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SSDs for the Virtualization Admin: What You Need To Know

If you are like me, you were probably dragged kicking and screaming into understanding and learning storage as you started your journey on "Route Virtual." Had you asked me six years ago about storage, I would have answered, "You mean the the LUN I ask the storage guy to provision?" Really, that would have been it. I knew nothing of Fiber Channel Arbitrated Loops, SAS, NAS/NFS or iSCSI, for that matter; I just understood, "I need a LUN for my Windows servers."

Since then, I have evolved with the technology and today, storage makes the virtual world go round. If you don't know storage, you cannot properly design and architect your environment across many technologies from server virtualization to desktop virtualization.

Storage is important to our careers, so I'm starting a series of storage blogs specifically around SSD, which is the hottest topic in storage right now. Please note these blogs will be more geared towards the virtualization admin, so I will have enough technical information that matters to the virtualization admin, things that you need to know when evaluating SSDs, what you will use them for, etc.

Let's start with a brief overview of the different types of SSD that exist, specifically, Single-Level and Multi-Level Cells, the pros of each and what different companies are doing to enhance the technology.

SLC SSDs are typically enterprise-class technology with the following characteristics:

  • Higher Cost per Bit
  • Higher Endurance
  • Low Power Consumption
  • Higher Write/Erase Speeds
  • Higher Write/Erase Endurance
  • Can endure operations at higher temperatures

MLC SSDs are similar, but provide higher capacities and lower cost per bit -- that's from a high-level, very basic overview. Now, let's look at some technical numbers:

Specification
SLC
MLC
Density
16Mbit
32Mbit
64Mbit
Read Speed
100ns
120ns
150ns
Block Size
64Kbyte
128Kbyte
Endurance
100,000
10,000
Operating Temperature
Industrial
Commercial

At first glance, you are probably thinking that SLC is the only way to go and MLC is just not yet enterprise-ready. You probably came to this conclusion by looking at the MLC's Endurance -- you just can't afford such a low life expectancy with MLC SSDs.

While the numbers I have provided in the table above are what I could find on the internet as somewhat of a standard, there really is not enough data to qualify the exact life expectancy of an MLC SSD. The number is a good guess at best. It doesn't take into consideration all the enhanced techniques that vendors are incorporating in their solutions to extend the endurance and reliability of MLC SSDs.

The fact of the matter, however, is that MLC technology has been advancing significantly and is now found in products that are enterprise-ready. Even IBM OEMs MLC technology from STEC and makes it available in their enterprise arrays. Manufacturers like Xiotech, Tintri, Whiptail and others all leverage MLC-based SSDs and make them available in the enterprise. But you may be wondering, "How do they do that? How can an MLC which is 10 times less reliable than an SLC from an endurance and longevity standpoint be viable and safe in the enterprise?"

MLC manufacturers and vendors are using a variety of techniques to compensate for the shortcomings in MLC, especially around endurance and longevity, including:

  • Error Correction Codes (ECC)
  • DRAM cache / Write Coalescing
  • Write Leveling (Wear leveling), spreads out the write distribution
  • Compression
  • Write Amplification

Note that all these techniques can be used with SLC or MLC SSDs, but they are more commonly used with MLC to compensate and extend its endurance and longevity. These techniques minimize the amount of data that has to be written to the flash memory. MLC SSDs are worn out by the number of writes. So, if there is a way to minimize the amount of writes, we inevitably extend the life of the SSD MLC disk.

There is significant effort under way to enhance MLC technology and it is getting better every day. Next time, I'll discuss the effects of using SSD MLC in VDI and what you can expect from it.

Posted by Elias Khnaser on 07/05/2011 at 12:49 PM


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