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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mtoma - Monday, August 31, 2009 - link
Here is an issue I think deserves to be adressed: could an conventional HDD (with 2-3 or 4 platters) slow down the performance of a PC , even if that PC boots from an excellent SSD drive, like an Intel X-25M? Let's say that on the SSD lies only the operating system, and that onto the conventional HDD lies the movie and music archive. But both drives run at the same time, and it is a well known fact that the PC runs at the speed of the slowest component (in our case the conventional HDD).I did not found ANYWHERE in the Web a review, or even an opinion regarding this issue.
I would appreciate if I get a competent answer.
Thanks a lot!
gstrickler - Monday, August 31, 2009 - link
That's a good question, and I too would like to see a report from someone who has done it.Some of your assertions/assumptions are not quite accurate. A PC doesn't "run at the speed of the slowest component", but rather it's performance is limited by the slowest component. Depending upon your usage patterns, a slow component may have very little effect on performance or it may make the machine nearly unusable. I think that's probably what you meant, I'm just clarifying it.
As for putting the OS on an SSD and user files on a HD, you would want to have not only the OS, but also your applications (at least your frequently used ones) installed on the SSD. Put user data (especially large files such as .jpg, music, video, etc.), and less frequently used applications and data on the HD. Typical user documents (.doc, .xls, .pdf) can be on either drive, but access might be better with them on the SSD so that you don't have to wait for the HD to spin-up. In that case, the HD might stay spun-down (low power idle) most of the time, which might improve battery life a bit.
Databases are a bit trickier. It depends upon how large the database is, how much space you have available on the SSD, how complex the data relations are, how complex the queries are, how important performance is, how much RAM is available, how well indexes are used, and how well the database program can take advantage of caching. Performance should be as good or better with the database on the SSD, but the difference may be so small that it's not noticeable, or it might be dramatically faster. That one is basically "try it and see".
Where to put the paging file/swap space? That's a tough one to answer. Putting it on the SSD might be slightly faster if your SSD has high write speeds, however,that will increase the amount of writing the the SSD and could potentially shorten it's usable life. It also seems like a waste to use expensive SSD storage for swap space. You should be able to minimize those by using a permanent swap space of the smallest practical size for your environment.
However, putting the swap space on a less costly HD means the HD will be spun-up (active idle) and/or active more often, possibly costing you some battery life. Also, while the HD may have very good streaming write speeds, it's streaming read speed and random access (read or write) speed will be slower than most SSDs, so you're likely to have slightly slower overall response and slightly shorter battery life than you will by putting the swap space on the SSD.
On a desktop machine with a very fast HD, it might make sense to put the paging file on the HD (or to put a small swap space on the SSD and some more on the HD), but on a machine where battery life is an important consideration, it might be better to have the swap space on the SSD, even though it's "expensive".
Pirks - Monday, August 31, 2009 - link
just turn the page file off, and get yourself 4 or 8 gigs of RAMgstrickler - Monday, August 31, 2009 - link
Windows doesn't like to operate without a page file.smartins - Tuesday, September 1, 2009 - link
Actually, I've been running without a page file for a while and never had any problems. Windows feels much more responsive. You do have to have plenty or ram, I have 6GB on this machine.mtoma - Thursday, September 3, 2009 - link
In my case, it's not a problem of RAM (I have 12 GB RAM and a Core i7 920),it's a problem of throwing or not 300 dolars down the window (on a Intel SSD drive). Currently I have a 1.5 TB Seagate Barracuda 11th generation, on wich I store ONLY movies, music and photos. My primary drive (OS plus programms) is a 300 GB Velociraptor.Do you think diffrent types of Windows behave difrent if you remove the page file? It seems to me if I remove this page file, I walk onto a minefield, and I don't want to do that.
Besides that, my real problem is to use (when I purachase the Intel drive) the Seagate Barracuda in a external HDD enclosure OR internally, and thus, possibly slow down my PC.
SRSpod - Thursday, September 3, 2009 - link
Adding a slow hard drive to your system will not slow your system down (well, apart from a slight delay at POST when it detects the drive). The only difference in speed will be that when you access something on the HDD instead of the SSD, it will be slower than if you were accessing it on the SSD. You won't notice any difference until you access data from the HDD, and if it's only music, movies and photos, and you're not doing complex editing of those files, then a regular HDD will be fast enough to view and play those files without issues.If you don't plan to remove it from your system, then attach it internally. Introducing a USB connection between the HDD and your system will only slow things down compared to using SATA.
Removing the pagefile can cause problems in certain situations and with certain programs (Photoshop, for example). If you have enough RAM, then you shouldn't be hitting the pagefile much anyway, so where it's stored won't make so much of a difference. Personally, I'd put it on the SSD, so that when you do need it, it's fast.
samssf - Friday, September 18, 2009 - link
Won't Windows write to the page file regardless of how much RAM you have? I was under the impression Windows will swap out memory that it determines isn't being used / needed at the moment.If you absolutely need to have a page file, I would use available RAM to create a RAM disk, and place your page file on this virtual disk. That way you're setting aside RAM you know you don't need for the page file, since Windows will write to that file anyway.
If you can, just turn it off.
minime - Monday, August 31, 2009 - link
Would someone please have the courtesy to test those things in a business environment? I'm talking about servers. Database, webapplication, Java, etc. Reliability? Maybe even enrich the article with a PCI-E SSD (Fusion-IO)?ciukacz - Monday, August 31, 2009 - link
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