The discard flag causes the filesystem to issue TRIM commands on each sector as they are deleted. There are varying schools of thought on whether this is a good idea, but it can have an impact on performance. The filesystem does not need to be mounted with -o discard to be able to run fstrim. In fact, some distros have a cron job that runs fstrim -av weekly in lieu of mounting with -o discard.
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For those interested in the technical details of The Dialup Sound: http://www.windytan.com/2012/11/the-sound-of-dialup-pictured.html
The discard flag causes the filesystem to issue TRIM commands on each sector as they are deleted. There are varying schools of thought on whether this is a good idea, but it can have an impact on performance. The filesystem does not need to be mounted with -o discard to be able to run fstrim. In fact, some distros have a cron job that runs fstrim -av weekly in lieu of mounting with -o discard.