Cache 'n' carry: What's the best config for your SSD?
How to gain solid-state performance with out losing hard drive capacity
Feature The idea of using a low-capacity SSD to store the most frequently accessed files or parts of files in order to access them more quickly than a mechanical hard drive can serve them up - a technique called SSD caching - has been around for some time, but it wasn’t until the arrival of Intel’s Smart Response Technology with the company’s Z68 chipset, released in 2011, that the technology began to be implemented in personal machines rather than servers.
Cache in your chips
Intel’s thinking was to get ordinary users into the SSD game by allowing then to put small, cheap solid-state drives into their systems alongside existing, large capacity HDDs rather than suggest they swap out the latter for a more expensive yet not as capacious SSD. The cache drive approach brings almost all of the benefits of SSD - fast boot times and file access - without having to break the bank to get a large storage space.
At the same time, Intel launched the 311, a 20GB capacity, 3Gb/s Sata SSD designed specifically to be used as a cache drive in Z68-based systems. Motherboard manufacturers were quick to take advantage of SRT, and a number of Z68 motherboards appeared sporting the connectors for an mSata drive. In certain Asus and Gigabyte boards, cache drives even came pre-installed.
However, what Intel and a great many others didn’t see coming was the very competitive, a polite way of saying cutthroat, pricing war that SSD suppliers are now engaged in. That has brought down the prices of mid-size SSDs to the point when many users are willing to take a punt. That said, SSDs approaching the kind of capacities we’ve come to take for granted with HDDs are still incredibly pricey. You can now pick up a fairly fast performing 120/128GB SSD for well under a hundred pounds to hold the OS and all the apps you need, but if you have a lot of data on your system, trading capacity for performance isn’t always attractive. Caching allows you to get the best of the both worlds.
At least, that’s how it’s being sold. But is it better? Let’s find out.
Intel Smart Response Technology
Intel’s Smart Response Technology (SRT) first appeared a couple of years ago in version 10.5 of the company’s Rapid Storage Technology (RST) Raid software for the Sandy Bridge chipset, but it was initially only enabled on the Z68 desktop chipset and a couple of mobile products. SRT has since been supported on the more recent Z77, H77 and Q77 chipsets. SRT works by caching the I/O data blocks of the most frequently used applications. It is able to discriminate between high value or multi-use bytes, such as boot, application and user data, and low-value data used in background tasking. The low-value data is ignored and left out of the caching process.
To enable SRT, you have to enter the motherboard’s Bios settings and switch the Sata controllers to Raid mode - SRT won’t work in either ACHI or IDE modes. The technology uses no more than 64GB of space on the SSD. Any extra space left over remains untouched, so there’s no point splashing out on a bigger drive. The SRT management software is an easy-to-use app which allows you to choose which of two types of caching you wish to employ.
The Two types are Enhanced (Write-Through) and Maximised (Write Back). In Enhanced mode, all the writes are sent to the SSD and HDD simultaneously which means the drives can be later separated without you having to worry about data preservation. However, there’s a hit on performance as all the writes slow to the speed of the HDD. In Maximised mode, the majority of the host writes are captured by the SSD and asynchronously copied to the HDD when the system is idle. Once again, there are good aspects to this approach – you get faster performance than Enhanced mode - and bad: SRT must be disabled before any of the drives can be removed otherwise the next time you boot up you’ll end up with the longest list of orphan files you're ever likely to see. Ask me how I know...
In 2012, Apple launched what it calls Fusion Drive technology on the latest incarnations of the iMac and the Mac Mini. Currently there are only two Fusion Drive options available, both using a 128GB MLC NAND SSD. The 1TB option is available for the latest Mac Mini or any new iMac, but the 3TB option is exclusively tied to the 27in iMac. Fusion Drive uses Mac OS X’s Core Storage Logical Volume Manager, available in version 10.7 Lion and up, to present multiple drives as a one single volume. The technology doesn’t behave in the same way as the usual SSD cache. Instead it moves data between the SSD and HDD and back again depending on how often the data is accessed and how much free space there is on any of the drives. The idea is to ensure that the most-read files are stored on the SSD, which is considered part of the host system’s overall storage capacity rather than a separate, ‘hidden’ buffer. El Reg will be digging deeper into Fusion in a future article.
Next page: Dedicated cache drives
Not a single mention of Linux anywhere
Sad to see that Linux isn't mentioned (or indeed benchmarked) here, even if it's just to suggest what the equivalent of Intel's SRT is for Linux. If you're building a new machine, I'd expect it to have SATA 3 because SATA 3 SSD prices are continuing to fall.
The only sensible combo, IMHO, is a Sata 3 SSD with 500MB+/sec read/write and a fast multi-terabyte HDD (I use Seagate 3TB's myself at 225Mbytes/sec read). Throw in a fast multi-core CPU and you're not far off 10 seconds boot time, meaning that multi-boot isn't tiresome any more.
Missing - hybrid drive?
I'd really have liked to see that review also compare a hybrid drive such as Seagate Momentus. Yes, it's a slow sleepy 2.5 inch laptop drive, but how much difference does that built-in flash cache make? And being out past the drive end of the SATA connector, it'll work with Linux or anything else you care to boot.
With Linux it is trivially easy to put the operating system and your own small / heavily accessed files onto a small SSD. One can configure a completely useful Linux system in 30Gb (about 15Gb of system files, 15Gb /home). Unfortunately 30Gb SSDs are slower than 256Gb ones, but they share the same near-zero seek time. Again it would be nice to see that benchmarked.
Enquiring minds would like to know...
Why is it that in all SSD-related discussions, PCI-E based versions are never mentioned.
From what I could see (and have been using) PCI-E based SSD's tend to have the best performance compared to SATA III (6 Gbps) and due to the slot, they can accommodate sizeable capacities without any space limitations.
And yet, reading the reg you'd think they don't yet exist.
(It's not for lack of demo units, is it?)
Re: No need for any cache - just go pure SSD
"And a 512GB SSD would be enough for what exactly ?
Booting the OS maybe and just that.
Nowadays a PC can't have less than 2TB."
The VAST majority of users don't need anything approaching 2TB - My Gaming machine for example has a 250Gig Samsumg 830, running Win7Pro, Libre Office, a few other apps, Steam and about a Dozen Games installed, and there's room to spare.
Granted I'm not storing Gigs and gigs of Videos, and if I want to install a new game I need to install another, but where's the problem with that?
The overwhelming majority of users run Windows with some antivirus, Office apps and everything else they do is through their browser - that would all fit in a 64Gig drive without any issues at all.
And your comment about building 12TB of storage using SSDs only is argumentative and distinctly trollish. This article is about devices which are aimed at home/laptop use - no-one is suggesting using hybrid / cache drives for enterprise storage where 19" racks of dozens of disks are the norm.
Go and crawl back under your bridge.
Make your Own
As far as the Fusion Drive is concerned. All the tools for rolling your own are built into the file systems in Lion and Mountain Lion. I built one for my Mac Pro ( which is only SATA 2 ) with a 256GB SSD and a 2TB WD Black. Very nice increase in performance and drops you straight into login after about 10-15 secs.