Investigating Samsung's RAPID Caching Technology

SSDs in Overdrive

Previously, in our feature of the Samsung SSD 840 EVO versus the Crucial M500, we noted that the drive featured something called RAPID mode and we briefly explained that it was a new caching technology that aims to boost random I/O performance with smaller size data at low queue depths. Today, we will be taking a closer look at this new technology and also find out if it is really effective.

RAPID actually stands for Real-time Accelerated Processing of I/O Data and it was introduced in July last year at the Samsung SSD Global Summit alongside the Samsung SSD 840 EVO. At that time, many believed that RAPID mode was introduced to negate the disadvantages of the SSD 840 EVO’s slower TLC NAND, but Samsung has categorically stated that this is not so, and what RAPID aims to do is to bring the performance of the SSD 840 EVO to a whole new level. In addition, Samsung has also confirmed that the summit that this new caching technology will be rolled out to its flagship SSD 840 Pro and also the SSD 840 at a later time.

To be sure, using cache to improve storage performance is not new - traditional mechanical hard disk drives have long used small amounts of flash memory onboard (typically 16MB to 64MB) to help improve performance. However, what’s unique about RAPID is in the way it determines what to cache and how it goes about doing it.

How it Works

First and foremost, it is important to note is that RAPID mode is a software-based technology and as such requires Windows 7 or 8 to work - sorry Mac users - and also the appropriate Samsung Magician software (version 4.2 and later) and firmware. Samsung does not explicitly specify what kind of a CPU is required, only that it must be at least 1GHz, but it did specify at least 2GB of RAM. This is crucial because unlike other caching technology, RAPID mode accelerates performance by using idle CPU cores and unused system memory.

When activated, RAPID mode inserts a filter driver in the Windows storage stack and it monitors all your storage-related activities between the operating system, user applications and the SSD. As it does, it learns what applications you use most often and caches them for faster access in future. According to Samsung, unlike typical caching algorithms which only concern themselves with application data, RAPID looks at every read request and caches both application and user data. In addition, it also looks at frequency of access, file types, system statuses and RAM availability to dynamically maintain the cache and accelerate read operations.

RAPID optimizes write operations too by caching write requests in DRAM before moving them in larger blocks to the SSD itself. This helps improve overall performance and also increases endurance of the drive by limiting writes to the SSD cells.

There has been concerns raised with caching writes to DRAM, especially since it is a volatile form of memory and could therefore result in data loss should power be suddenly cut. However, Samsung has assured that RAPID adheres strictly to Windows conventions and the cache is frequently flushed so that data does not remain on the DRAM for longer than is necessary.

For now, RAPID mode limits itself to 1GB of system memory and between reboots, this cache is automatically transferred to the SSD. Naturally, this costs some storage space, but this move preserves contents of the cache and ensures that it constantly and effectively learns your usage patterns.

For those who are concerned about RAPID mode taking up excessive CPU and system memory resource, Samsung is quick to state that RAPID technology was designed to specifically make use of excess system resources and will automatically scale down resource usage and even free up memory for other tasks. In our own usage, we noticed that while system memory and CPU load does go up on storage intensive workloads, it was not so much as to degrade the overall user experience. Furthermore, performance gains from RAPID mode more than makes up for the extra resources it consumes, as you will see later in the benchmarks.

Test Setup

The drives will be tested on our recently revamped storage testbed. The main changes are the faster Core i5-2500K processor and an accompanying Z77 motherboard which has native Thunderbolt connectivity.

• Intel Core i5-2500K (3.3GHz)
• ASUS P8Z77 Pro Thunderbolt (Intel Z77 chipset)
• 2 x 2GB DDR3-1600 memory
• MSI GeForce 8600 GTS
• Windows 7

We have also revised our benchmarks, ditching older benchmarks such as HD Tune and also including an all new timing test to better evaluate the drive’s real world performance.

The list of benchmarks used are as follows:

• AS-SSD benchmark 1.7.4739
• CrystalDiskMark 3.0.1
• PCMark 7 (Storage suite)
• Iometer (version 2006.07.27)
• Timing Tests (Cold start, Reboot, Apps Launching)

What we are concerned here primarily is the effects of RAPID mode caching technology on the drive’s performance. In Samsung’s white paper, the company claims to be able to achieve transfer speeds exceeding that of SATA’s practical bandwidth and says that speeds in excess of 1GB/s is possible. Very impressive and lofty claims, and we are interested to see if it is true. Here's the list of drives we'll be comparing in our results chart:-

• Crucial M500
• Samsung SSD 840 EVO
• Samsung SSD 840 Pro
• Plextor M5 Pro Extreme
• OCZ Vector 150

Timing Tests

With RAPID mode enabled, we noticed that both boot and restart times suffered slightly, the reason for this is that the system needs to load the filter driver first as well as retrieve and reinstate the cache that was stored in the drive from your previous session - and all of this takes up some time. However, as a result of RAPID mode, application launch times did see improvements. On the Samsung SSD 840 EVO, RAPID mode enabled it to launch the entire Adobe CS6 suite a good half a second faster. This is an appreciable gain considering the small differences seen when comparing SSDs in general.

CrystalDiskMark 3.0.1 Results

CrystalDiskMark is an easy-to-run and quick utility to use to gauge a drive’s performance. It measures sequential read and write performance and random read and write speeds of random 4KB, 4KB (queue depth 32) and 512KB data.

We began with CrystalDiskMark and first impressions were very good. Sequential read and write speeds went through the roof as both the Samsung SSD 840 EVO and SSD 840 Pro managed speeds in excess of 1GB/s. These numbers exceed even those from PCIe-based SSDs that we see on notebooks. 512k read and writes saw massive improvements as once again speeds beyond 1GB/s were attained. 4k read speeds saw modest improvements but write speeds were once again in many order of magnitudes faster. Finally, on the intensive 4k, 32 queue depth workload, improvements of over 100% was achieved. 

AS SSD 1.7.4739 Results

AS SSD is a benchmark that uses non-compressible and completely random data. What this means is that the drives using controllers such as the SandForce SF-2281 cannot compress the data first, which takes away one of their strong advantages and help us score it more evenly against other drives.

On AS SSD’s Copy benchmark, we noticed that the ISO copy workload received the biggest boost as transfer speeds increased by around 50%. The Program and Game copy scenarios received more modest boosts to performance. And again, we saw notable increases in sequential read and write speeds. Performance increases on 4K reads were pretty modest, only around 30%; but 4K writes saw a whopping 400% increase. Finally, on the intensive 4K, 64-thread workloads, we recorded amazing improvements of around 230% and 1500% on read and write operations respectively.  

PCMark 7 Results

PCMark 7 is a benchmarking suite from FutureMark that evaluates the performance of Windows 7 machines. It tests a wide range workloads and aspects of the system ranging from computation, image and video manipulation and storage. We’ll be looking solely at the storage test here.

With RAPID mode enabled, both drives enjoyed substantial improvements to performance - the SSD 840 EVO saw a 11% increase, whereas the SSD 840 Pro enjoyed a 9% increase in scores. Looking at the performance breakdown chart, we can see that the biggest improvements are seen in the "Starting Applications" workload, and to a smaller extent, the "Importing Pictures" workload. 

Iometer Results (Part 1)

Lastly, we put the drives through the rigorous grind of Iometer, with different workloads and I/O queue depths. We have chosen to show results from a queue depth of 1 to 5 as this better represents the workloads a typical consumer might face.

While RAPID mode has proved thus far to be very beneficial and useful, its benefits are not readily apparent on Iometer. In some cases, there was hardly any difference in performance, and if there were, the gains were usually quite modest. 

Iometer Results (Part 2)

Finally, we look at the I/O response times for the workloads reported on the previous page. Since IOPS did not vary greatly with or without RAPID mode, average response times were mostly the same too. 

No Contest

Looking at the results of our benchmarking, we think readers would readily agree that it is puzzling why Samsung would keep the launch of RAPID mode so hush-hush and low-key. Even if the benefits are not immediately apparent in our timing tests, actual transfer speeds in benchmarks such as CrystalDiskMark and AS SSD saw very significant increases to performance. As it is, we do not think that it would be overstatement at all to say that RAPID mode works very well.

While the results are overwhelmingly positive for RAPID mode, we have to wonder why is it not enabled by default? As the old saying goes, "You cannot have your cake and eat it", and the fact that it is disabled by default suggests that there could be some risk in using it. Consider that for write operations, RAPID mode works by caching data first in your system memory, which is a volatile form of memory, meaning that it requires power to store information, what happens then in the event of a sudden power outage?

Although Samsung has reassured that its caching technology frequently flushes the cache and adheres strictly to Windows conventions, accidents can and may occur - it is just a matter of luck (or the lack of it). In the event that a power outage happens at an inopportune moment, there is the off chance that you might just lose your data that wasn't yet written to the drive. That said, we do believe the chance of this happening to be very slight, especially if you live in Singapore, as our power grid is pretty reliable and robust. And if you are worried about this, a good UPS (Uninterruptible Power Supply) should do the trick, which is pretty much a requirement in some countries like Indonesia and India. Notebook users should have no issues so long as you keep your battery pack in your notebook when you are using it.

If we had any gripes it is that RAPID mode is currently only available on Windows machines and not Macs. We know of many owners of older MacBooks who have upgraded their mechanical hard disk drives to Samsung SSDs and this is one group of users that would also benefit greatly from this caching technology.

All in all, RAPID mode is an impressive caching technology that not only works well but also cements Samsung’s reputation and standing as one of the leaders in the consumer SSD space. Already, their SSD 840 Pro and SSD 840 EVO drives are amongst the quickest in the market, but with RAPID mode enabled, their performance shoots to a whole new level that's unmatched by anything else in their competitive playing field at this point of time.

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