Hi all, (Also posting on DC404)
I've been watching the SSD market evolve for a while, and wishing I could buy one of decent size. I periodically study the market, product, specs, and prices to see how things are going. Being an engineer who's paranoid about the security, longevity, and reliability of my data storage; I believe that the devil is in the details, and some of the details of SSD failure modes have always bugged me. In my latest review of the state of affairs, I've noted that SSD prices have dropped to very attractive ranges in the sub $ 0.50 / GB range. But, I've also discovered that the really cheap drives are missing some key features that I'd personally want to see in my drives. These are things you have to pay extra for. For the mix of features I'd like to see, I've come to realize that I'll have to pay $ 1 - $ 2 / GB (in current market conditions). This either means I wait and save up longer, or buy smaller, or both. I wanted to share what I've found. While at least two of these features are security specific, in my mind, they're almost all related to data security, in that, if your drive doesn't reliably store your data for the time period that you need it to, then your data is not very secure. Caveat 1, almost everything here is my opinion. I will talk about some facts, design options, specs, etc. But, what you want to do about those facts, what you think about whether one particular design element is there or not, is up to you. Only you can say what kind of risk factors or implications you think that has for you. Caveat 2, this is not meant to be a rigorously documented white paper. It's meant to be a synopsis of my observations after reading a fair number of ads, articles, and data sheets. As far as I know, everything I'm saying is true. Here are 23 important or critical features or specs that I would want to see in my SSD, if not all of them, at least most of them. The main point is that you just won't get most of these on a cheap drive. And, you won't necessarily get them on an expensive drive. You have to read the spec sheets and reviews to find out. Many of these things are described in Wikipedia, should you be inclined to look. Here's what's discussed below, is substantial detail: 01) Data Endurance 02) Data Retention 03) SLC or MLC, not TLC flash cells 04) 5 Year Warranty 05) Overprovisioning 06) TRIM Command 07) Background Garbage Collection 08) Power Loss Protection 09) SMART - Self-Monitoring, Analysis and Reporting Technology 10) Wear Indicator, Life Remaining, Data Written 11) Exotic Diagnostic Data 12) Wear Leveling 13) NCQ - Native Command Queuing 14) Full Drive Encryption 15) Extra Robust ECC (Error Correction Code) 16) Compressible and Uncompressible Performance 17) Secure Erase 18) Data Center Design 19) SATA 3 - 6 Gbps Interface 20) Data Cloning Software (4K Sector Support) 21) Physical Size - 2.5" x 7mm 22) 3.5" Mounting Adapter 23) Intel or Samsung Preferably Here are the details: 01) Data Endurance - This is the rated life of the flash memory cells. How many writes can they handle without excessive degradation? Sometimes, this data is hard to find. You want to look for the TBW or Total Bytes Written spec. This tells you the total amount of data you can write to the drive. There is also a DWPD or Drive Writes Per Day spec. This tells you how many times you can completely write the drive's capacity per day. Finally, there is a GB / Day of data written spec. None of the per day specs mean anything unless you know how many days are under consideration. That number may or may not be the same as the warranty. Consumer drives are typically rated for 20 - 30 GB of writes per day for 3 to 5 years. Prosumer and entry level data center drives are usually rated for 50 - 70 GB of writes per day. Having your system cache on the SSD, using it as a scratch drive or temp drive, or doing things like continuously rewriting video files you're editing will use up the quota faster. So, how much data endurance is enough? Well, short of running some detailed analytics software to track your data writing (reading is not a problem), it's hard to say. As a consumer, and someone who pushes pc components hard at times and likes to run them till they drop, I'd rather err on the side of a prosumer type of device. One option is to deploy SSD's which have built in analytics on a test machine and see how fast the usage quota gets used. All SSD's have a data endurance spec. Some manufacturers and spec sheets may make it hard to find. If you get a drive that is dramatically too low, you may find your data at risk much sooner than you thought. 02) Data Retention - This is a whole different animal. This spec is VERY hard to find. It refers to the time that the drive will retain your data with power off. When the drive is new, this can be many years, even a decade. However, when the drive has reached its write endurance limit, this number can drop to as little as 3 months. You want this number to be as high as possible for as long as possible. Tech report has been doing some data endurance testing and data retention testing on several drives since August 2013. At this point, they have stored 600 TB of data (which is about double most drives' rated lifespan). The very popular Samsung 840 (which uses TLC flash, see below) showed some data retention problems after 300 TB of storage and after being left powered down for only a week. The Samsung also showed over 2000 reallocated sectors after 600 TB of storage. For these reasons, I would not buy that drive. See this article: http://techreport.com/review/26058/the-ssd-endurance-experiment-data-retention-after-600tb One thing this means to me is that I would not want to use an SSD as an archival backup that might sit on the shelf for months or years. 03) SLC or MLC, not TLC flash cells - Every cell of a flash memory device is essentially a capacitor. An SLC (Single Level Cell) device stores one voltage in the capacitor which can represent one binary bit. An MLC (Multi Level Cell) device stores 4 voltages (or possibly 3 plus zero) which can represent two binary bits. A TLC (Triple Level Cell, a bit of a misnomer) device stores 8 voltages, which can represent three binary bits. So, theoretically, with TLC, you can store triple the data in the same space on the silicon chip (more or less). As such, you can make bigger drives much cheaper, hence the appeal. The drives that Tech Report is testing are all MLC except for the Samsung 840 (which is not the same as the 840 Pro). The 840 uses TLC, although Samsung tries to avoid using that verbiage. In the test, the 840 was the only one documented to have retention problems and had FAR more reallocated sectors than the other drives. This is why I say to avoid TLC. SLC is the most reliable technology, but the prices for those really are sky high. MLC represents a good compromise between reliability and price. 04) 5 Year Warranty - Almost every drive you'll find on retail store shelves, and many at online sellers, will have a 2 - 3 year warranty. If you search at higher price levels, around $ 1 / GB, you'll start to find drives with 5 year warranties. This is my preference. If I'm going to go to the trouble to move all my stuff to an SSD, or especially if I have to go to the trouble of splitting things among two drives, or reinstalling the OS; then I don't want to mess with it too often. So, I'd go for the longer warranty. However, warranties are not simple. They're often tied to the data endurance rating, and the warranty will expire sooner timewise if you exceed the stated quota. Sometimes, warranties include tech support for the warranty period. Sometimes, they don't. 05) Overprovisioning - The drive makers often include extra space in the flash memory, compared to the rating of the drive. So, it may actually have 128 GB of flash, but the rated capacity may be 120 GB. So, they've actually reserved 6 - 7% for spare space. This is a good thing. The drive uses this space for managing it's wear leveling activities, garbage collection, trimming unused sectors, and other housekeeping things. One very important thing it uses this for is to provide spare sectors to put into usage when it reallocates one that's not reading or writing properly. So, in the case of the Samsung 840 I mentioned above that had 2000+ reallocated sectors, assuming the drive has that much or more overprovisioned area, then the usable capacity of the drive will not have been decreased. Having to USE that many spare sectors still makes me nervous. Even if you can buy a drive without overprovisioning, or even if you an configure it to eliminate overprovisioning, I woul dn't do that. If you're looking at a drive that has a capacity that's an even multiple of 2, such as 128 GB or 256 GB, as opposed to 120 GB or 240 GB, I would question to see if it is overprovisioned. 06) TRIM Command - This is a fairly common but not universal (as far as I know) feature that you definitely want. If both the OS and the drive are TRIM compatible, then the OS can tell the drive what sectors are no longer needed. Flash memory has to be erased in large chunks before it can be reused. By allowing the OS to tell the drive what chunks are not needed, it can do this erasure in the background so that, when a write is required, it will have erased flash cells ready and waiting. This means write performance is improved, particularly if the drive is fairly full. 07) Background Garbage Collection - This allows the drive to recover unneeded sectors and prepare them for reuse in the background while the drive is doing other things. Background Garbage Collection and TRIM interact, but are not the same. You may have TRIM, but not Background Garbage Collection, for example. TRIM helps the garbage collector know what to zone in on, but the drive with garbage collection may be able to do more on its own even when it's not getting TRIM requests from the OS. 08) Power Loss Protection - This is a very important feature. It is rare on most low end drives. SSD's have ram (volatile) caches just like HDD's. Sometimes, writes are queued up in the ram prior to being written to flash. If the power fails abruptly, those pending writes can be lost, which can corrupt files and data. Power Loss Protection provides capacitors on the power bus in the drive. This maintains the drive long enough to clear the write cache and commit all pending writes to flash in the event of a power failure. 09) SMART - Self-Monitoring, Analysis and Reporting Technology - Most hard drives have SMART, and most SSD's do too, but not necessarily all. This subsystem allows the computer to monitor various aspects of the health and status of the drive. Reallocated sectors is one thing typically monitored by the SMART system, along with many other parameters. You definitely want your SSD to have SMART. 10) Wear Indicator, Life Remaining, Data Written - Most of these attributes are passed through the SMART system, although some drives use proprietary software to monitor this. This, essentially, tells you in various ways how much of the drive's endurance rating you've used up and how much drive life is left. A very desirable feature. 11) Exotic Diagnostic Data - (That's my term for it.) Some drives can monitor and report more exotic and less frequently used, but still useful, data. This might include write amplification statistics, wear leveling statistics, compression statistics, and temperature, etc. This data can be valuable to more advanced users or enterprise users. 12) Wear Leveling - Pretty much all flash storage systems have this. It distributes the writes to the drive fairly evenly amongst all available memory cells. This prevents any one cell or group of cells from wearing out substantially before the others. There are variations in the way this is accomplished, and some manufacturers rave on about how their way is better. 13) NCQ - Native Command Queuing - (I'll just copy from Wikipedia here.) "NCQ is also used in newer solid-state drives where the drive encounters latency on the host, rather than the other way around. For example, Intel's X25-E Extreme solid-state drive uses NCQ to ensure that the drive has commands to process while the host system is busy processing CPU tasks. NCQ also enables the SSD controller to complete commands concurrently (or partly concurrently, for example using pipelines) where the internal organisation of the device enables such processing." Bottom line, it improves performance, which is always good. 14) Full Drive Encryption - This feature automatically encrypts all data written to the drive, and decrypts it on the fly when needed. This is generally 128 bit or 256 bit AES encryption. This is similar to what you can do with something like TrueCrypt, but is faster and more seamless. If the drive or PC is stolen, and the password or passphrase or other key is not known to the attacker, the drive just looks like gibberish. I don't know, and have not explored, how this is accessed by the user. I'm assuming you need a compatible bios and that it pops up during boot and asks for a password. I also don't know if there are some computers that cannot use an encrypted drive. 15) Extra Robust ECC (Error Correction Code) - All storage drives include error correction codes in the data stream to compensate for one or multiple single bit errors in a row due to flaky storage media. Sometimes they can correct for more continuous errors, sometimes less. Some manufacturers like to rave on about their more extensive error correction in certain product lines. 16) Compressible and Uncompressible Performance - Almost all, if not all, SSD's compress data for storage to allow a given amount of flash cells to do more and thus reduce the cost of the drive. That's great if your data is compressible, like a text file. However, it's not so great if the data is not compressible, like executables, or already compressed data. Sometimes, the performance of the drive will greatly decrease when fed incompressible data. You want a drive that maintains good performance regardless. 17) Secure Erase - This feature allows you to completely and securely erase the SSD in preparation for new usage, decommissioning, or sale, etc. This is better than running something like DBAN on it for two reasons. First, it's probably much faster. Second, if the drive has reallocated sectors, you won't be able to access them from the normal api and interface. This function SHOULD include those sectors which are no longer accessible to the user. While I know that secure erase has been in the ATA spec for some time, I don't know if all SSD's support this. You would want to verify it if it's important to you. 18) Data Center Design - This is a sort of nebulous thing that some manufacturers and some reviewers rave on about with some products. An example would be the Intel 730 series of SSD's. While this doesn't have a specific meaning, it probably means the drive has more of the features I've been discussing. It also probably means the drive is more expensive. 19) SATA 3 - 6 Gbps Interface - This is an almost universal feature on SSD's. The simple reason is that if you make the drive with a SATA 2 - 3 Gbps Interface, you won't get anywhere near its maximum performance. So, when you read data transfer specs in the 550 MBps range, then you're getting close to saturating the SATA 3 bus. That's one reason the max data transfer speed on most drives is similar. If you put a SATA 3 drive into a SATA 2 or USB 2 enclosure, you'll severely cripple it. 20) Data Cloning Software (4K Sector Support) - You're going to have to get your data from your old drive to your new one somehow, probably including the OS, the registry (for Windows), user files, etc. Many SSD packages include or include a link to download Data Cloning Software, such as Acronis. This is a handy feature. You must use newer software which fully supports drives with 4K sectors. If you use older software which only supports 512 byte sectors, and your partitions are misaligned, your performance could suffer dramatically. I know this applies to newer hard drives and believe it applies to SSD's. There is some debate about it, and I couldn't prove it while writing this. I'm assuming SSD's partitions have to be aligned on 4K boundaries. 21) Physical Size - 2.5" x 7mm - This is obviously not mandatory for some applications. However, this size of SSD is the most versatile and universal. This form factor allows the drive to fit in almost anything that will accept a SATA drive. This includes almost all notebook / laptop computers. Some won't take a 9mm thickness. Very very small devices might need an mSATA drive or memory card. As far as I know, most memory cards don't include most of these features I've discussed. 22) 3.5" Mounting Adapter - Many drives include this, and it's a handy addition to allow mounting in desktop drive bays. Some desktop cases include drive cages which automatically adapt to either a 2.5" or 3.5" drive. I've seen some youtube pc building videos where someone just velcros the drive to the computer case. Since it's not mechanical, I guess that could be an option, although it might create static, which would be bad. 23) Intel or Samsung Preferably - Finally, while I would consider any major brand name of drive, I've found overwhelmingly in my reading that Intel and Samsung drives are supposed to be the most reliable. So, I would have a potential preference for one of those, if my other criteria were met. I hope you find this info helpful. I know I'm going to be revisiting this list when I finish saving enough money to buy an SSD. Unfortunately, I also know I won't be buying the cheapest thing in town. Sincerely, Ron -- Sent from my Android Acer A500 tablet with bluetooth keyboard and K-9 Mail. Please excuse my potential brevity if I'm typing on the touch screen. (PS - If you email me and don't get a quick response, you might want to call on the phone. I get about 300 emails per day from alternate energy mailing lists and such. I don't always see new email messages very quickly.) Ron Frazier 770-205-9422 (O) Leave a message. linuxdude AT techstarship.com _______________________________________________ tech-chat mailing list [email protected] http://lists.linuxmoose.com/mailman/listinfo/tech-chat
