dillondf

There is something called read-disturb, where reading a row of NAND over and over again can disturb adjacent rows. The effect is many orders of magnitude lower than the erase wear effect and data caching within the drive (as well as data caching in the OS) mitigates it significantly. So for all intents and purposes you don't have to worry about it.

The SSD's normal internal scans will detect the bits when they start to get flaky and rewrite the block then. It might do some rough tracking to trigger a scan sooner than it would otherwrise, but it is more a firmware issue and not so much a wearout issue.

-Matt

Durability has steadily decreased as flash density has increased. 10000, then 5000, then 2000 for standard MLC parts over the last few years as densities have increased. I'm not sure what Samsung's new 3D process is. At the same time, the voltage regulation and comparators used on-chip has gotten a lot better, making it easier to detect leaky cells so reliability has significantly improved for the erase cycles the flash does have.

The original Intel 40G SSDs could handle an average of 10000 erase cycles (for each block of course), giving the 40G SSD around a 200TB minimum life if you divide it out and then divide by 2 for safety. (Intel spec'd 20TB or 40TB or something like that, 400TB @ 10000 erase cycles, divide by 2 gives you ~200TB or so).

A modern 512G Crucial SSD sits somewhere around a 2000 erase cycle durability, or around 512TB of relatively reliable wear (1PB / 2 for safety).

I would not necessarily trust an SSD all the way to the point where it says it is completely worn out, I would likely replace it well before that point or when the Hardware_ECC_Recovered counter starts to look scary. But I would certainly trust it at least through the 50% mark on the wear status. Remember that shelf life degrades as wear increases. I don't know what that curve looks like but we can at least assume somewhere in the ballpark of ~10 years new, and ~5 years at 50% wear. Below ~5 years and I would start to worry (but that's still better than a shelved HDD which can go bad in 6 months and is unlikely to last more than a year or two shelved).

-Matt

No, nobody is magicking up any fairy dust here. The author (which is me) is looking at things realistically. My maintenance costs just from a time perspective are already radically lower.

In anycase, so as long as the SSD is powered and thus able to rewrite blocks as bits start to go bad (creating some wear in the process but not a whole lot compared to normal operation), the actual data will be retained for significantly longer than 10 years.

Unpowered data retention (where there is nothing there checking for and rewriting blocks whos bits start to go bad) depends on cell wear. I didn't pull up the chip specs but my recollection is that it is 10 years for new cells and 1-2 years for a relatively worn cell. Depends on temperature of course. That means I can pull a SSD and shelve it without too much worry for a relatively long time, something that cannot be said for any hard drive.

CD's corrode from the inside out, primarily due to air trapped in the holes I believe. Even though it is laser burned there is no real care taken in constructing the plastic sides or metal film to keep up air so when it gets vaporized the oxygen sticks around and starts corroding the metal. Or the plastic gets fuzzy from UV exposure or age. Totally different ballgame.

Hard drives have even worse problems if left unpowered. I used to pull backup drives and shelve them, but their life spans are radically lowered if left unpowered on a shelf for 6 months and then replugged into a machine at a later time. I'm sure some HDD expert can say why. So I stopped doing that. Now the backup drives stay powered on 24x7.

Hard drives seem to have a limited life span whether you use them or not. I have tons of old HDDs sitting around, some well over 20 years old in fact. If I plug an old HDD in I can sometimes read the data off before the whole thing turns into a brick. I mostly use them to test disk driver error handling.

SSDs left sitting around can always be reformatted (i.e. just using TRIM to clean out the whole thing then start writing to it fresh). Their wear limit is based on rewrite cycles rather than how long they've been sitting around. HDDs are physically destroyed, you don't get a fresh start by reformatting an old HDD.

-Matt

Well, I probably don't have to tell people to never run copper between buildings. Always run fiber. The longer the runs, the higher the common mode voltage between grounds and the higher the stress on the isolation circuitry. Plus lightning strikes don't have to hit the cable directly, they can pull up the ground for the whole building and suddenly instead of having 200V of common mode you have 4000V for a few milliseconds.

Anyone who has ever wired up a T1 in the basement of a highrise knows what I mean.

-Matt

It's really unclear whether explicitly initiated tests actually help for a SSD. The SSD has its own internal mechanisms to scan the flash chips which (I presume) is unrelated to SMART, since they are required for normal operation... primarily detecting weak cells before the bits actually go bad. Whole chips can go bad out of the box but after an SSD has been running for a few months there isn't much left that can go south other than normal wear or a firmware failure.

firmware issues dominated SSD problems in the early years. Those issues are far less prevalent today though Samsung got its ass bitten by not adding enough redundancy and having data go bad on some of its newer offerings. Strictly a firmware issue. Which is another way of saying that one should always buy the not-so-bleeding edge technologies that have had the kinks worked out of them rather than the bleeding-edge-technologies that haven't.

If it starts to bite me I may change my tune. But until that happens I put it in the wasted-cycled category.

-Matt

Basically only buy SSD brands who either are chip fabs or have a relationship with a single chip fab. So. Intel, Crucial, Samsung, maybe one or two others. And that's it. And frankly I have to say that only Intel and Crucial have been really proactive about engineered fixes for failure cases. Never buy SSDs from second-tier vendors such as Kingston who always use the cheapest third-party flash chips they can find. There are literally dozens of those sorts of vendors. Hundreds, even.

-Matt

You do understand that flash erase blocks are around 128KB now, right? Random writes to a SSD can only be write-combined to a point. Once the SSD has to actually start erasing blocks and doing collections of those randomly combined sectors things can get dicey real fast in the face of further random writes. It doesn't have a magic wand. The point is that the SSD has no way to determine ahead of time what the rewrite order is going to be as random blocks continue to get written. You can rationalize it all you want, the write amplification is going to happen everywhere in the chain to one degree or another. For that matter, filesystems are ganging blocks together now too (and have been for a long time). It is not uncommon to see 16KB-64KB filesystem block sizes.

Nobody gives a shit about a mere 100GB in physical writes to a storage subsystem in a day. A single consumer 512GB crucial can handle a rate like that for almost 30 years.

Write amplification effects are not to be underestimated, but fortunately there are only a very few situations where it's an issue. And as I said, the situations can be largely mitigated by database writers getting off their butts and fixing their backends to push the complexity to the indexes and away from trying to optimize the linear layout of the database by seek-writing into it.

-Matt

To be clear here, what the poster means is that piecemeal database writes of, say, 128-byte records can cause a huge amount of write amplification, so 100GB/day worth of database writes can end up being 1000GB/day worth of flash rewrites. This issue largely goes away if the database backend appends replacement records rather then rewriting them in place, and uses the index to point to the new copy of the record. At that point the battery-backed ram the RAID system has combined with the appends results in much lower write amplification and a SSD could probably handle it.

-Matt

There are certainly workloads that will wear out a SSD, random database writes being the most common. But that is only a very small portion of the storage ecosystem, not to mention that there are plenty of ways to retool SQL backends to not do random writes any more, particularly since it doesn't gain you anything on a modern copy-on-write style filesystem verses indexing the new record yourself as an append. So I expect this particular issue will take care of itself in the future. It's a matter of not blindly using someones database backend and expecting it to be nice.

The vast majority of information stored these days is write-once-read-never, followed by write-once-read-occasionally. I expected our developer box which is chock full of uncompressed crash dumps and many, many copies of the source tree in various states to have more wear on it than it did, but after thinking about it a bit I realized that most of that data was write-once-read-hardly-at-all.

In terms of hardware life, for servers there are only a few things which might cut short a computer's life, otherwise it would easily last 50 years. (1) Electrolytic capacitors. (2) Any spinning media or low frequency transformers. (3) On-mobo flash or e2 that cannot be updated.

(1) Electrolytic capacitors have largely disappeared from motherboards in favor of solid caps which, if not over-volted, should last 30 years. Electrolytic caps are not sealed well and evaporate over time, as well as slowly burn holes in the insulator. They generally go bad 10-30 years later depending on how much you undervolt them (the more you undervolt, the longer they last). Even so I still have boards with 30+ year old electrolytics in them that work.

(2) Spinning media obviously has a limited life. That's what we are getting rid of now :-). Low frequency transformers have mostly gone away. Transformers in general... anything with windings that is, have a limited life due to the wire insulation breaking down over time but most modern use cases in a computer (if there are any at all) likely have huge errors of margin.

(3) Firmware stored in E2 and flash, or OTP eprom, rather than fuse-based proms, will become corrupt over time. 10 years is a minimum life, 20-30 years is more common. It depends on a number of factors.

Other than that there isn't much left that can go bad. All motherboards these days have micro coatings which effectively seal even the chip leads, so corrosion isn't as big a factor as it was 20 years ago. The actual chip logic basically will not fail and since the high-speed clocks on the whole mobo can be controlled, so aging effects which degrade junction performance for most of the chip can be mitigated. I suppose an ethernet port might go bad if it gets hit by lightning but I've never had a mobo ethernet go bad in my life. Switch ethernet ports going bad is usually just due to poor parts selection or overvolting which would not be present in a colocation facility or machine room.

In anycase, there is no reason under the sun that a modern computer with a SSD wouldn't last 30 years with only fan, real-time clock battery, and PSU replacements.

-Matt