The Impact of Spare Area on SandForce, More Capacity At No Performance Loss?
by Anand Lal Shimpi on May 3, 2010 2:08 AM ESTNo, it’s not the new Indilinx JetStream controller - that’ll be in the second half of the year at the earliest. And it’s definitely not Intel’s 3rd generation X25-M, we won’t see that until Q4. The SSD I posted a teaser of last week is a modified version of OCZ’s Agility 2.
The modification? Instead of around 28% of the drive’s NAND set aside as spare area, this version of the Agility 2 has 13%. You get more capacity to store data, at the expense of potentially lower performance. How much lower? That’s exactly what I’ve spent the past several days trying to find out.
The drive looks just like a standard Agility 2. OCZ often makes special runs of drives for testing with no official labels or markings, in fact that's what my first SandForce drive came as late last year. Internally the drive looks identical to the Agility 2 we reviewed not too long ago.
OCZ lists the firmware as 1.01 compared to the standard 1.0 firmware on the shipping Agility 2. The only difference I'm aware of is the amount of NAND set aside as spare area.
SandForce and Spare Area
When you write data to a SandForce drive the controller attempts to represent the data you’re writing with fewer bits. What’s stored isn’t your exact data, but a smaller representation of it plus a hash or index so that you can recover the original data. This results in potentially lower write amplification, but greater reliance on the controller and firmware.
SandForce stores some amount of redundant information in order to deal with decreasing reliability of smaller geometry NAND. The redundant data and index/hash of the actual data being written are stored in the drive’s spare area.
While most consumer SSDs dedicate around 7% of their total capacity to spare area, SandForce’s drives have required ~28% until now. As I mentioned at the end of last year however, SandForce would be bringing a more consumer focused firmware to market after the SF-1200 with only 13% over provisioning. That’s what’s loaded on the drive OCZ sent me late last week.
| SandForce Overprovisioning Comparison | |||||
| Advertised Capacity | Total Flash | Formatted Capacity (28% OP) | Formatted Capacity (13% OP) | ||
| 50GB | 64GB | 46.6GB | 55.9GB | ||
| 100GB | 128GB | 93.1GB | 111.8GB | ||
| 200GB | 256GB | 186.3GB | 223.5GB | ||
| 400GB | 512GB | 372.5GB | 447.0GB | ||
As always, if you want to know more about SandForce read this, and if you want to know more about how SSDs work read this.
When Does Spare Area Matter?
In addition to the SandForce-specific uses of spare area, all SSDs use it for three purposes: 1) read-modify-writes, 2) wear leveling and 3) bad block replacement.
If a SSD is running out of open pages and a block full of invalid data needs to be cleaned, its valid contents is copied to a new block allocated from the spare area and the two blocks swap positions. The old block is cleaned and tossed into the spare area pool and formerly spare block is now put into regular use.
Recreated from diagram originally produced by IBM's Zurich Research Lab
The spare area is also used for wear leveling. NAND blocks in the spare area are constantly being moved in and out of user space to make sure that all parts of the drive wear evenly.
And finally, if a block does wear out (either expectedly or unexpectedly), its replacement comes from the spare area.
Facebook
Twitter
RSS
30 Comments
View All Comments
Spoelie - Monday, May 3, 2010 - link
One area I think that might still be affected is reliability. SandForce stated that (1) smaller geometries introduce more defects and (2) manufacturers could use cheaper, less reliable flash in drives with their controllers.Does the reduction of spare area impart reduced lifetime/reliability in the above scenarios or is its responsibility purely one for performance? I reckon it's not something one would be able to measure though.
GeorgeH - Monday, May 3, 2010 - link
Reliability will go down. 28% wasn't a random choice, it was selected to deliver a certain MTBF. AFAIK the "enterprise" drives use the same 28%, though, so "consumer" usage models should be able to get by with less.The real question is how they arrived at 13% - is it Bean Counter Bob's number or Engineer Eric's number? Until they answer that question and release their methodology for arriving at 13%, I wouldn't touch one of these with a thousand foot pole. The chance that 13% was the misguided result of some accountant waddling over to the R&D department for 5 minutes is just too great relative to the small benefit of 10-20 "free" GB's.
softdrinkviking - Monday, May 3, 2010 - link
i wonder how much of a role the spare area plays in maintaining the compression algorithms for the sandforce controller.it's seems like, with such a complex controller, it would be wise to have plenty of "hash or index" space to work with, or is that all stored somewhere else?
jleach1 - Monday, May 3, 2010 - link
IDK about you...but i dont plan on keeping a drive this small for that long. A few years is reasonable. Right now, what people want is: a cheap drive that performs well. I'll gladly trade 6 months of the life of my drive for some badly needed space. In 6 months, theyll likely have a set of firmware options that increase the amount of usable space, and improved algorithms that offset the normal reliability problems.Good job OCZ. Less $/GB= a happier public.
softdrinkviking - Wednesday, May 5, 2010 - link
my question was about how the amount of spare area would effect the short term reliability of the drive. assuming that these drives are relatively unproven, who's to say that they won't start losing data because of the complex compression used by the controller?i want to know if lessening the spare area could contribute to controller errors, leading to the loss of data.
Belard - Monday, May 3, 2010 - link
Looking at your benchmarks, other than SATA 3/6GB/s system, the Intel X25-M (G2) are still constantly the fastest and most reliable on the market. Personally, I can't wait for the SSD market to have SATA-3 drives as standard.Seq. Read
OCZ = 264 MB/s * (okay a bit faster)
X25 = 256 MB/s
Seq. Write
OCZ = 252 MB/s * (Destroys the intel)
X25 = 102 MB/s
But most operations are random... So if you're doing Video encode/decode or copy, the OCZ kills.
Random. Read
OCZ = 52 MB/s
X25 = 64 MB/s * Intel wins easily. Even the top 6GB/s is barely faster.
Random Write
OCZ = 44 MB/s
X25 = 46 MB/s * (not bad for an OLD drive)
Half the drives are much slower, but some of the best easily faster.
It will be intresting to see what happens to the SSD market in 12 months.
7Enigma - Monday, May 3, 2010 - link
I kinda feel the same way. Since we have not yet reached the point where a large portion of our data is stored on these (most of us at least), these sequential writes just don't blow me away the same way the X25 changed the HD scene. After the intial setup (OS, programs, a couple games), the drive is basically going to be a random read/write drive with occassional install, and for that I can wait the extra time that a faster drive would have saved if the end result (gaming/bootup/etc.) is nearly the same.What I want to see is the game-changing performance the X25 did to the traditional HD in the random read/write metric. Get those into the 200-300MB/sec and THEN I'll get excited again.
The0ne - Monday, May 3, 2010 - link
Reading all the latest Anandtech SSD reviews feels like I'm reading someone's hobby work :) So many changes. Can't wait til it stabilizes A LOT more.sgilmore1962 - Monday, May 3, 2010 - link
Random WriteOCZ = 44 MB/s
X25 = 46 MB/s * (not bad for an OLD drive)
Half the drives are much slower, but some of the best easily faster.
Conveniently omitting the part where if you are using Windows 7 4k random writes are aligned on 4k boundries. The Sandforce random 4k writes become 162mb/s a whopping margin over Intel G2.
Belard - Monday, May 3, 2010 - link
Do you know that there is a REPLY button? That way your COMMENT would be attached to the post, rather than starting a whole new dis-connected thread.So look to the left, there my name is and you'll see the word REPLY. Give it a shot.
- - - - -
Man, wish there was a QUOTE function as well as the ability to save my LOG-IN on this revised site.
"Conveniently omitting the part where if you are using Windows 7 4k random writes are aligned on 4k boundries. The Sandforce random 4k writes become 162mb/s a whopping margin over Intel G2."
Er... no. I *DID* go with the Win7 performance test. I was comparing the REVIEWED drive to the Intel X25-M. And I ALSO said "but some of the best easily faster."... so I was NOT disregarding the SF drives.
I was expecting people to be able to figure this out.
And when it comes to RANDOM reads... All those SF drives your so concerned with, are easily SLOWER than the X25-M.
Intel X25-M G2 160/80 = 64.3~5 MB/s
Intel X25-M G1 160/80 = 57.9 MB/s
SF 1200~1500s = 49.4~52.1 MB/s... Ouch, SF is slower than the year old G2 and even older G1!! Even losing up to 15MB.s! About 25% slower than intels!
The intel drives were the most expensive... now they are generally cheaper (cost per GB).
I will continue to buy G2 drives (even those without the intel label) for my clients until something that is better across the board comes out. As far as I am concerned, Random Reads are somewhat more important than random reads... and both are about Sequential. This is why Windows7 boots up in about 10~12 seconds vs 35~50sec for a HD on the same same desktop.
And I am not even a big fan of intel. I usually build AMD systems. But I'll buy what is good.
Intel X25-M G2 wins in:
A - price
B - Availability (Many of the OCZs are not even available. Some stores carry older models)
C - Performance Random
D - Performance Sequential (okay, at 256 vs 265.... intel is a bit slower)
E - Reliability
F - TRIM support (Its unclear if all the other drives support TRIM - depending on the Firmware)
From the looks of things, the G2 will lose its position when the G3 comes out.
I plan to get a G3 for my next build... Hopefully it'll be $150~200 for 80GB with SATA 3.0 delivering 375+MB/s Seq Read/Write and 200MB/s for random R/W. That, I would really drool over!