The SSD Relapse: Understanding and Choosing the Best SSD
by Anand Lal Shimpi on August 30, 2009 12:00 AM EST- Posted in
- Storage
Live Long and Prosper: The Logical Page
Computers are all about abstraction. In the early days of computing you had to write assembly code to get your hardware to do anything. Programming languages like C and C++ created a layer of abstraction between the programmer and the hardware, simplifying the development process. The key word there is simplification. You can be more efficient writing directly for the hardware, but it’s far simpler (and much more manageable) to write high level code and let a compiler optimize it.
The same principles apply within SSDs.
The smallest writable location in NAND flash is a page; that doesn’t mean that it’s the largest size a controller can choose to write. Today I’d like to introduce the concept of a logical page, an abstraction of a physical page in NAND flash.
Confused? Let’s start with a (hopefully, I'm no artist) helpful diagram:
On one side of the fence we have how the software views storage: as a long list of logical block addresses. It’s a bit more complicated than that since a traditional hard drive is faster at certain LBAs than others but to keep things simple we’ll ignore that.
On the other side we have how NAND flash stores data, in groups of cells called pages. These days a 4KB page size is common.
In reality there’s no fence that separates the two, rather a lot of logic, several busses and eventually the SSD controller. The latter determines how the LBAs map to the NAND flash pages.
The most straightforward way for the controller to write to flash is by writing in pages. In that case the logical page size would equal the physical page size.
Unfortunately, there’s a huge downside to this approach: tracking overhead. If your logical page size is 4KB then an 80GB drive will have no less than twenty million logical pages to keep track of (20,971,520 to be exact). You need a fast controller to sort through and deal with that many pages, a lot of storage to keep tables in and larger caches/buffers.
The benefit of this approach however is very high 4KB write performance. If the majority of your writes are 4KB in size, this approach will yield the best performance.
If you don’t have the expertise, time or support structure to make a big honkin controller that can handle page level mapping, you go to a larger logical page size. One such example would involve making your logical page equal to an erase block (128 x 4KB pages). This significantly reduces the number of pages you need to track and optimize around; instead of 20.9 million entries, you now have approximately 163 thousand. All of your controller’s internal structures shrink in size and you don’t need as powerful of a microprocessor inside the controller.
The benefit of this approach is very high large file sequential write performance. If you’re streaming large chunks of data, having big logical pages will be optimal. You’ll find that most flash controllers that come from the digital camera space are optimized for this sort of access pattern where you’re writing 2MB - 12MB images all the time.
Unfortunately, the sequential write performance comes at the expense of poor small file write speed. Remember that writing to MLC NAND flash already takes 3x as long as reading, but writing small files when your controller needs large ones worsens the penalty. If you want to write an 8KB file, the controller will need to write 512KB (in this case) of data since that’s the smallest size it knows to write. Write amplification goes up considerably.
Remember the first OCZ Vertex drive based on the Indilinx Barefoot controller? Its logical page size was equal to a 512KB block. OCZ asked for a firmware that enabled page level mapping and Indilinx responded. The result was much improved 4KB write performance:
| Iometer 4KB Random Writes, IOqueue=1, 8GB sector space | Logical Block Size = 128 pages | Logical Block Size = 1 Page |
| Pre-Release OCZ Vertex | 0.08 MB/s | 8.2 MB/s |
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CList - Tuesday, September 1, 2009 - link
Don't be disgusted at Newegg, be disgusted at the people who are willing to pay the premium price! Newegg is simply playing a reactionary role in the course of natural free-market economics and cannot be blamed. The consumers, on the other hand, are willing participants and are choosing to pay those prices. When no one is left who is willing to pay those prices, Newegg will quickly lower them.Cheers,
CList
gfody - Tuesday, September 1, 2009 - link
I don't understand how consumers have any control over what Newegg is charging for the 160gb that's not even in stock yet.If Newegg wants to get the absolute most anyone is willing to pay for every piece of merchandise they may as well just move to an auction format.
DrLudvig - Tuesday, September 1, 2009 - link
Yeah, if you look at intel's website, http://www.intel.com/cd/channel/reseller/asmo-na/e...">http://www.intel.com/cd/channel/reselle...na/eng/p..., you will se that the R5 includes "3.5" desktop drive bay adapter to 2.5" SSD adapter bracket, screws, installation guide, and warranty documentation.Why on earth Newegg is charging that much more for it i really don't know, here in denmark the R5 retails for about 15 bucks more than the C1.. Which really isn't that bad..
Mr Perfect - Tuesday, September 1, 2009 - link
Whoa. That's it? An adapter kit? With that kind of price difference, I expected it to be the D0 stepping of SSDs or something.Thanks for clearing that up.
NA1NSXR - Monday, August 31, 2009 - link
The reason not being that performance or longevity is not good enough, but because improvements are still coming too quickly, and prices falling fast still. Once the frequency of significant improvements and price drops slow down, I will more seriously consider an SSD. I suppose it depends on how much waiting on the I/O you do though. For me, it is not so much that a Velociraptor is intolerable.bji - Tuesday, September 1, 2009 - link
Perhaps this is what you meant, but you should really clarify. It's still not time for YOU to buy an SSD. SSDs represent an incredible performance improvement that is well worth the money for many people.DragonReborn - Monday, August 31, 2009 - link
say i wanted to go crazy (it happens)...should i get two 80gb intel g2's or the 160gb intel g2? same space...is the RAID 0 performance worth it?i have all my important data backed on a big 2tb drive so the two ssd's (or 1 160gb) will just hold my OS/progs/etc.
thoughts?
kensiko - Monday, August 31, 2009 - link
I would say that in real world usage, you won't notice a huge difference between RAID and not RAID, SSD are already fast enough for the rest of the system. Also, TRIM may not work for now in RAID configuration.Just look at Windows Start up, no difference between Gen2 SSD!
Gc - Monday, August 31, 2009 - link
This is a nice article, but the numbers leave an open question.What is Samsung doing right? Multiprocess/multithread performance?
The article finds Samsung drives performance is low on 2MB reads,
(new 2MB sequential reads not given, assume same as 'used')
used 2MB sequential reads (low rank, 79% of top)
good on 2MB writes:
new 2MB sequential writes (middle rank, 89% of top)
used 2MB sequential writes (2nd place, 91% of top)
and horrible on 4KB random files:
(new 4KB random reads not given, assume same as 'used')
used 4KB random read (bottom ssd ranked, only 36% of top)
new 4KB random write (low rank, only 9% of top)
used 4KB random write (bottom ssd ranked, only 3% of top, < HD)
Yet somehow in the multitasking Productivity test and Gaming test, it was surprisingly competitive:
multitasking productivity (mid-high rank, 88% of top)
gaming (mid-high rank, 95% of top)
The productivity test is described as "four tasks going on at once, searching through Windows contacts, searching through Windows Mail, browsing multiple webpages in IE7 and loading applications". In other words, nearly all READS (except maybe for occasionally writing to disk new items for the browser history or cache).
The gaming test is described as "reading textures and loading level data", again nearly all READS.
Q. Given that the Samsung controller's 2MB read performance and
4KB read performance are both at the bottom of the pack, how
did it come out so high in the read-mostly productivity test
and gaming test?
Does this indicate the Samsung controllers might be better than Indilinx for multiprocess/multithreaded loads?
(The Futuremark pdf indicates Productivity 2 is the only test with 4 simultaneous tasks, and doesn't say whether the browser tabs load concurrently. The Gaming 2 test is multithreaded with up to 16 threads. [The Samsung controller also ranks well on the communications test, but that may be explained: Communications 1 includes encryption and decompression tasks where Samsung's good sequential write performance might shine.])
Since many notebooks/laptops are used primarily for multitasking productivity (students, "office"-work), maybe the Samsung was a reasonable choice for notebook/laptop OEMs. Also, in these uses the cpu and drive are idle much of the time, so the Samsung best rank on idle power looks good. (But inability to upgrade firmware is bad.)
(The article doesn't explain what the load was in the load drive test, though it says the power drops by half if the test is switched to random writes; maybe it was sequential writes for peak power consumption. It would have been helpful to see the power consumption rankings for read-mostly loads.)
Thanks!
rcocchiararo - Monday, August 31, 2009 - link
Your prices are way off, newegg is charging ludicrous ammounts right now :(also, the 128 agility was 269 last week, i was super exited, then it went back to 329, and its now 309.