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Any reference about the 'no discard' advice ? I used it for a while on an Intel 520 80GB SSD, it was fine at first but when I reached low free space (~1GB) the SSD became sluggish, which isn't unexpected, but even after freeing 4GB it was still slow. Manually trimming reported '0 Bytes trimmed'. Only after remounting the disk without `discard` and then `fstrim --all` that the SSD got decent performances back.

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I'd guess your SSD was doing a synchronous discard which would be slow. I know anything less than SATA 3.1 does it synchronously, and maybe there are other situations too. E.g., maybe async trimming requires a decent amount of free space?

It was fine before I reach low free space and I don't have SATA 3.1, not even 3, possible 1 lol, so it was synchronous ?

I guess the controller needs some temporary space to do its magic and below that point it will struggle.

The odd fact is that freeing space didn't allow for normal behavior until "forcing" an actual trim.

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