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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IPL - Monday, August 31, 2009 - link
I first started reading anandtech when I got seriously interested in SSDs and honestly, you write the best SSD articles around! Thank you for all the help you gave me in deciding which SSD to buy.I ordered online the new G2 last week and should be getting it in a few days. I live in Greece and the re-launched G2 has been available here for about a week now.
I am planning on replacing the HDD on my Feb 08 Macbook Pro (last refresh pre-unibody) as soon as I get it. I am just a consumer with a little bit of knowledge on tech but not a pro at all. I just thought of asking all a few questions that I have pre-drive swapping.
1. Will TRIM be supported on macs? Any news if and when?
2. When then new TRIM firmware is out, do I have to just install the firmware or will I need to format everything and start from fresh in order to get it to work?
3. I have bought a 2,5'' SATA USB enclosure in order to put my G2 in there first, connect it to the laptop via the USB and install Snow Leopard from there. After I am done, I will remove the G2 from the enclosure, swap the drives and hopefully, everything will be working. Does this sound logical? I am worried about the h/w drivers to be honest.
Thanks in advance for your help. I will post some non-scientific time results as soon as get this done. Cant wait.
gstrickler - Monday, August 31, 2009 - link
The simplest way to swap the HD on most Mac OS machines is:1. Connect both the old and the new drive to the machine (internally or in an external USB or FireWire case).
2. Use Disk Utility (included in Mac OS X) to set the appropriate partitioning scheme (GUID for Intel based Macs, Apple Partition Scheme for PPC Macs) on the new drive.
3. Partition and format the new drive.
4. Use Carbon Copy Cloner (shareware) to clone the old drive to the new drive.
5. Try booting off the new drive. Note that PPC Macs can't boot from USB drives, but Intel based Macs can. All PPC and Intel Macs with a built-in FireWire port can boot from a FireWire drive.
6. If not already done, physically swap the drives to the desired locations, boot and set the preferred startup drive.
IPL - Tuesday, September 1, 2009 - link
Awesome, thanks for the help.I have checked Carbon Copy Cloner and it is already one of my options. Never tried it before but looked easy enough.
I havent decided yet which way I will do it (fresh install or clone existing drive) but I will make my mind up when everything is ready!
Anand Lal Shimpi - Monday, August 31, 2009 - link
Thank you for reading and saying such wonderful things, I really do appreciate it :)1) I don't believe TRIM is presently supported in Snow Leopard. I've heard that Apple may be working on it but I don't think it's there now.
2) From what I've seen, it should preserve your data. It's still worth backing up just in case something ridiculous happens.
3) What you're describing should work, although if I were you I'd just swap the drives and install. Hook your old drive up via USB and pull any data you need off of that.
Take care,
Anand
sunbear - Monday, August 31, 2009 - link
Another fantastic article. I just wanted to draw your attention to recent reports that the majority of currently available laptops (including the MacBookPro) are unable to support transfer rates greater than SATA-150 (http://www.hardmac.com/news/2009/06/16/new-macbook...">http://www.hardmac.com/news/2009/06/16/...imited-1....Since most laptops can't even use the full performance of these SSD's, do you have any recommendation regarding which one would be the best bang-for-the-buck to speed up a laptop?
Personally, I am interested in putting SSD's in a laptop not only for the speed improvements, but I'm also hoping that it reduces the amount of heat that my laptop will put out so that I can finally find a laptop that you can use comfortably on your lap!
Incidentally, it would be really great if laptop reviewers checked to see if they could comfortably work with a laptop at full load on their lap as a standard test.
Anand Lal Shimpi - Monday, August 31, 2009 - link
Even on a SATA-150 interface, you're generally only going to be limiting your sequential read speed and perhaps your sequential write speed a bit. Random read/write speeds don't really go above 60MB/s so you're fine there.They recommendations remain the same; Intel at the top end, anything Indilinx MLC to save a bit. If anything, a SATA-150 interface makes the Intel drive look a bit better since its 80MB/s sequential write limit isn't as embarrassing :)
Take care,
Anand
Dobs - Monday, August 31, 2009 - link
I hope Seagate / Western Digital etc. bring even more innovation / competition in SSD's next year... and not just Enterprise products.And one thing I don't fully understand is why there aren't more dedicated 3.5" drives. Patriot has the adapter but what about the rest??? No money in desktops anymore???
Anand Lal Shimpi - Monday, August 31, 2009 - link
OCZ is making a 3.5" Vertex drive, waiting on it for review :)Take care,
Anand
kisjoink - Monday, August 31, 2009 - link
Now that the good performing SSDs are half the price of last year, I'd really like to see a 2xSSD in RAID 0 article!mgrmgr - Monday, August 31, 2009 - link
I second the request for a 2xSSD RAID-0 article...with specific discussions about which applications it benefits (Photoshop?) and which ones it doesn't.Before October 22nd when I buy a new Win7 computer? Please. :-)