The SSD Relapse: Understanding and Choosing the Best SSD
by Anand Lal Shimpi on August 30, 2009 12:00 AM EST- Posted in
- Storage
A Quick Flash Refresher
DRAM is very fast. Writes happen in nanoseconds as do CPU clock cycles, those two get along very well. The problem with DRAM is that it's volatile storage; if the charge stored in each DRAM cell isn't refreshed, it's lost. Pull the plug and whatever you stored in DRAM will eventually disappear (and unlike most other changes, eventually happens in fractions of a second).
Magnetic storage, on the other hand, is not very fast. It's faster than writing trillions of numbers down on paper, but compared to DRAM it plain sucks. For starters, magnetic disk storage is mechanical - things have to physically move to read and write. Now it's impressive how fast these things can move and how accurate and relatively reliable they are given their complexity, but to a CPU, they are slow.
The fastest consumer hard drives take 7 milliseconds to read data off of a platter. The fastest consumer CPUs can do something with that data in one hundred thousandth that time.
The only reason we put up with mechanical storage (HDDs) is because they are cheap, store tons of data and are non-volatile: the data is still there even when you turn em off.
NAND flash gives us the best of both worlds. They are effectively non-volatile (flash cells can lose their charge but after about a decade) and relatively fast (data accesses take microseconds, not milliseconds). Through electron tunneling a charge is inserted into an N-channel MOSFET. Once the charge is in there, it's there for good - no refreshing necessary.
N-Channel MOSFET. One per bit in a NAND flash chip.
One MOSFET is good for one bit. Group billions of these MOSFETs together, in silicon, and you've got a multi-gigabyte NAND flash chip.
The MOSFETs are organized into lines, and the lines into groups called pages. These days a page is usually 4KB in size. NAND flash can't be written to one bit at a time, it's written at the page level - so 4KB at a time. Once you write the data though, it's there for good. Erasing is a bit more complicated.
To coax the charge out of the MOSFETs requires a bit more effort and the way NAND flash works is that you can't discharge a single MOSFET, you have to erase in larger groups called blocks. NAND blocks are commonly 128 pages, that means if you want to re-write a page in flash you have to first erase it and all 127 adjacent pages first. And allow me to repeat myself: if you want to overwrite 4KB of data from a full block, you need to erase and re-write 512KB of data.
To make matters worse, every time you write to a flash page you reduce its lifespan. The JEDEC spec for MLC (multi-level cell) flash is 10,000 writes before the flash can start to fail.
Dealing with all of these issues requires that controllers get very crafty with how they manage writes. A good controller must split writes up among as many flash channels as possible, while avoiding writing to the same pages over and over again. It must also deal with the fact that some data is going to get frequently updated while others will remain stagnant for days, weeks, months or even years. It has to detect all of this and organize the drive in real time without knowing anything about how it is you're using your computer.
It's a tough job.
But not impossible.
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sunbear - Monday, August 31, 2009 - link
Even though most laptops are now SATA-300 compatible, the majority are not able to actually exceed SATA-150 transfer speeds according to some people who have tried. I would imagine that sequential read/write performance would be important for swap but the SATA-150 will be the limiting factor for any of the SSD's mentioned in Anand's article in this case.Here's the situation with Thinkpads:
http://blogs.technet.com/keithcombs/archive/2008/1...">http://blogs.technet.com/keithcombs/arc...vo-think...
The new MacBookPro is also limited to SATA-150.
smartins - Tuesday, September 1, 2009 - link
Actually, The ThinkPad T500/T400/W500 are fully SATA-300 compatible, it's only the drives that ship with the machines that are SATA-150 capped.I have a Corsair P64 on my T500 and get an average of 180MB/read which is consistent with all the reviews of this drive.
mczak - Monday, August 31, 2009 - link
article says you shouldn't expect it soon, but I don't think so. Several dealers already list it, though not exactly in stock (http://ht4u.net/preisvergleich/a444071.html)">http://ht4u.net/preisvergleich/a444071.html). Price tag, to say it nicely, is a bit steep though.Seramics - Monday, August 31, 2009 - link
Another great articles from Anandtech. Kudos guys at AT, ur my no. 1 hardware site! Anyway, its really great that we have a really viable competitor to Intel- Indilinx. They really deserve the praise. Now we can buy a non Intel SSD and have no nonsensical stuttering issue! Overall, Intel is still leader but its completely nonsensical how bad their sequential write speed is! I mean, its even slower than a mechanical hard disk! Thats juz not acceptable given the gap in performance is so large and Intel SSD's actually can suffer a significantly worst performance in real world when sequential write speed performance matters. Intel, fix your seq write speed nonsence please!Seramics - Monday, August 31, 2009 - link
Sorry for double post. Its unintentional and i duno how to delete the 2nd post.Seramics - Monday, August 31, 2009 - link
Another great articles from Anandtech. Kudos guys at AT, ur my no. 1 hardware site! Anyway, its really great that we have a really viable competitor to Intel- Indilinx. They really deserve the praise. Now we can buy a non Intel SSD and have no nonsensical stuttering issue! Overall, Intel is still leader but its completely nonsensical how bad their sequential write speed is! I mean, its even slower than a mechanical hard disk! Thats juz not acceptable given the gap in performance is so large and Intel SSD's actually can suffer a significantly worst performance in real world when sequential write speed performance matters. Intel, fix your seq write speed nonsence please!Shadowmaster625 - Monday, August 31, 2009 - link
Subtle. Very subtle. Good article though.3 questions:
1. Is there any way to read the individual page history off the SSD device so I can construct a WinDirStat style graphical representation of the remaining expected life of the flash? Or better yet is there already a program that does this?
2. Suppose I had a 2 gigabyte movie file on my 60gb vertex drive. And suppose I had 40GB of free space. If I were to make 20 copies of that movie file, then delete them all, would that be the same as running Wiper?
3. Any guesses as to which of these drives will perform best when we make the move to SATA-III?
4. (Bonus) What is stopping Intel from buying Indilinx (and pulling their plug)? (Or just pulling their plug without buying them...)
SRSpod - Thursday, September 3, 2009 - link
3. These drives will perform just as they do now when connected to a 6 GBps SATA controller. In order to communicate at the higher speed, both the drive and the controller need to support it. So you'll need new 6 GBps drives to connect to your 6 GBps controller before you'll see any benefit from the new interface.heulenwolf - Monday, August 31, 2009 - link
Yeah, once the technology matures a little more and drives become more commoditized, I'd like to see more features in terms of feedback on drive life, reliability, etc. When I got my refurb Samsung drives from Dell, for example, they could have been on the verge of dying or they could have been almost new. There's no telling. The controller could know exactly where the drive stands, however. Some kind of controller-tracked indication of drive life left would be a feature that might distinguish comparable drives from one another in a crowded marketplace.While they're at it, a tool to allow adjusting of values such as the amount of space not reported to the OS with output in terms of write amplification and predicted drive life would be really nifty.
Sure, its over the top, but we can always hope.
nemitech - Monday, August 31, 2009 - link
I picked up an Agility 120 Gb for $234 last week from ebay ($270 list price - - 6% bing cashback - $20 pay pal discount). I am sure there will be similar deals around black Friday. $2 per Gb is possible for a good SSD.