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A closer look at RAPID DRAM caching on the Samsung 840 EVO SSD

A RAM disk for your SSD

Samsung's 840 EVO is a breath of fresh air in a sea of largely cookie-cutter SSDs. It's a true original—the first solid-state drive to combine TLC main storage with a faster SLC write cache. Thanks in part to this TurboWrite cache, the EVO is quick enough to keep pace with high-end SSDs. In some tests, it's even faster than Samsung's flagship 840 Pro.

Yet the 840 EVO is priced firmly in budget territory. The 250GB model has dropped to $185 already, and the terabyte variant sells for 65 cents per gig. Meanwhile, the 500GB drive we reviewed last month is under $400.

Our first look at the 840 EVO was admittedly put together in a bit of a hurry. We got the drive only a few days before the product launch, and there was barely enough time to test the thing, let alone write about it. As a result, we weren't able to explore thoroughly the EVO's secondary caching layer, otherwise known as RAPID mode. This optional, software-based solution commandeers a portion of system memory for use as a separate drive cache. It's also coming to Samsung's 840 Pro later this year.

Since the EVO review, we've been putting RAPID mode through its paces across our entire storage test suite. We now have a better sense of where this reimagined RAM disk improves performance—and where it has the opposite effect.

Before diving into our results, let's spend a moment to, ahem, refresh our memory about what RAPID mode is all about. RAPID stands for Real-time Accelerated Processing of I/O Data, so we should probably honor the all caps. You can enable the feature via Samsung's SSD Magician utility, and you'll need to be running Windows 7 or 8 for it to work. When enabled, RAPID mode takes up to a gigabyte of system memory. DRAM is even faster than the flash memory used in SSDs, so there's some wisdom in using it as a high-speed cache for solid-state drives.

Samsung says RAPID mode is used primarily to accelerate read performance. Data is speculatively loaded into the cache based on user access patterns. The caching intelligence considers several factors, including how frequently and recently the data has been accessed. It also discriminates against large media files to avoid polluting the cache with data that may not benefit from quicker access times.

If this all of sounds familiar, you may be thinking of Windows' SuperFetch routine, which does something similar. However, Samsung says SuperFetch only considers application data. RAPID mode looks at each and every read request, and it's capable of caching both application and user data.

In addition to accelerating read performance, RAPID mode offers "write optimization." Caching writes in DRAM before moving them to the 840 EVO's flash-based TurboWrite cache helps maintain performance at high queue depths, according to Samsung. This approach evidently conveys other benefits, too, but Samsung isn't talking specifics. It's possible RAPID mode collates incoming data and writes it to the flash in larger blocks to make more efficient use of the NAND's limited endurance.

Of course, caching writes in volatile DRAM introduces the potential for data loss due to an unexpected power failure. RAPID mode transfers the contents of its write cache to the SSD every time the Windows write cache is flushed, so it doesn't hang on to the data for too long. There's still some risk attached, which is probably why RAPID mode is disabled by default.

Between reboots, Samsung's software automatically copies the contents of the RAPID cache to main storage. This seamless step preserves the contents of the cache, but it'll cost you a gig of SSD capacity.

Although RAPID mode is limited to 1GB right now, Samsung tells us future versions of the software may allow users to allocate even more memory for caching. Plenty of enthusiast rigs have gobs of RAM, and it would be nice to be able to dedicate more of it to the RAPID cache. The software already uses compression to make the most of the available space, though.

Now that we've covered the basics, let's see what RAPID mode can do. We tested the Samsung 840 EVO with and without the RAPID cache enabled. Both configs were tested using the same system described on this page of our 840 EVO review.

Load times
Load time tests are usually prime candidates for cache-based acceleration, but there's a caveat attached for RAPID mode. Because the cache relies on software that loads after the OS, it can't speed up the boot process. Our Windows 7 boot duration test highlights this fact nicely.

The RAPID config actually takes a fraction of a second longer than the standard 840 EVO. We measure the Win7 boot duration using the OS's built-in performance monitoring tools, which tell us how long it takes for the system to become idle after the OS first begins to load. The extra time required to load Samsung's Magician utility in the background probably explains the 0.4-second delay associated with RAPID mode in this test.

RAPID mode isn't meant to accelerate OS load times, but it should load games faster... right?

Not these ones, at least according to our stopwatch. There was essentially no difference between the EVO's standard and RAPID configs in our usual level load tests. We repeated the tests five times with the RAPID config, providing ample opportunity for the software to pick up on the repetitive access pattern. The later runs weren't consistently faster than the earlier ones, though.

Note that all the SSDs are pretty evenly matched in these tests. They're all within about a second of each other, which suggests that moving away from mechanical storage may effectively eliminate storage as a bottleneck.

Now, let's see what happens in our other storage tests.