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SSD, HOT OR NOT

Written by Roy Mikes on Tuesday, 09 April 2013. Posted in General

Solid State Drives (SSDs) are immens populair these day's. They are known for extreme performance. This is where SSDs shine. From that perspective it's more then reasonable to think that there is a good future role for solid-state drives. The downside still is that SSDs are frakking expensive. But this is just a matter of time. Maybe we we'll see a future were SSDs are Less expensive then HDDs. Just because we'll consuming more SSDs then HDDs. Time will tell.

A SSD uses what is called NAND-based flash memory. NAND Flash is a special form of Flash memory. This is a non-volatile type of memory, which means that you can turn off the disk and it don't loses data. This is of course an essential characteristic of any type of permanent memory. But writing to Flash memory is pretty slow compared to many other 'volatile' memory technologies such as DRAM. Flash also has a limited number of write-cycles. Flash memory can only be written and erased a limited number of times. This is often referred to the maximum program/erase cycles (P/E cycles). Single-level cell (SLC) flash, designed for higher performance and longer endurance, can typically operate between 50,000 and 100,000 cycles. Enterprise SSD's like STEC's are touching 1 million cycles. Multi-level cell (MLC) flash is designed for lower cost applications and has a greatly reduced cycle count of typically between 3,000 and 5,000. Now you know the differents between desktop and Enterprise SSDs.

So what's hot on SSD and why are all Storage vendors and a lot of Startups using SSD?

Hot

A SSD has no moving parts, so it is more likely to keep your data safe in the event of an earthquake while it's operating. Slightly exaggerated but there are regions where this argument stands. Unlike in a HDD, the SSD does not produce heat. Even the quietest HDD will emit a bit of noise when it is in use from the drive spinning or the read arm moving back and forth. SSDs make no noise at all, since they're non-mechanical. Good for your energy bills and so Green IT. Definitely if you have to deal with storing, but also processing large amounts of data.

But the biggest advantage is speed. SSDs are in general 30 times faster than HDDs. So performance wise you get less capacity but high speed in return. But speed is just a word and can mean anything. 

The lack of consistency in how SSDs are configured has lead to confusion on how the performance data are reported to companies. Many SSD storage vendors will report their performance numbers when the drive is empty. This is known as the Fresh Out of Box state and in the case of SSD can be quite misleading because of the way data is written to flash drives. Due to the nature of flash memory's operation, data cannot be directly overwritten as it can in a HDD. A process commonly referred to as “garbage collection”. Data is written to the flash memory in units called pages, often 4–8 kilobytes in size. However, the memory can only be erased in larger units called blocks, often 256 kilobytes in size. Watch your SSD write cliff. I'll discus this later. But in any case, this impacts performance.

 

Or Not 

Every advantage has its disadvantage! So what's the downside of SSDs? Many factors will affect the write amplification of an SSD a typically used term by Intel and Western Digital.

When an SSD is writing data sequentially, the write amplification is equal to none. The reason is as the data is written, the entire block is filled sequentially. When an SSD is writing data random, you deal with completely garbage collected, secure erased, 100% TRIMed, Write Cliff or newly installed. A lot of overhead as you can see. So the overall performance of an SSD is dependent upon a number of factors. Writing to a flash memory device takes longer than reading from it. It's almost negligible, but deletion of cells takes in average 9 times as long as read and write.

How do we deal with high performance applications, real time information or batch processes? This is typically data that needs SSD. Because HDD probably doesn't meet the requirements.  But what if write performance drops after all the free flash memory pages in an SSD have been initially written and the device cannot provide enough free pages to keep up with write requests? If a block that needs to be erased contains active data, the active data must be written to a new location to free up the block to be erased. This process of copying valid data from one block to a new block is the primary cause of the write cliff. The TRIM Command Can Help. The TRIM command is to be used to extend SSD life and improve performance. When the TRIM commands mark entire pages of data as ‘deleted’ before the SSD runs out of free data pages, the deleted page can be erased and overwritten with new data without having to copy any data thereby reducing write amplification, increasing performance and prolonging SSD life.

So, TRIM soothes your pain and don't take it away.

 

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About the Author

Roy Mikes

Roy Mikes

Roy Mikes has developed a deep knowledge of virtualization, storage and Cloud in a broad perspective over the past 15 years, but also streamlines his focus on VMware and EMC Technology. Because of that knowledge and focus, Roy works as a Cloud and Storage Technologist Specialist, Data Center Architect Specialist and VMware Certified Professional.

Comments (1)

  • 50mu

    50mu

    10 April 2013 at 13:32 |
    Nice story about optimizing performance and wear leveling on SSDs.

    reply

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