The SSD Anthology: Understanding SSDs and New Drives from OCZ
by Anand Lal Shimpi on March 18, 2009 12:00 AM EST- Posted in
- Storage
Strength in Numbers, What makes SSDs Fast
Given the way a single NAND-flash IC is organized one thing should come to mind: parallelism.
Fundamentally the flash that’s used in SSDs cut from the same cloth as the flash that’s used in USB drives. And if you’ve ever used a USB flash drive you know that those things aren’t all that fast. Peak performance to a single NAND-flash IC is going to be somewhere in the 5 - 40MB/s range. You get the faster transfer rates by reading/writing in parallel to multiple die in the same package.
The real performance comes from accessing multiple NAND ICs concurrently. If each device can give you 20MB/s of bandwidth and you’ve got 10 devices you can access at the same time, that’s 200MB/s of bandwidth. While hard drives like reads/writes to be at the same place on the drive, SSDs don’t mind; some are even architected to prefer that data be spread out all over the drive so it can hit as many flash devices as possible in tandem. Most drives these days have 4 - 10 channel controllers.
The Recap
I told you I’d mention this again because it’s hugely important, so here it is:
A single NAND flash die is subdivided into blocks. The typical case these days is that each block is 512KB in size. Each block is further subdivided into pages, with the typical page size these days being 4KB.
Now you can read and write to individual pages, so long as they are empty. However once a page has been written, it can’t be overwritten, it must be erased first before you can write to it again. And therein lies the problem, the smallest structure you can erase in a NAND flash device today is a block. Once more, you can read/write 4KB at a time, but you can only erase 512KB at a time.
It gets worse. Every time you erase a block, you reduce the lifespan of the flash. Standard MLC NAND flash can only be erased 10,000 times before it goes bad and stops storing data.
Based on what I’ve just told you there are two things you don’t want to do when writing to flash: 1) you don’t want to overwrite data, and 2) you don’t want to erase data. If flash were used as a replacement for DVD-Rs then we wouldn’t have a problem, but it’s being used as a replacement for conventional HDDs. Who thought that would be a good idea?
It turns out that the benefits are more than worth the inconvenience of dealing with these pesky rules; so we work around them.
Most people don’t fill up their drives, so SSD controller makers get around the problem by writing to every page on the drive before ever erasing a single block.
If you go about using all available pages to write to and never erasing anything from the drive, you’ll eventually run out of available pages. I’m sure there’s a fossil fuel analogy somewhere in there. While your drive won’t technically be full (you may have been diligently deleting files along the way and only using a fraction of your drive’s capacity), eventually every single block on your drive will be full of both valid and invalid pages.
In other words, even if you’re using only 60% of your drive, chances are that 100% of your drive will get written to simply by day to day creation/deletion of files.
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SunSetSupaNova - Wednesday, March 18, 2009 - link
Just wanted to say great job Anand on a great article, it took me a while to read it from start to finish but it was well worth it!FHDelux - Wednesday, March 18, 2009 - link
That was the best review i have read in a long time. I originally bought an OCZ Core drive when they first came out. It was the worst piece of garbage i had ever used. Newegg wouldn't let me send it back and OCZ support forums told me all sorts of junk to get me to fix it but it was just a poorly designed drive. I eventually ended up getting the egg to take it back for credit and i wrote OCZ off as a company blinded by the marketing department. I currently own an Intel SSD and its wonderfull, everytime i see OCZ statements saying their drive competes with the Intel drive i would laugh and think back to the OCZ techs telling me i need to update my bios, or i need to install vista service pack 1 before it would work right.I am thankful that you slapped that OCZ big wig around until they made a good product. All of us out there that wasted our time and money on Pre-vertex generation drives are greatfull to you and the whole industry should be kissing your butt right now.
One thing these companies need to learn is that marketing isn't the answer, creating solid products is. Hopefully OCZ has learned their lesson, and because of your article i will give them another chance.
THANK YOU!
kelstertx - Wednesday, March 18, 2009 - link
I didn't want to worry about eventual failure of the Flash chips of an SSD, and went with an SDRAM based Ramdrive from Acard. These drives have no latency of any kind, since they use SDRAM, and no lifespan of write cycles. I've been using mine for a couple of weeks now, and I like it a lot. I put Ubuntu on mine, and had 2G left for my small home folder. The standard HDD is my long-term storage for data files, music, etc. As SDRAM gets more affordable over time, I can add DIMMs and bump up the size.I know this review was about SSDs strictly, so an SDRAM drive doesn't technically fit, but it would have been interesting to see a 9010 or 9010b in there for comparison. It beat the Intel SSD in almost all the tests. http://techreport.com/articles.x/16255/1">http://techreport.com/articles.x/16255/1
7Enigma - Wednesday, March 18, 2009 - link
I've been eying these guys ever since the announced their first press release. Every time I always was drawn away by the constant need for power (4h max on battery scares the bejeezus out of me if I was to be gone on vacation during a storm), high power usage at all times, and high cost of entry (after factoring in all of the ram modules).I really dislike that article as well, since I think the bottlenecks were much less apparent with such a horribly slow cpu. The majority of that review's data is extremely compressed. I mean a P4, and 1 gig of memory; are you F'ing kidding me? This article was written in Jan of this year!? Why didn't they just use my old 486DX?
tirez321 - Wednesday, March 18, 2009 - link
What would a drive zeroing tool do to write performance, like if you used acronis privacy expert to zero only the "free space" regularly? Would it help write performance due to the drive not having to erase pages before writing?tirez321 - Wednesday, March 18, 2009 - link
I can kinda see that it wouldn't now.Because there would still be states there regardless.
But if you could inform the drive that it is deleted somehow, hmm.
strikeback03 - Wednesday, March 18, 2009 - link
The subjective experiences with stuttering are more important to me than most of the test numbers. Other tests I have found of the G.Skill Titan and similar have looked pretty good, but left out mention of stuttering in use.Too bad, as the 80GB Intel is too small and the ~$300 for a 120GB is about the most I am willing to pay. Maybe sometime this year the OCZ Vertex or similar will get there.
strikeback03 - Tuesday, March 24, 2009 - link
When I wrote that, the Newegg price for the 120GB Vertex was near $400. Now they have it for $339 with a $30 MIR. Now that's progress.kamikaz1k - Wednesday, March 18, 2009 - link
the latency times are switched...incase u wanted to kno.also, first post ^^ hallo!
GourdFreeMan - Wednesday, March 18, 2009 - link
It seems rather premature to assume the ATA TRIM command will significantly improve the SSD experience on the desktop. If you were to use TRIM to rewrite a nonempty physical block, you do not avoid the 2ms erase penalty when more data is written to that block later on and instead simply add the wear of another erase cycle. TRIM, then, is only useful for performance purposes when an entire 512 KiB physical block is free.A well designed operating system would have to keep track of both the physical and logical maps of used space on an SSD, and only issue TRIM when deletion of a logical cluster coincides with the freeing of an entire physical block. Issuing TRIMs at any other time would only hurt performance. This means the OS will have significantly fewer opportunities to issue TRIMs than you assume. Moreover, after significant usage the physical blocks will become fragmented and fewer and fewer TRIMs will be able to be issued.
TRIM works great as long as you only deal with large files, or batches of small files contiguously created and deleted with significant temporal locality. It would greatly aid SSDs in the "used" state Anand artificially creates in this article, but on a real system where months of web browsing, Windows updates and software installing/uninstalling have occurred the effect would be less striking.
TRIM could be mated with periodic internal (not filesystem) defragmentation to mitigate these issues, but that would significantly reduce the lifespan of the SSD...
It seems the real solution to the SSD performance problem would be to decrease the size of the physical block... ideally to 4 KiB, as that is the most common cluster size on modern filesystems. (This assumes, of course, that the erase, read and write latencies could be scaled down linearly.)