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Here's what LSFS (WAFL, CASL) is about. Where Log-Structuring concept came from, what it's good for and why.

Log-Structured File System is obviously effective, but not for everyone. As the "benefits vs. drawbacks" list shows, Log-Structuring is oriented on virtualization workload with lots of random writes, where it performs like a marvel. It's won't work out as a common file system for everyday tasks. Check out this overview and see what LSFS is all about.

Log-Structured File System

This overview is about the file system concepts used in NetApp (WAFL), Nimble Storage (CASL) and StarWind (LSFS). Those who are completely new to the idea of logging for I/O sequentializing, can find the basic info with the following link, because what's below is still kind of a "deeper dive":

https://en.wikipedia.org/wiki/Log-structured_file_system

The idea that eventually evolved into Log-Structuring concept lies in aggregating small random writes into one big page and putting it in cyclic log to copy them to the "final destination" later, when disk subsystem workload is lower. This design already works in many databases, such as Microsoft SQL Server. Here's a short insight into the matter:

https://en.wikipedia.org/wiki/Transaction_log

Logically, the next step after the databases would be to implement the same principle for common file systems. ZFS Intent Log (ZIL) is a good example of this approach:

https://en.wikipedia.org/wiki/ZFS#ZFS_cache:_ARC_.28L1.29.2C_L2ARC.2C_ZIL

Even before ZFS, there was an idea to make ONLY the log, without no other on-disk structures. This way all the metadata is also kept on the log and there's no copying from it anywhere. Basically, the log is just being constantly overwritten throughout the disk in a cyclical manner.

This Log-Structuring approach has numerous benefits, as well as drawbacks. Let's go through them.

...MORE

Read the entire article here at StarWind.

Published Wednesday, October 28, 2015 2:08 PM by David Marshall
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