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TRIM allows the OS to "inform a solid-state drive (SSD) which blocks of data are no longer considered in use and can be wiped internally". Without this, the write speed on SSDs becomes very slow over time. More info @ Wikipedia.

At UDS there was a discussion about the method that was going to be used for trimming: online discard (adding a "discard" option to /etc/fstab) or fstrim with a cron job. Based on some benchmarks, the developers decided to go with fstrim because, like we also pointed out a while back, with online discard there's a performance hit, for example when deleting a large number of small files.

So starting with Ubuntu 14.04, SSDs that support it will be trimmed out of the box by using fstrim with a weekly cron job, as long as you're using a supported filesystem: ext3, ext4, xfs and btrfs.

According to the blueprint, TRIM will also be enabled by default for Ubuntu Touch soon.

Update: Only Intel and Samsung SSDs will have TRIM enabled by default in Ubuntu 14.04 because some cheap SSDs can even brick themselves when running TRIM. This doesn't mean TRIM should only be used with Samsung and Intel SSDs, but to avoid running into issues, this is the only option for now.