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SSD Myths and Legends - "write endurance" This, below, is the original text of my SSD endurance article published in March 2007 Does the fatal gene of "write endurance" built into flash SSDs prevent their deployment in intensive server acceleration applications? It was certainly true as little as a few years ago (2005). What's the risk with today's devices? Flash based solid state disks would seem to be the ideal virtual storage device... In every other respect you can treat them in exactly the same way as a hard drive:- same interface, same software model. They even fit mechanically into the same standard hard drive slots. And in many ways they are better - significantly faster, consuming less electric power and more tolerant of ambient temperature and vibration extremes. You mostly don't need to know about what's inside them. They are the perfect "fit and forget" storage product. In the smaller form factors like 1.8" and 2.5" - the gap in capacity between SSDs and hard drives has disappeared. If it wasn't for the price you'd use them - right? (The price advantage of SSDs in particular applications is discussed in another article.) What's wrong with this utopian vision? And why is it that even if you were offered a flash SSD accelerator for your server absolutely FREE you might still hesitate about installing it? The answer explains why the flash SSD server acceleration market still isn't a billion dollar plus market - even 4 years after I first posed this exact same question. When you look in more detail at flash SSDs there is just one skinny dark stormcrow hanging around the edge of this picture which makes you feel uneasy about a technology which in other respects is acquiring an untarnished reputation. That's the prickly issue of write endurance. Write Endurance: - The number of write cycles to any block of flash is limited - and once you've used up your quota for that block - that's it! The disk can become unreliable. In the early days of flash SSDs managing this was a real headache for oems and users. The maximum number of write cycles to an address block - the endurance - was initially small (about 10,000 write cycles in 1994, rising to 100,000 in 1997). And the capacity of flash storage was small too. So the write endurance limit was more than just a theoretical consideration. In the worst case - you could destroy a flash SSD in less than a week! But in those days the SSD was being designed in by electronics engineers who knew exactly how the SSD was going to be used. If it helped solve the problem they could even rewrite the software a different way to lessen the risk. But when you buy an SSD for use in a notebook or server - you don't write the software. You don't control the data. So how do you know in advance if you're going to hit that brick wall? This fear is an issue which has slowed down the adoption of flash SSDs in commercial server acceleration applications. Write endurance doesn't affect RAM based SSDs - which have until now dominated that part of the market - mainly due to their superior speed. But the speed of flash SSDs has improved to the point where they could replace RAM based SSDs in many server acceleration slots at a much lower price - if it wasn't for the worry about endurance. Write endurance has been a FUD issue for potential enterprise server users. They know it's lurking there - but who can they trust to quantify the problem in their own language? Server makers didn't want users to know about SSDs (any type - period) during 2000 to 2006 - because more SSDs meant selling less servers. In the 2005 edition of the SSD Buyers Guide I wrote about the problem... "One disadvantage, compared to RAM SSDs is that flash has an intrinsic limit on the total number of write cycles to a particular destination. The limit varies, according to manufacturer but is over millions of cycles in the most durable products. Internal controllers within the flash SSD manage this phenomenon and can reallocate physical media transparently to prolong media life. In most applications, high endurance flash SSDs can have a reliable operating life which is typically 3 times as high as that of a hard drive. But I would hesitate about installing a flash SSD as a server speedup in a university maths research department, for example, or in other applications where the ratio of data writes to data reads is unusually high." In May 2006 I came to the conclusion that my earlier doubts may need to be revised. It was clear from reader emails and negative comments about SSDs which I saw in other publications that fear and doubt about the impact of write endurance was slowing down adoption of flash SSDs in the server acceleration market. It was also clear that most users didn't know how to interpret the kind of data being offered by SSD oems - which was designed for an elite audience of electronics designers - and not for managers of storage systems. So I contacted all flash SSD oems with the idea of setting up a standard way of presenting endurance life expectancy data - with a proposal which I called the "SSD Half Life." That dialog met with some enthusiasm but there wasn't enough vendor support to take it further. The SSD oems I talked to took reliability very seriously - but didn't want their own proprietary reliability schemes and models swamped by a general industry wide scheme. The way that SSD oems deal with the management of write endurance internally within their products varies but they all have the common theme of scoring how many times a block of memory has been written to, and then reallocating physical blocks to logical blocks dynamically and transparently to spread the load across the whole disk. In a well designed flash SSD you would have to write to the whole disk the endurance number of cycles to be in danger. Some manufacturers go a step further. SiliconSystems has a patented algorithm which delivers a lifetime which it claims is better than simplistic wear levelling. Another manufacturer Adtron actually has a percentage of spare flash blocks in the SSD - which are invisible to the host interface and don't show up as spare storage. But internally - when blocks get close to the limit - the data is transparently switched over to the spare parts of the disk to give an additional breathing space. The precise numbers are a proprietary secret but are based on analyzing the software from real customers' SSD applications over many years. OEMs, like these, which target high reliability applications, are also more picky about which flash chips they use, and qualify them according to the results they see from testing. the Flash SSD Application from Hell* - the Rogue Data Recorder In most real-life applications the computer does a lot more reads from disk than writes - and the duty cycle (that's the percentage of time that the disk is being accessed at all) is low. But to estimate whether you should be worried about write endurance with today's SSD technology I've chosen a worst case example - the Rogue Data Recorder. Real hard disk based data recorders from companies like Conduant can record data continuously in an endless loop. They are useful for a bunch of applications such as capturing pre-trigger data in seismic events, capturing unpredictable data for modelling and bugging phone calls. I managed a company in the mid 80s which pushed storage technology to its limits to get wire speed continuous recording onto disk and massive memory systems with inbuilt real-time trigger processors, embedded workstations and array processors for various types of industries and agencies. That was a good education for my day job now of cutting and pasting. Most of you wouldn't set out to design a real-time data recorder - and if you are doing that - this article isn't going to tell you anything you don't already know. But by looking at the worst thing which could happen and estimating a confidence boundary from that - it can tell you how much you need to worry. The nightmare scenario for your new server acceleration flash SSD is that a piece of buggy software written by the maths department in the university or the analytics people in your marketing department is launched on a Friday afternoon just before a holiday weekend - and behaves like a data recorder continuously writing at maximum speed to your disk - and goes unnoticed. How long have you got before the disk is trashed? For this illustrative calculation I'm going to pick the following parameters:- | ||||||||
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| To get that very high speed the process will have
to write big blocks (which also simplifies the calculation). We assume perfect wear leveling which means we need to fill the disk 2 million times to get to the write endurance limit. 2 million (write endurance) x 64G (capacity) divided by 80M bytes / sec gives the endurance limited life in seconds. That's a meaningless number - which needs to be divided by seconds in an hour, hours in a day etc etc to give... The end result is 51 years! But you can see how just a handful of years ago - when write endurance was 20x less than it is today - and disk capacities were smaller. For real-life applications refinements are needed to the model which take into account the ratio and interaction of write block size, cache operation and internal flash block size. I've assumed perfect cache operation - and sequential writes - because otherwise you don't get the maximum write speed. Conversely if you aren't writing at the maximum speed - then the disk will last longer. Other factors which would tend to make the disk last longer are that in most commercial server applications such as databases - the ratio of reads to writes is higher than 5 to 1. And as there is no wear-out or endurance limit on read operations - the implication is to increase the operating life by the read to write ratio. As a sanity check - I found some data from Mtron (one of the few SSD oems who do quote endurance in a way that non specialists can understand). In the data sheet for their 32G product - which incidentally has 5 million cycles write endurance - they quote the write endurance for the disk as "greater than 85 years assuming 100G / day erase/write cycles" - which involves overwriting the disk 3 times a day. How to interpret these numbers? With current technologies write endurance is not a factor you should be worrying about when deploying flash SSDs for server acceleration applications - even in a university or other analytics intensive environment. How about RAID systems stuffed with flash SSDs? The calculation above gives the worst case (shortest) operating life based on stuffing data into a single disk at the fastest possible speed. Having a faster interface coming into the a box stuffed with SSDs doesn't make the life shorter - because the data can only be striped to any individual disk at the limiting rate for that disk. Au contraire:- not only can an SSD RAID array offer a multiple of a single SSD's throughput, and IOPs, just as with hard disks but depending on the array configuration the operating life can be multiplied as well - because not all the disks will operate at 100% duty cycle. That means that MTBF and not write endurance will be the limiting factors. And although oem published MTBF data for hard disks has been discredited recently - the MTBF data for flash SSDs has been verified for over a decade in more discriminating applications in high reliability embedded systems. I've been waiting years for storage oems to start marketing flash SSD based storage arrays - as alternatives to RAM based systems. What's held that market back has been the looming shadow of write endurance. That myth - that flash SSDs wear out - now belongs to the past. ...Later:- in May 2008 - in an exclusive interview with STORAGEsearch.com - AMCC 3ware confirmed it is working with leading SSD oems to develop products which will support the unique needs of the flash SSD RAID market. * clarifying why the Rogue Data Recorder is the Worst Case Application I didn't need to explain this choice to those who design SSDs, but it's clear from some comments I've seen that some readers who don't have an electronics / semiconductor education or don't know enough about SSD internals have queried this choice. Why, for example, does the data recorder example stress a flash SSD more than say continuously writing to the same sector? The answer is that the data recorder - by writing to successively sectors - makes the best use of the inbuilt block erase/write circuits and the external (to the flash memory - but still internal to the SSD) buffer / cache. In fact it's the only way you can get anywhere close to the headline spec data write throughput and write IOPS. This is because you are statistically more likely to find that writing to different address blocks finds blocks that are ready to write. If you write a program which keeps rewriting data to exactly the same address sector - all successive sector writes are delayed until the current erase / write cycle for that part of the flash is complete. So it actually runs at the slowest possible write speed. If you were patient enough to try writing a million or so times to the same logical sector - then at some point the internal wear leveling processor would have transparently assigned it to a different physical address in flash by then. This is invisible to you. You think you're still writing to the same memory - but you're not. It's only the logical address that stays the same. In fact you are stuffing data throughout the whole physical flash disk - while operating at the slowest possible write speed. It will take orders of magnitude longer wearing out the memory in this way than in the rogue data recorder example. That's because writing to flash is not the same as writing to RAM, and also because writing to a flash SSD sector is not the same as writing to a block of dumb flash memory. There are many layers of virtualization between you and the raw memory in an SSD. If you write to a dumb flash memory chip successively to the same location - then you can see a bad result quite quickly. But comparing dumb flash storage to intelligent flash SSDs is like comparing the hiss on a 33 RPM vinyl music album to that on a CD. They are quite different products - even though they can both play same music. ...Later:- Clarifying Flash Endurance Specifications I've added this footnote in response to some reader emails which asked about the variation in flash endurance specs quoted by different flash SSD oems. Like any semiconductor related spec (such as memory speed, or analog offset voltage in an op-amp, or failed memory blocks in a high density RAM chip) - there's a spread of performance which depends on the process and may vary over time in the same wafer fab, or at the same time when chips are made in different fabs within the same company. A spec such as 100k or 1 million or 10 million erase-write cycles - is a business decision made according to market conditions - which gives generic semiconductor buyers a confidence level that if they buy 1 million chips - then the reject rate - of those that will fail due to process tolerances - will be acceptably low. The shape of the distribution curve may not actually be gaussian - but there is a distribution curve in there which is implied by the published specs. Due to process variations between oems (some designs will be automatically shrunk from old designs, other layout geometries may be recompiled or optimized for that particular process point) there will be vast differences between the endurance from different chipmakers. As the generic semiconductor flash market doesn't place a premium on this spec - the "datasheet" published standard number will gradually improve at a slow pace (every 2-3 years) even if some oems are making chips today which are 10x better. If I was designing a high reliability flash SSD - I would want to get into the process details - qualify devices and order them to my own spec. Currently SSD volumes are too low - to give much buying power with flash chipmakers. Therefore few SSD oems are able to buy flash chips qualified to their own specs. (This is done by batch testing samples and by negotiation with the fab where the chips are made.) Some SSD oems make their own flash chips - and while this gives them more control over the end to end process - it does not necessarily mean that they start with the best chips. See also:- is eMLC the true successor to SLC in enterprise flash SSD?- which so called "enterprise MLC" tastes the sweetest? How come there are so many different and contradictory reliability claims? | ||||||||
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For more information about SSDs take a look
at these resources
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