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SCSI versus MLC versus SLC

Posted: Fri Dec 12, 2008 2:44 am
by JAB Creations
I don't just love code, I love hardware too. So I'm curious about what people think when comparing SCSI to MLC and especially to SLC based flash hard drives. I'm getting the idea that write performance is very important for MySQL in example clearly SLC drives are going to be more competitive to SCSI. However I'm only running a couple of RAID 1's for my system so I'm curious what the folks working for (or who own their own) businesses have to say about high end flash drives versus SCSI drives right now especially since flash drives will be matching high end SCSI drives in capacity most likely by sometime next year.

Re: SCSI versus MLC versus SLC

Posted: Sat Dec 13, 2008 7:33 pm
by josh
You mean solid state? It needs constant power ( battery backups ) and its more expensive then normal drives. Honestly I don't see the need for it ( yet - in enterprise apps ). Some of the servers we use for our site are dual quad core xeons, with 15k rpm SCSI drives in raid-5, sites nearly 100% idle all day, page benchmarking is 200ms with no caching or anything implemented. With 32GB of ram it basically is solid state storage, if you tune mysql caching.

Re: SCSI versus MLC versus SLC

Posted: Sun Dec 14, 2008 4:36 am
by arjan.top
SSD does not need constant power

Re: SCSI versus MLC versus SLC

Posted: Sun Dec 14, 2008 6:09 am
by Eran
SSD is a storage format, SCSI is an interface
And SSD doesn't need constant power - the battery backups are just for flushing the cache if power is turned off suddenly

Re: SCSI versus MLC versus SLC

Posted: Sun Dec 14, 2008 6:30 am
by josh
SSDs based on volatile memory such as DRAM are characterized by ultra fast data access, generally less than 0.01 milliseconds, and are used primarily to accelerate applications that would otherwise be held back by the latency of Flash SDDs or traditional HDDs. DRAM-based SSDs usually incorporate internal battery and backup storage systems to ensure data persistence while no power is being supplied to the drive from external sources. If power is lost, the battery provides power while all data is copied from random access memory (RAM) to back-up storage, or to allow the data's transfer to another computer. When the power is restored, the data are copied back to RAM from the back-up storage, and the SSD resumes normal operation. (Similar to the hibernate function used in modern operating systems.)
With 32GBs of ram though, I'm essentially running solid state though. If the load gets high enough then we start paging onto our 15k RPM SCSI drives in raid 5 configuration. If you ever sysadmin'd a linux box you'd realize linux automatically intelligently allocates as much memory as possible to block caching, with these kinds of resources I can also have mysql hold the entire index & key structure in cache as well. The system hardly ever has to hit disk, and if it does the drives are crazy fast anyways. We couldn't go pure solid state because we have a TB and a half of data.

SS memory is based on a flip-flop pattern in electronics. http://en.wikipedia.org/wiki/Flip-flop_(electronics)

If you plug in your TV press the on button usually its a "button" not a switch, pull the plug and plug it back in the TV doesn't remember that it was "on". There's no way the project stakeholders would let business success be hinged on some faulty battery 4,000 miles away powering 1TB+ of data for a production site.

Re: SCSI versus MLC versus SLC

Posted: Sun Dec 14, 2008 6:44 am
by Eran
If power is lost, the battery provides power while all data is copied from random access memory (RAM) to back-up storage, or to allow the data's transfer to another computer. When the power is restored, the data are copied back to RAM from the back-up storage, and the SSD resumes normal operation. (Similar to the hibernate function used in modern operating systems.)
An SSD drive is a little more advanced than the on/off button on your TV. On average, an SDD drive will consume much less power than a magnetic drive (which what makes them ideal for laptops). I agree though that SSD is not yet the optimal solution for production servers, but it's an interesting development.

Re: SCSI versus MLC versus SLC

Posted: Sun Dec 14, 2008 9:11 am
by josh
pytrin wrote:An SSD drive is a little more advanced than the on/off button on your TV.
They both use the same technology on a granular level, but yeah. Flip flop is a "pattern", implemented by transistors, logic gates, RAM chips, etc...