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TRIM Command and the SSD Write Cliff

Well, we may have recently avoided the ‘fiscal cliff’ in Washington DC, but some cliffs cannot be avoided.  Just ask Norm of the TV show “Cheers” how many times he wished he could avoid Cliff Clavin.  In the world of SSDs, the cliff that we want to avoid is the SSD “write cliff.”

Why the Write Cliff Matters

The SSD write cliff is the effect where SSD write performance drops off after all the free flash memory pages in an SSD have been initially written to and the device cannot provide enough free pages to keep up with subsequent write requests.  Each new write request then requires the SSD locate a block that can be erased for the new data.  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, called ‘write amplification’ increases SSD wear and is the primary cause of the write cliff.

The SSD write cliff and ways to avoid it are discussed in depth in the VeloBit White Paper: SSD Performance Tips: Avoid The Write Cliff.  Figure 1 shows an example of the write cliff observed in laboratory testing of a high performance, 320 GB PCI-E SSD at VeloBit.  What this shows is that the SSD achieves maximum bandwidth (500 MB/s) until the SSD is filled with data (320 GB on the x-axis).  However, once the device is full, the IO bandwidth rapidly drops off for additional write requests, hence the term ‘cliff’.  In this test, the “__% size” lines represent the allowable address space for data being written to the device.  We can see that bandwidth drops rapidly once the SSD is full and sustaining maximum IO is not possible.

Figure 1. SSD write cliff observed in lab testing

The TRIM Command Can Help

When a file is stored by an operating system and the OS “deletes” the file, the operating system file structure simply removes the file name from its file list and records that the disk space associated with the file is now available for writing new data into.  The delete command does not, however physically erase/remove the data from the storage media.  For SSDs, this means that a deleted file is still active in the SSD file mapping system and the memory cells used by the ‘deleted’ file are unavailable for use. As SSDs became more widely used, the problem of having ‘deleted’ files retained in valuable SSD storage became a big issue.  The TRIM command was recently developed specifically to address this problem.  TRIM is used to inform the SSD which file has been deleted and hence which blocks of data are no longer considered in use.  The SSD can then make these blocks available for erasure/reuse, reducing write amplification and SSD wear.

TRIM Does Not Create World Peace

While the TRIM command is a valuable tool to be used to extend SSD life and improve performance, it really can only help postpone the write cliff somewhat. The SSD hits the write cliff when it runs out of free SSD data pages and starts encountering write amplification. When 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 written with new data without having to copy any valid data thereby reducing write amplification, increasing performance and prolonging SSD life.  TRIM can help delay, but can’t eliminate the write cliff.

TRIM Requires New Software

So, good programming practices would include using the TRIM to help manage SSD life and performance.  However, many existing applications and installations were written before TRIM was available.  Using SSD caching software like VeloBit is an easier to deploy SSDs and properly manage them in legacy systems without having to rewrite applications.

Read the original blog entry...

More Stories By Peter Velikin

Peter Velikin has 12 years of experience creating new markets and commercializing products in multiple high tech industries. Prior to VeloBit, he was VP Marketing at Zmags, a SaaS-based digital content platform for e-commerce and mobile devices, where he managed all aspects of marketing, product management, and business development. Prior to that, Peter was Director of Product and Market Strategy at PTC, responsible for PTC’s publishing, content management, and services solutions. Prior to PTC, Peter was at EMC Corporation, where he held roles in product management, business development, and engineering program management.

Peter has an MS in Electrical Engineering from Boston University and an MBA from Harvard Business School.

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