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Basic Differences between Solid State Flash Drives - MLC vs SLC . Comparison to Spindle Hard-Drives

Updated on January 22, 2015

Installing Solid-State (SSD) MLC Drives In a Server is Not Recommended

Almost looks just like an average standard drive. It is much lighter and not quite as thick.
Almost looks just like an average standard drive. It is much lighter and not quite as thick.
The Corsair Force Series 3 SSD SATA 3 for example, is a drive I have used. It is a great budget SSD drive for  a desktop applications, but it is not recommended for servers or other multi-user environments.
The Corsair Force Series 3 SSD SATA 3 for example, is a drive I have used. It is a great budget SSD drive for a desktop applications, but it is not recommended for servers or other multi-user environments.

Use the Right Drive for the Right Purpose.


Using SLC Solid State SSD Drives in Servers?

There are a multitude of manufacturers of solid state drives. Most manufacturers of these drives have familiar names in the hard-drive industry that we have seen for many years past like Western Digital and Seagate. Then there are others that we've seen over the years producing cassette tapes (remember those?), CDs, and DVDs to USB flash drives but now, they also make hard-drives. I have used solid state drives and they perform absolutely great compared to spindle based drives. I would like to note however that I have only used the MLC solid state drives. Don't have the funds for the expensive SLC drives? The IOP (Input Output) gain over other technologies gives a system a noticeable boost in performance. The price of SLC drives is much, much less than MLC and are at such a good point that it is very temping to use them for server applications. They are so inexpensive it's also very enticing and luring for thinking about and actually adding them into storage array appliances.

MLC Solid State Drives for Servers?

The mean time to failure using SSD drives is much less than standard spindle based drives. It is absolutely not recommended to use these drives in servers, especially as part of the storage for a hyperviser like VMware, Citrix XenServer or XenDesktop, or Microsoft Hyper-V. I know of a company that knowingly installed this level drive into their SAN/NAS. They wanted the predictable performance but was aware the drives were desktop grade SLC drives and not enterprise MLC drives that cost four times the amount or more.

As expected there was drive failures. They had 2 drive failures within 8 months. They did and still keep spares in their inventory and watch diligently for drive failures.


MLC vs. SLC Flash Drive Technology

A mentioned there many vendors of solid state drive technology. All built on the same premise of performance with no moving parts. Interestingly enough, I have had to replace many drives over the years due to failure. The failure perhaps out of the 200 or so drives I have replaced was only about 8 to 10 times where the mechanical components have failed. it is usual the pci board components, the electronics, that fail. Failure of the mechanics, such as the arm, gives us a clanking sound when the drive is shaken.



 

Solid-State Drive Introduction Video. This is NOT a promotion for Intel. Their video is excellent however in describing the Solid State Drive properties.

 
Specially designed SSD controller cards can extend the life of a Solid State Drive.
Specially designed SSD controller cards can extend the life of a Solid State Drive.

MLC vs. SLC Flash for Enterprise Server and Storage SSDs

Enterprise Server and Storage SSDs

In a nutshell, wear and tear. That's what it boils down to. The number of write cycles that the media type, SLC or MLC, can endure before failure. The general accepted rule is that SLC NAND can endure 100,000 write cycles while MLC can endure about one tenth of that at only 10,000 cycles. That’s just a generalization of the media's design. Controller and array vendors are implementing methods to compensate for the differences in raw NAND reliability. Examples include wear leveling (or write leveling) to spread out across different parts of the flash disk the write distributions, DRAM cache and write coalescing, minimizing write amplification, over-provisioning capacity, enhanced error correction codes (ECC), compression, and other techniques, some others of which are proprietary, for minimizing the amount of data that has to be written to the flash memory and also how often it is written. Most of these techniques can be used with SLC and/or MLC devices, although they are becoming more prevalent in the MLC space. These techniques are giving way to an eMLC category of SSD drives.



Performance and Pricing is So Luring, Temping to use SSD for Servers.

It’s possible to get almost near-equivalent read performance with MLC drives, but MLC NAND really suffers on write performance compared to SLC flash. As such, by its design, SLC flash technology will always be inferior to SLC but vendors are using a variety of techniques to improve the write performance on MLC SSDs. Generally, there’s a 2X to 3X difference in write performance between SLC and MLC at the NAND level.

Performance At the SSD NAND Level

At the SSD level, performance depends mainly on the controller implementation, DRAM caching, and factors such as firmware efficiency (to, among other things, minimize write amplification), the ability to place data in previously erased blocks, over-provisioning, and read/write pattern factors such as data transfer size and random vs. sequential I/O.

 

Flash Basics - NAND Flash

NAND Flash memory, like many other types of memory, stores the data in a large array of cells. Each cell holds one or more bits of data. In a typical NAND Flash device, “a high voltage of 1.8V is applied to the control gate to draw electrons from the substrate to tunnel through the gate oxide into a polysilicon floating gate layer.To store one bit, two charge levels in the floating gate layer can be stored to distinguish between a ‘1’ and a ‘0’."2 Multilevel cells, discussed later, store additional charge levels within each bit cell.

Main Difference Campared to "Regular" Traditional Drives

The components inside of regular HHD drives is very different from SSD drives.
The components inside of regular HHD drives is very different from SSD drives.

Traditional Spindle Based Drives


Drives such those that have been in use for many decades and still are very widely used more than SSD drives have a platter on which is stored. The metal platter spins and the arm moves to the various locations of the drive to write and read data.

Data is written or read on SSD drives by electronically charging individual memory cells. On spindle based drives, the data is written and read magnetically,




SAS / SATA Connections

SATA and SAS spindle drives will look the same . Their connector differs so at a glance one can see what type of drive it is.
SATA and SAS spindle drives will look the same . Their connector differs so at a glance one can see what type of drive it is. | Source

HD or SSD - The Connections are the Same

Within the spindle drive world there are two major categories of drives.

The two are:

  1. SATA Serial Attached
  2. SAS Serial Attached SCSI

Unlike solid state drives and spindle drives where one can easily see and *feel* the difference with the SSD drives being considerably lighter, these two drive technologies look the same except for their connections.

The difference between the SAS and SATA may be another hubpage as going into the differences is not part of this hub's scope. In addition, it is rare that a general computer user will see a SASA drive. They are most common in servers and storage arrays. Desktops and laptop use SATA technology with the exception of high-end workstation desktops (at least at the time of this writing)

The key thing to remember is that the connectors for flash (Solid State Drives ) and spindle drives are the same. The media differs. The SSD drives are flash memory and the spindle drives are platters. The connectors and the drive access technology is the same - SAS or SATA.


Warranty and Support

Online PC Remote Support Software

The warranty on many of the drives is most often just a year but there are also drive that have a three year warranty and support. Support will come in various flavors and although I have not utilized all brands of solid state drives, some will use remote support software to assist customer with issues when possible. Remote support is usually the method for manufactures to assist and aid their clients and customers over the web, Not often however, the support and return policy is most important because when solid state drives fail, it is not gradual, they just stop working. The failure make it impossible for any web based remote support software to be helpful and requires the drive to be replaced.



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