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
Once More, With Feeling
Ryan said we’d lose some sequential write performance. The drive would no longer be capable of 230MB/s writes, perhaps only down to 80 or 90MB/s now. I told him it didn’t matter, that write latency needed to come down and if it were at the sacrifice of sequential throughput then it’d be fine. He asked me if I was sure, I said yes. I still didn’t think he could do it.
A couple days later and I got word that OCZ just got a new firmware revision back from Korea with the changes I’d asked for. They were going to do a quick test and if the results made me happy, they’d overnight a drive to me for Saturday delivery.
He sent me these iometer results:
The New Vertex was looking good, but was it too good to be true?
I couldn’t believe it. There was no way. “Sure”, I said, “send the drive over”. He asked if I’d be around on Saturday to receive it. I would be, I’m always around.
This was the drive I got:
No markings, no label, no packaging - just a black box that supposedly fixed all of my problems. I ran the iometer test first...it passed. I ran PCMark. Performance improved. There’s no way this thing was fixed. I skipped all of the other tests and threw it in my machine, once again cloning my system drive. Not a single pause. Not a single stutter.
The drive felt slower than the Intel or Summit drives, but that was fine, it didn’t pause. My machine was usable. Slower is fine, invasive with regards to my user experience is not.
I took the Vertex back out and ran it through the full suite of tests. It worked. Look at the PCMark Vantage results to see just what re-focusing on small file random write latency did to the drive’s performance:
The Vertex went from performing like the OCZ Apex (dual JMicron JMF602B controllers) to performing more like an Intel X25-M or OCZ Summit. I’ll get to the full performance data later on in this article, but let’s just say that we finally have a more affordable SSD option. It’s not the fastest drive in the world, but it passes the test for giving you the benefits of a SSD without being worse in some way than a conventional hard drive.
As the Smoke Cleared, OCZ Won Me Over
Now let’s be very clear what happened here. OCZ took the feedback I gave them, and despite it resulting in a product with fewer marketable features implemented the changes. It’s a lot easier to say that your drive is capable of speeds of up to 230MB/s than it is to say it won’t stutter, the assumption is that it won’t stutter.
As far as I know, this is the one of the only reviews (if not the only) at the time of publication that’s using the new Vertex firmware. Everything else is based on the old firmware which did not make it to production. Keep that in mind if you’re looking to compare numbers or wondering why the drives behave differently across reviews. The old firmware never shipped thanks to OCZ's quick acting, so if you own one of these drives - you have a fixed version.
While I didn’t really see eye to eye with any of the SSD makers that got trashed in the X25-M review, OCZ was at least willing to listen. On top of that, OCZ was willing to take my feedback, go back to Indilinx and push for a different version of the firmware despite it resulting in a drive that may be harder to sell to the uninformed. The entire production of Vertex drives was held up until they ended up with a firmware revision that behaved as it should. It’s the sort of agility you can only have in a smaller company, but it’s a trait that OCZ chose to exercise.
They were the first to bring an Indilinx drive to the market, the first to produce a working drive based on Samsung’s latest controller, and the company that fixed the Indilinx firmware. I’ve upset companies in the past and while tempers flared after the X25-M review, OCZ at least made it clear this round that their desire is to produce the best drive they could. After the firmware was finalized, OCZ even admitted to me that they felt they had a much better drive; they weren’t just trying to please me, but they felt that their customers would be happier.
I should also point out that the firmware that OCZ pushed for will now be available to all other manufacturers building Indilinx based drives. It was a move that not only helped OCZ but could help every other manufacturer who ships a drive based on this controller.
None of this really matters when it comes to the drive itself, but I felt that the backstory was at least just as important as the benchmarks. Perhaps showing you all a different side of what goes on behind the scenes of one of these reviews.
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Kary - Thursday, March 19, 2009 - link
Why use TRIM at all?!?!?If you have extras Blocks on the drive (NOT PAGES, FULL BLOCKS) then there is no need for TRIM command.
1)Currently in use BLOCK is half full
2)More than half a block needs to be written
3)extra BLOCK is mapped into the system
4)original/half full block is mapped out of system.. can be erased during idle time.
You could even bind multiple continuous blocks this way (I assume that it is possible to erase simultaneously any of the internal groupings pages from Blocks on up...they probably share address lines...ex. erase 0000200 -> just erase block #200 ....erase 00002*0 -> erase block 200 to 290...btw, did addressing in base ten instead of binary just to simplify for some :)
korbendallas - Wednesday, March 18, 2009 - link
Actually i think that the Trim command is merely used for marking blocks as free. The OS doesn't know how the data is placed on the SSD, so it can't make informed decision on when to forcefully erase pages. In the same way, the SSD doesn't know anything about what files are in which blocks, so you can't defrag files internally in the drive.So while you can't defrag files, you CAN now defrag free space, and you can improve the wear leveling because deleted data can be ignored.
So let's say you have 10 pages where 50% of the blocks were marked deleted using the Trim command. That means you can move the data into 5 other pages, and erase the 10 pages. The more deleted blocks there are in a page, the better a candidate for this procedure. And there isn't really a problem with doing this while the drive is idle - since you're just doing something now, that you would have to do anyway when a write command comes.
GourdFreeMan - Wednesday, March 18, 2009 - link
This is basically what I am arguing both for and against in the fourth paragraph of my original post, though I assumed it would be the OS'es responsibility, not the drive's.Do SSDs track dirty pages, or only dirty blocks? I don't think there is enough RAM on the controller to do the former...
korbendallas - Wednesday, March 18, 2009 - link
Well, let's take a look at how much storage we actually need. A block can be erased, contain data, or be marked as trimmed or deallocated.That's three different states, or two bits of information. Since each block is 4kB, a 64GB drive would have 16777216 blocks. So that's 4MB of information.
So yeah, saving the block information is totally feasible.
GourdFreeMan - Thursday, March 19, 2009 - link
Actually the drive only needs to know if the page is in use or not, so you can cut that number in half. It can determine a partially full block that is a candidate for defragmentation by looking at whether neighboring pages are in use. By your calculation that would then be 2 MiB.That assumes the controller only needs to support drives of up to 64 GiB capacity, that pages are 4 KiB in size, and that the controller doesn't need to use RAM for any other purpose.
Most consumer SSD lines go up to 256 GiB in capacity, which would bring the total RAM needed up to 8 MiB using your assumption of a 4 KiB page size.
However, both hard drives and SSDs use 512 byte sectors. This does not necessarily mean that internal pages are therefore 512 bytes in size, but lacking any other data about internal pages sizes, let's run the numbers on that assumption. To support a 256 MiB drive with 512 byte pages, you would need 64 MiB of RAM -- which only the Intel line of SSDs has more than -- dedicated solely to this purpose.
As I said before there are ways of getting around this RAM limitation (e.g. storing page allocation data per block, keeping only part of the page allocation table in RAM, etc.), so I don't think the technical challenge here is insurmountable. There still remains the issue of wear, however...
GourdFreeMan - Wednesday, March 18, 2009 - link
Substitute "allocated" for "dirty" in my above post. I muddled the terminology, and there is no edit function to repair my mistake.Also, I suppose the SSD could store some per block data about page allocation appended to the blocks themselves at a small latency penalty to get around the RAM issue, but I am not sure if existing SSDs do such a thing.
My concerns about added wear in my original post still stand, and doing periodic internal defragmentation is going to necessitate some unpredictable sporadic periods of poor response by the drive as well if this feature is to be offered by the drive and not the OS.
Basilisk - Wednesday, March 18, 2009 - link
I think your concerns parallel mine, allbeit we have different conclusions.Parag.1: I think you misunderstand the ERASE concept: as I read it, after an ERASE parts of the block are re-written and parts are left erased -- those latter parts NEED NOT be re-erased before they are written, later. If the TRIM function can be accomplished at an idle moment, access time will be "saved"; if the TRIM can erase (release) multiple clusters in one block [unlikely?], that will reduce both wear & time.
Parag.2: This argument reverses the concept that OS's should largely be ignorant about device internals. As devices with different internal structures have proliferated over the years -- and will continue so with SSD's -- such OS differentiation is costly to support.
Parag 3 and onwards: Herein lies the problem: we want to save wear by not re-writing files to make them contiguous, but we now have a situation where wear and erase times could be considerably reduced by having those files be contiguous. A 2MB file fragmented randomly in 4KB clusters will result in around 500 erase cycles when it's deleted; if stored contiguously, that would only require 4-5 erase cycles (of 512KB SSD-blocks)... a 100:1 reduction in erases/wear.
It would be nice to get the SSD blocks down to 4KB in size, but I have to infer there are counter arguments or it would've been done already.
With current SSDs, I'd explore using larger cluster sizes -- and here we have a clash with MS [big surprise]. IIRC, NTFS clusters cannot exceed 4KB [for something to do with file compression!]. That makes it possible that FAT32 with 32KB clusters [IIRC clusters must be less than 64KB for all system tools to properly function] might be the best choice for systems actively rewriting large files. I'm unfamiliar with FAT32 issues that argue against this, but if the SSD's allocate clusters contiguously, wouldn't this reduce erases by a factor of 8 for large file deletions? 32KB clusters might ham-string caching efficiency and result in more disk accesses, but it might speed-up linear reads and s/w loads.
The impact of very small file/directory usage and for small incremental file changes [like appending to logs] wouldn't be reduced -- it might be increased as data-transfer sizes would increase -- so the overall gain for having fewer clusters-per-SSD-block is hard to intuit, and it would vary in different environments.
GourdFreeMan - Wednesday, March 18, 2009 - link
RE Parag. 1: As I understand it, the entire 512 KiB block must always be erased if there is even a single page of valid data written to it... hence my concerns. You may save time reading and writing data if the device could know a block were partially full, but you still suffer the 2ms erase penalty. Please correct me if I am mistaken in my assumption.RE Parag. 2: The problem is the SSD itself only knows the physical map of empty and used space. It doesn't have any knowledge of the logical file system. NTFS, FAT32, ext3 -- it doesn't matter to the drive, that is the OS'es responsibility.
RE Parag. 3: I would hope that reducing the physical block size would also reduce the block erase time from 2ms, but I am not a flash engineer and so cannot comment. One thing I can state for certain, however is that moving to smaller physical block sizes would not increase wear across the surface of the drive, except possibly for the necessity to keep track of a map of used blocks. Rewriting 128 blocks on a hypothetical SSD with 4 KiB blocks versus 1 512 KiB block still erases 512 KiB of disk space (excepting the overhead in tracking which blocks are filled).
Regarding using large filesystem clusters: 4 KiB clusters offer a nice tradeoff between filesystem size, performance and slack (lost space due to cluster size). If you wanted to make an SSD look artificially good versus a hard drive, a 512 KiB cluster size would do so admirably, but no one would use such a large cluster size except for a data drive used to store extremely large files (e.g. video) exclusively. BTW, in case you are unaware, you can format a non-OS partition with NTFS to cluster sizes other than 4 KiB. You can also force the OS to use a different cluster size by first formating the drive for the OS as a data drive with a different cluster size under Windows and then installing Windows on that partition. I have a 2 KiB cluster size on a drive that has many hundreds of thousands of small files. However, I should note that since virtual memory pages are by default 4 KiB (another compelling reason for the 4 KiB default cluster size), most people don't have a use for other cluster sizes if they intend to have a page file on the drive.
ssj4Gogeta - Wednesday, March 18, 2009 - link
Thanks for the wonderful article. And yes, I read every single word. LOLrudolphna - Wednesday, March 18, 2009 - link
Hey anand, page 3, the random read latency graph, they are mixed up. it is listed as the WD Velociraptor having a .11ms latency, I think you might want to fix that. :)