In part one of this Write Amplification (WA) series, I examined how WA works in basic solid-state drives (SSDs). Part two now takes a deeper look at how SSDs that use some form of data reduction technology can see a very big and positive impact on WA.
Data reduction technology can master data entropy
The performance of all SSDs is influenced by the same factors – such as the amount of over provisioning and levels of random vs. sequential writing – with one major exception: entropy. Only SSDs with data reduction technology can take advantage of entropy – the degree of randomness of data – to provide significant performance, endurance and power-reduction advantages.
Data reduction technology parlays data entropy (not to be confused with how data is written to the storage device – sequential vs. random) into higher performance. How? When data reduction technology sends data to the flash memory, it uses some form of data de-duplication, compression, or data differencing to rearrange the information and use fewer bytes overall. After the data is read from flash memory, data reduction technology, by design, restores 100% of the original content to the host computer This is known as “loss-less” data reduction and can be contrasted with “lossy” techniques like MPEG, MP3, JPEG, and other common formats used for video, audio, and visual data files. These formats lose information that cannot be restored, though the resolution remains adequate for entertainment purposes.
The multi-faceted power of data reduction technology
My previous blog on data reduction discusses how data reduction technology relates to the SATA TRIM command and increases free space on the SSD, which in turn reduces WA and improves subsequent write performance. With a data-reduction SSD, the lower the entropy of the data coming from the host computer, the less the SSD has to write to the flash memory, leaving more space for over provisioning. This additional space enables write operations to complete faster, which translates not only into a higher write speed at the host computer but also into lower power use because flash memory draws power only while reading or writing. Higher write speeds also mean lower power draw for the flash memory.
Because data reduction technology can send less data to the flash than the host originally sent to the SSD, the typical write amplification factor falls below 1.0. It is not uncommon to see a WA of 0.5 on an SSD with this technology. Writing less data to the flash leads directly to:
Each of these in turn produces other benefits, some of which circle back upon themselves in a recursive manner. This logic diagram highlights those benefits.
So this is a rare instance when an amplifier – namely, Write Amplification – makes something smaller. At LSI, this unique amplifier comes in the form of the LSI® DuraWrite™ data reduction technology in all SandForce® Driven™ SSDs.
This three-part series examines some of the details behind write amplification, a key property of all NAND flash-based SSDs that is often misunderstood in the industry.
In today’s solid state drives (SSDs), the NAND flash memory must be erased before it can store new data. In other words, data cannot be overwritten directly as it is in a hard disk drive (HDD). Instead, SSDs use a process called garbage collection (GC) to reclaim the space taken by previously stored data. This means that write demands are heavier on SSDs than HDDs when storing the same information.
This is bad because the flash memory in the SSD supports only a limited number of writes before it can no longer be read. We call this undesirable effect write amplification (WA). In my blog, Gassing up your SSD, I describe why WA exists in a little more detail, but here I will explain what controls it.
It’s all about the free space
I often tell people that SSDs work better with more free space, so anything that increases free space will keep WA lower. The two key ways to expand free space (thereby decreasing WA) are to 1) increase over provisioning and 2) keep more storage space free (if you have TRIM support).
As I said earlier, there is no WA before GC is active. However, this pristine pre-GC condition has a tiny life span – just one full-capacity write cycle during a “fresh-out-of-box” (FOB) state, which accounts for less than 0.04% of the SSD’s life. Although you can manually recreate this condition with a secure erase, the cost is an additional write cycle, which defeats the purpose. Also keep in mind that the GC efficiency and associated wear leveling algorithms can affect WA (more efficient = lower WA).
The other major contributor to WA is the organization of the free space (how data is written to the flash). When data is written randomly, the eventual replacement data will also likely come in randomly, so some pages of a block will be replaced (made invalid) and others will still be good (valid). During GC, valid data in blocks like this needs to be rewritten to new blocks. This produces another write to the flash for each valid page, causing write amplification.
With sequential writes, generally all the data in the pages of the block becomes invalid at the same time. As a result, no data needs relocating during GC since there is no valid data remaining in the block before it is erased. In this case, there is no amplification, but other things like wear leveling on blocks that don’t change will still eventually produce some write amplification no matter how data is written.
Calculating write amplification
Write Amplification is fundamentally the result of data written to the flash memory divided by data written by the host. In 2008, both Intel and SiliconSystems (acquired by Western Digital) were the first to start talking publically about WA. At that time, the WA of all SSDs was something greater than 1.0. It was not until SandForce introduced the first SSD controller with DuraWrite™ technology in 2009 that WA could fall below 1.0. DuraWrite technology increases the free space mentioned above, but in a way that is unique from other SSD controllers. In part two of my write amplification series, I will explain how DuraWrite technology works.
This three-part series examines some of the details behind write amplification, a key property of all NAND flash-based SSDs that is often misunderstood in the industry.
It seems like our smartphones are getting bigger and bigger with each generation. Sometimes I see people holding what appears to be a tablet computer up next to their head. I doubt they know how ridiculous they look to the rest of us, and I wonder what pants today have pockets that big.
I certainly do like the convenience of the instant-on capabilities my smart phone gives me, but I still need my portable computer with its big screen and keyboard separate from my phone.
A few years ago, SATA-IO, the standards body, added a new feature to the Serial ATA (SATA) specification designed to further reduce battery consumption in portable computer products. This new feature, DevSleep, enables solid state drives (SSDs) to act more like smartphones, allowing you to go days without plugging in to recharge and then instantly turn them on and see all the latest email, social media updates, news and events.
Why not just switch the system off?
When most PC users think about switching off their system, they dread waiting for the operating system to boot back up. That is one of the key advantages of replacing the hard disk drive (HDD) in the system with a faster SSD. However, in our instant gratification society, we hate to wait even seconds for web pages to come up, so waiting minutes for your PC to turn on and boot up can feel like an eternity. Therefore, many people choose to leave the system on to save those precious moments… but at the expense of battery life.
Can I get this today?
To further extend battery life, the new DevSleep feature requires a signal change on the SATA connector. This change is currently supported only in new Intel® Haswell chipset-based platforms announced this June. What’s more, the SSD in these systems must support the DevSleep feature and monitor the signal on the SATA connector. Most systems that support DevSleep will likely be very low-power notebook systems and will likely already ship with an SSD installed using a small mSATA, M.2, or similar edge connector. Therefore, the signal change on the SATA interface will not immediately affect the rest of the SSDs designed for desktop systems shipping through retail and online sources. Note that not all SSDs are created equal and, while many claim support for DevSleep, be sure to look at the fine print to compare the actual power draw when in DevSleep.
At Computex last month, LSI announced support for the DevSleep feature and staged demonstrations showing a 400x reduction in idle power. It should be noted that a 400x reduction in power does not directly translate to a 400x increase in battery life, but any reduction in power will give you more time on the battery, and that will certainly benefit any user who often works without a power cord.
Not likely. But you might think that solving your computer data security problems is very well possible when someone tells you that TCG Opal is the key. According to its website, “The Trusted Computing Group (TCG) is a not-for-profit organization formed to develop, define and promote open, vendor-neutral, global industry standards, supportive of a hardware-based root of trust, for interoperable trusted computing platforms.”
That might take a bit to digest, but think about TCG as a group of companies creating standards to simplify deployment and increase adoption of data security. The consortium has two better known specifications called TCG Enterprise and TCG Opal.
Sorting through the alphabet soup of data security
“Our SED with TCG Opal provides FDE.” While this might look like a spoonful of alphabet soup, it is music to the ears of a corporate IT manager. Let me break it down for those who just hear fingernails on the chalkboard. A self-encrypting drive (SED) is one that embeds a hardware-based encryption engine in the storage device. One chief benefit is that the hardware engine performs the encryption, preserving full performance of the host CPU. An SED can be a hard disk drive (HDD) or a solid state drive (SSD). True, traditional software encryption can secure data going to the storage device, but it consumes precious host CPU bandwidth. The related term, full drive encryption (FDE), is used to describe any drive (HDD or SSD) that stores data in an encrypted form. This can be through either software-based (host CPU) or hardware-based (an SED) encryption.
Most people would assume that if their work laptop were lost or stolen, they would suffer only some lost productivity for a short time and about $1,500 in hardware costs. However, a study by Intel and the Ponemon Institute found that the cost of a lost laptop totaled nearly $50,000 when you account for lost IP, legal costs, customer notifications, lost business, harm to reputation, and damages associated with compromising confidential customer information. When the data stored on the laptop is encrypted, this cost is reduced by nearly $20,000. This difference certainly supports the need for better security for these mobile platforms.
When considering a security solution for this valuable data, you have to decide between a hardware-based SED and a host-based software solution. The primary problem with software solutions is they require the host CPU to do all of the encryption. This detracts from the CPU’s core computing work, leaving users with a slower computer or forcing them to pay for greater CPU performance. Another drawback of many software encryption solutions is that they can be turned off by the computer user, leaving data in the clear and vulnerable. Since hardware-based encryption is native to the HDD or SSD, it cannot be disabled by the end user.
In April 2013, LSI and a few other storage companies worked with the Ponemon Institute to better understand the value of hardware-based encryption. You can read about the details in the study here, but the quick summary is that hardware-based encryption solutions can offer a 75% total cost savings over software-based solutions, on average.
When is this available?
At the Computex Taipei 2013 show earlier this month, LSI announced availability of a firmware update for SandForce® controllers that adds support for TCG Opal. The LSI suite at the show featured TCG Opal demonstrations using self-encrypting SSDs provided by SandForce Driven™ member companies, including Kingston, A-DATA, Avant and Edge. (Contact SSD manufacturers directly for product availability.)
Imagine a bathtub full of water and asking someone to empty the tub while you turn your back for a moment. When you look again and see the water is gone, do you just assume someone pulled the drain plug?
I think most people would, but what about the other methods of removing the water like with a siphon, or using buckets to bail out the water? In a typical bathroom you are not likely to see these other methods used, but that does not mean they do not exist. The point is that just because you see a certain result does not necessarily mean the obvious solution was used.
I see a lot of confusion in forum posts from SandForce Driven™ SSD users and reviewers over how the LSI® DuraWrite™ data reduction and advanced garbage collection technology relates to the SATA TRIM command. In my earlier blog on TRIM I covered this topic in great detail, but in simple terms the operating system uses the TRIM command to inform an SSD what information is outdated and invalid. Without the TRIM command the SSD assumes all of the user capacity is valid data. I explained in my blog Gassing up your SSD that creating more free space through over provisioning or using less of the total capacity enables the SSD to operate more efficiently by reducing the write amplification, which leads to increased performance and flash memory endurance. So without TRIM the SSD operates at its lowest level of efficiency for a particular level of over provisioning.
Will you drown in invalid data without TRIM?
TRIM is a way to increase the free space on an SSD – what we call “dynamic over provisioning” – and DuraWrite technology is another method to increase the free space. Since DuraWrite technology is dependent upon the entropy (randomness) of the data, some users will get more free space than others depending on what data they store. Since the technology works on the basis of the aggregate of all data stored, boot SSDs with operating systems can still achieve some level of dynamic over provisioning even when all other files are at the highest entropy, e.g., encrypted or compressed files.
With an older operating system or in an environment that does not support TRIM (most RAID configurations), DuraWrite technology can provide enough free space to offer the same benefits as having TRIM fully operational. In cases where both TRIM and DuraWrite technology are operating, the combined result may not be as noticeable as when they’re working independently since there are diminishing returns when the free space grows to greater than half of the SSD storage capacity.
So the next time you fill your bathtub, think about all the ways you can get the water out of the tub without using the drain. That will help you remember that both TRIM and DuraWrite technology can improve SSD performance using different approaches to the same problem. If that analogy does not work for you, consider the different ways to produce a furless feline, and think about what opening graphic image I might have used for a more jolting effect. Although in that case you might not have seen this blog since that image would likely have gotten us banned from Google® “safe for work” searches.
I presented on this topic in detail at the Flash Memory Summit in 2011. You can read it in full here: http://www.lsi.com/downloads/Public/Flash%20Storage%20Processors/LSI_PRS_FMS2011_T1A_Smith.pdf
Tags: bathtub drain, controller, data reduction technology, DuraWrite, flash, flash controller, flash memory, Flash Memory Summit, NAND, over-provisioning, SandForce, SandForce Driven SSD, SATA, Serial ATA, solid state drive, TRIM
It may sound crazy, but hard disk drives (HDDs) do not have a delete command. Now we all know HDDs have a fixed capacity, so over time the older data must somehow get removed, right? Actually it is not removed, but overwritten. The operating system (OS) uses a reference table to track the locations (addresses) of all data on the HDD. This table tells the OS which spots on the HDD are used and which are free. When the OS or a user deletes a file from the system, the OS simply marks the corresponding spot in the table as free, making it available to store new data.
The HDD is told nothing about this change, and it does not need to know since it would not do anything with that information. When the OS is ready to store new data in that location, it just sends the data to the HDD and tells it to write to that spot, directly overwriting the prior data. It is simple and efficient, and no delete command is required.
However, with the advent of NAND flash-based solid state drives (SSDs) a new problem emerged. In my blog, Gassing up your SSD, I explain how NAND flash memory pages cannot be directly overwritten with new data, but must first be erased at the block level through a process called garbage collection (GC). I further describe how the SSD uses non-user space in the flash memory (over provisioning or OP) to improve performance and longevity of the SSD. In addition, any user space not consumed by the user becomes what we call dynamic over provisioning – dynamic because it changes as the amount of stored data changes.
When less data is stored by the user, the amount of dynamic OP increases, further improving performance and endurance. The problem I alluded to earlier is caused by the lack of a delete command. Without a delete command, every SSD will eventually fill up with data, both valid and invalid, eliminating any dynamic OP. The result would be the lowest possible performance at that factory OP level. So unlike HDDs, SSDs need to know what data is invalid in order to provide optimum performance and endurance.
Keeping your SSD TRIM
A number of years ago, the storage industry got together and developed a solution between the OS and the SSD by creating a new SATA command called TRIM. It is not a command that forces the SSD to immediately erase data like some people believe. Actually the TRIM command can be thought of as a message from the OS about what previously used addresses on the SSD are no longer holding valid data. The SSD takes those addresses and updates its own internal map of its flash memory to mark those locations as invalid. With this information, the SSD no longer moves that invalid data during the GC process, eliminating wasted time rewriting invalid data to new flash pages. It also reduces the number of write cycles on the flash, increasing the SSD’s endurance. Another benefit of the TRIM command is that more space is available for dynamic OP.
Today, most current operating systems and SSDs support TRIM, and all SandForce Driven™ member SSDs have always supported TRIM. Note that most RAID environments do not support TRIM, although some RAID 0 configurations have claimed to support it. I have presented on this topic in detail previously. You can view the presentation in full here. In my next blog I will explain how there may be an alternate solution using SandForce Driven member SSDs.
Have you ever run out of gas in your car? Do you often risk running your gas tank dry? Hopefully you are more cautious than that and you start searching for a gas station when you get down to a ¼ tank. You do this because you want plenty of cushion in case something comes up that prevents you from getting to a station before it is too late.
The reason most people stretch their tank is to maximize travel between station visits. The downside to pushing the envelope to “E” is you can end up stranded with a dead vehicle waiting for AAA to bring you some gas.
Now most people know you don’t put gas in a solid state drive (SSD), but the pros and cons associated with how much you leave in the “tank” is very relevant to SSDs.
To understand how these two seemingly unrelated technologies are similar, we first need to drill into some technical SSD details. To start, SSDs act, and often look, like traditional hard disk drives (HDDs), but they do not record data in the same way. SSDs today typically use NAND flash memory to store data and a flash controller to connect the memory with the host computer. The flash controller can write a page of data (often 4,096 bytes) directly to the flash memory, but cannot overwrite the same page of data without first erasing it. The erase cycle cannot expunge only a single page. Instead, it erases a whole block of data (usually 128 pages). Because the stored data is sometimes updated randomly across the flash, the erase cycle for NAND flash requires a process called garbage collection.
Garbage collection is just dumping the trash
Garbage collection starts when a flash block is full of data, usually a mix of valid (good) and invalid (older, replaced) data. The invalid data must be tossed out to make room for new data, so the flash controller copies the valid data of a flash block to a previously erased block, and skips copying the invalid data of that block. The final step is to erase the original whole block, preparing it for new data to be written.
Before and during garbage collection, some data – valid data copied during garbage collection and the (typically) multiple copies of the invalid data – is in two or more locations at once, a phenomenon known as write amplification. To store this extra data not counted by the operating system, the flash controller needs some spare capacity beyond what the operating system knows. This is called over-provisioning (OP), and it is a critical part of every NAND flash-based SSD.
Over-provisioning is like the gas that remains in your tank
While every SSD has some amount of OP, some will have more or less than others. The amount of OP varies depending on trade-offs made between total storage capacity and benefits in performance and endurance. The less OP allocated in an SSD, the more information a user can store. This is like the driver who will take their tank of gas clear down to near-empty just to maximize the total number of miles between station visits.
What many SSD users don’t realize is there are major benefits to NOT stretching this OP area too thin. When you allocate more space for OP, you achieve a lower write amplification, which translates to a higher performance during writes and longer endurance of the flash memory. This is like the driver who is more cautious and visits the gas station more often to enable greater flexibility in selecting a more cost-effective station, and allows for last-minute deviations in travel plans that end up burning more fuel than originally anticipated.
The choice is yours
Most SSD users do not realize they have full control of how much OP is configured in their SSD. So even if you buy an SSD with “0%” OP, you can dedicate some of the user space back to OP for the SSD.
A more detailed presentation of how OP works and what 0% OP really means was presented at the Flash Memory Summit 2012 and can be viewed with this link for your convenience: http://www.lsi.com/downloads/Public/Flash%20Storage%20Processors/LSI_PRS_FMS2012_TE21_Smith.pdf
It pays to be the cautious driver who fills the gas tank long before you get to empty. When it comes to both performance and endurance, your SSD will cover a lot more ground if you treat the over-provisioning space the same way – keeping more in reserve.
Most people fully understand that electronics are useless without power, but what happens when devices lose power in the middle of operating? That answer is highly dependent on a number of variables including what type of electronic device is in question.
For solid state drives (SSDs) the answer depends on factors such as whether an uninterruptable power supply (UPS) is connected, what controller or flash processor is used, the design of the power circuit of the SSD, and the type of memory. If an SSD is in the middle of a write operation to the flash memory and power to the SSD is disconnected, many bad things could happen if the right safety measures are not in place. Many users do not think about all the non-user initiated operations the SSDs may be performing like background garbage collection that could be under way when the power fails. Without the correct protection, in most cases data will be corrupted.
According to the Nielsen company, 108.4 million viewers were tuned into the 2013 Super Bowl in New Orleans only to be shocked to witness the power go down for 34 minutes in the middle of the game. If power can be lost during such an incredibly high profile event such as this, it can happen just about anywhere.
Inside the New Orleans Superdome stadium operations and broadcast server rooms
Enterprise computing environments typically have multiple servers with data connections and lots of storage. Over the past few years, a larger percentage of the storage is kept on SSDs for the very active or “hot” data. This greatly improves data access time and reduces overall latency of the system. Enterprise servers are often connected to UPS systems that supply the connected devices with temporary power during a power failure.
Usually this is enough power to support uninterrupted system operations until power is restored, or at least until technicians can complete their current work and shut down safely. However there are many times when UPS systems are not deployed or fail to operate properly themselves. In those cases, the server will experience a power failure as abrupt as if someone had yanked the power cord from the wall socket.
The LSI® SandForce® flash controllers are at the heart of many popular SSDs sold today. The flash controller connects the host computer with the flash memory to store user data in fast non-volatile memory. The SandForce flash controllers are specifically engineered to operate in different environments, and the SF-2500/2600 FSPs are designed to provide the high level of data integrity required for enterprise applications. In the area of power failure protection, they include a combination of firmware (FW) and hardware circuitry that monitors the power coming into the SSD. In the event of a power failure or even a brown-out, the flash controller is alerted to the situation and hold-up capacitors in the SSD provide the necessary power and time for the controller to complete pending writes to the flash memory. This same circuit is also designed to prevent the risk of lower page corruption with Multi-level Cell (MLC) memory.
Watch out for SSD solutions that provide backup capacitors, but lack the necessary support circuitry and special firmware to ensure the data is fully committed to the flash memory before the power runs out. Even if these other special circuits are present, only truly enterprise SSDs that are meticulously designed and tested to withstand power failures are up to the task of storing and protecting highly critical data.
In the control room and down on the field
The usage patterns of non-enterprise systems like notebooks and ultrabooks call for a different power failure support mechanism. Realize that when you have a notebook or ultrabook system, you have a built in mini-UPS system. A power outage from the wall socket has no impact to the system until the battery gets low. At that point the operating system will tell the computer to shut down and that will be ample warning for the SSD to safely shut down and ensure the integrity of the data. But what if the operating system locks up and does not warn the SSD or the system is an A/C-powered desktop without a battery?
The LSI SF-2100/2200 FSPs are purpose-built for these client environments and operate with the assumption that power could disappear at any point in time. They use special FW techniques so that even without a battery present, as is the case with desktop systems, they greatly limit the potential for data loss.
The naked facts
It should be clear that the answer to the original question is highly dependent upon the flash controller at the heart of the SSD. Without having the critical features discussed above and designed into the LSI SandForce flash controllers, it is very possible to lose data during a power failure. The LSI SandForce flash controllers are engineered to withstand power failures like the one that hit New Orleans at the Super Bowl, but don’t expect them to fix wardrobe malfunctions.