﻿ LSI Blog | over-provisioning Archives - LSI Blog

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


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Enter SSDs


Keeping your SSD TRIM


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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.

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