a 64. (They are cheep enough)

 

[desura]

If your going to use Intel Rapid Storage your max is 64gb. So if your going to buy, just get a 64. (They are cheep enough)

but like, they're all the same?

 

[kbp]

I would not slice anything off a 128gb drive. What I would do is buy a 64gb drive to cache your 2tb drive via IRST.

 

[sub.mesa]

HP thinks 10% .

That is for server workloads. Regular customers store much more static data with large files, instead of databases and other data that has much highly share of random I/O.

For consumers, it is more like 0,01% to 1,5%. For example, just caching the metadata, which is a fraction of a multi terabyte volume, will lead to a decent boost in overall performance. Browsing directories, searching files all get enhanced when caching metadata, while the metadata itself is tiny compared to the actual data.

Many people think SSD caching works the way they think is simple: most used files get stored on SSD. But this is wrong, at least for the majority of SSD caching technologies. Proper caching technologies, Intel SRT included, will only cache non-contiguous I/O. It will cache those fragments of big files that get accessed in such a way, while the rest of the file is not cached even if accessed frequently.

The whole reason for SSD caching is that the haddrives can focus on sequential I/O, not having to be slow down for random seeks all the time. Harddrives are very fast when they read a large file from A to Z, the SSD cache is to allow this to happen more frequently, without having to seek for metadata.

If your going to use Intel Rapid Storage your max is 64gb. So if your going to buy, just get a 64. (They are cheep enough)

Don't do this. Buy a 128GB SSD and use it as 64GB cache drive. Or, buy a 64GB SSD and use it as 40GB cache - no more! Your SSD needs more spare space or your write amplification will be very high and you will wear the SSD much faster than you should.

Special SSD for caching are only different in the regard that they have more overprovisioning. I.e. a 20GB cache SSD is actually 32GiB; much more than the regular 6,7% of spare space that normal SSDs have.

Personally I like the idea of using your SSD for multiple purposes. Install your operating system, use as cache and leave the rest as overprovisioned space. A 128GB or even 256GB SSD is most suitable to this.

Some other pointers:
- you do not need SLC
- you do not need special cache SSDs (but do overprovision regular ones!)
- more is not better; you are sacrificing RAM memory the larger your cache gets. This can be multiple gigabytes for only 64GB of cache.
- you can circumvent the weird protection in Intel SRT that prevents you from using an SSD as SRT cache if Windows has been installed to the SSD instead of the HDD.
- a small cache of only 10GB is worth it already, like the OP said, the benefit will diminish as the cache gets bigger.
- most caching technologies only work until the next reboot; so the acceleration effect is (temporarily) gone after a reboot; it will be like you just configured the cache. This means caching is most effective for 24/7 systems.