Comments on: On-the-fly Data Compression for SSDs

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By: hemarcin

hemarcin — Thu, 21 Jul 2011 19:04:08 +0000

I’m not sure I agree.

“While it may feel incorrect, CD’s is actually no less valid than CDs when constructing a plural. What matters is preference by the writer/editor and consistency.”

Actually, the rules of English grammar (btw, it’s grammar, not grammer) state it clearly that apostrophes have nothing to do with marking the plural form of a noun. Hence, the form CD’s is unacceptable and it’s really nothing to do with a writer’s preferences.

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By: rattusrattus

rattusrattus — Wed, 09 Mar 2011 08:42:42 +0000

jc2it
No not a lock in. SSDs use the same interface as their hard disk counterparts. The controllers we are talking about are on the SSD, just like there is a controller on every hard disk…

By: Job Cacka

Job Cacka — Wed, 02 Mar 2011 23:53:13 +0000

So if my controller gets ruined I would need to ensure I purchase the exact controller in order to ensure reliable access my data. Sounds like YAFOVLI Yet Another Form Of Vendor Lock-in. Hmmm, perhaps it might be even necessary to purchase a spare for the shelf from the get go.

By: Jeffrey Layton

Jeffrey Layton — Fri, 25 Feb 2011 16:26:11 +0000

But there is a tradeoff in your approach. Allowing btrfs to compress the data for you uses CPU time whereas SandForce uses the processor on the SSD. If CPU time is important (it is in HPC), then you might want to offload the compression to the SSD.

However if you do this you may not save any space since the SSD can’t report a variable capacity and btrfs allows you to save space before it hits the storage device. But at the very least, the SandForce controller also increases the “life” of the SSD depending upon your data.

One other observation – there aren’t many file systems that do compression. If you rely on the file system then you are stuck with that file system. But SandForce based SSDs allow you to use any file system.

It’s all a trade IMHO. The great thing that SandForce has done for us to give us some cool technology allowing us to do the things we like to do (more performance, more life in our SSD, etc). Pretty great idea IMHO.

Jeff

By: Jeffrey Layton

Jeffrey Layton — Fri, 25 Feb 2011 16:21:38 +0000

Let me try to state things differently. There are literally thousands of applications that all have different IO patterns. Trying to develop an SSD that handles all of them well is impossible. So you have to select a target set of applications or perhaps a target market. Then you have to select representative applications and understand their IO pattern. Then you can tune the SSD firmware to perform well on those patterns.

You also couple this will the reality of running several applications at the same time which all may have different IO patterns.

The downside is that the performance may stink for other applications outside what is being targeted (as well as different loads).

So how do you market such a device? Do you tell the world that on these types of applications it will work fantastic? Do you tell the world that it will stink on other applications so don’t buy it?

My guess is that SandForce did the best they could and chose a representative set of applications that was part of their target market. Then they report the performance based on the IO patterns and applications they targeted. I don’t think there is anything wrong with that at all – in fact it was I would have done as well.

Benchmarks are a completely different subject. Do benchmarks represent real world applications? If so, which ones? Part of the difficulty is understanding the “compressibility” of the data for the applications versus the “compressibility” of the data in the benchmark. Are they comparable? Can you make any correlation between them? Is there a casual relationship?

Lots of details in designing and testing SSDs. Personally I think the technology behind SandForce is really great. You now have the opportunity to change your applications (I tend to work with applications that have source or that I write myself) to allow them to run better with SandForce based SSDs. Think of it as tuning your application for a particular chip architecture. We’ve never before had the opportunity to tune and application for storage hardware (it’s usually for memory, processor, or NICs). I personally think this is pretty cool and applaud SandForce for doing this.

Jeff

By: cweberusa

cweberusa — Thu, 24 Feb 2011 23:55:55 +0000

For me these discussions miss the boat almost entirely. The real breakthrough with SSDs is their low latency, two orders of magnitude lower than for hard drives. Also, in my tests with a Sandforce-based SSD and an Intel G2 SSD the latency was much more consistent and predictable than for my various 7200 rpm SATAII drives. All told, SSDs address the age old issue of slow disk vs. fast CPU and memory at the latency level, and as a result they universally speed up computational work regardless of the actual workload (except for very long running number crunching jobs). Everyone around me who has started to use an SSD won’t ever give it back, and it is not due to the overall read or write throughput- although we can quibble all day about nuances of such, it’s because of the much improved responsiveness of the system. We finally have a semblance of balanced systems back in our hands.

By: jlinton

jlinton — Thu, 24 Feb 2011 17:30:42 +0000

I have a drive with one of these controllers, this was one of the first things I checked (compressed vs uncompressed data performance).

I’m not sure his results are 100% valid. What he is describing sounds more like a fragmentation problem caused lack of TRIM/secure erase in his setup.

I’m betting that if he secure erases the drive, creates/mounts a FS, and reruns the test, the performance between compressed and uncompressed will be similar until he reaches the native capacity of the drive. Then he needs to run wiper.sh to restore it.

Basically, once the drive becomes full/fragmented the write performance goes to hell. This fact is masked by writing 100% compressible data. He should rerun the tests with 2:1 data. I’m betting his numbers are still going to be significantly less than the advertised drive speeds until he wipes the drive. Once he wipes the drive and runs wiper.sh (the linux TRIM workaround) on a frequent basis the performance will stay steady regardless of the compression ratio.