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Myth Busting video: Caches are unreliable

Perhaps the most widely held belief about write caching is that it makes a system unreliable. Something along the lines of "Data written to a cache instead of to the media will be lost on an unexpected power interruption, leaving the system in a damaged state and rendering data useless." There are a variety of options busting this myth.


Myth Busting: Using a Write Cache to Improve Performance Means Sacrificing Data Integrity

The type of cache referred to above is also known as a write-back cache. Data is stored in a memory location, then written to the media at a later time. With volatile memory, this can be a hazard - but there are other options available. Better SSD controllers have a battery-backed cache, where the power is maintained long enough to commit the writes to non-volatile storage. Another option becoming more common is a Magnetic RAM (MRAM) style cache or trickle feed Static RAM (SRAM), where the data can persist for a short time until the unit is powered back on.

The write-back cache isn’t the only type of cache either. A type of cache known as the write-through cache is common in RAID controller cards. Here the data is stored on the drive immediately, with a copy remaining in memory for read and update purposes.

With no access to those specialized options, the only other solution for full reliability is to disable the write cache, right? On a file system with no data reliability mechanisms, such as FAT, the write caching at the disk level can greatly increase the risk. This file system can require several sector writes to get from one consistent file system state to another, and the longer those writes remain in the cache, the larger the window of vulnerability.

Datalight's Reliance Nitro provides a write cache which doesn’t decrease reliability, primarily due to the transactional model we use. Data is committed to the media from both buffers and external cache during a transaction point. Unlike other "reliable" solutions, the system designer and application developer have complete control over how often these commits occur. Datalight software can minimize the data-at-risk without actually disabling the disk cache.



Looks like another myth is busted!


For more information on Datalight's Relaince Nitro please contact us.