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systemd / zram-generator

Systemd unit generator for zram devices
MIT License
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Mount filesystems with "discard" option by default #102

Closed keszybz closed 3 years ago

keszybz commented 3 years ago

As discussed in https://github.com/systemd/zram-generator/pull/95, when files are removed from the filesystem, without "discard", the blocks remain allocated and consume memory. With the option, they are freed. So mounting without discard doesn't make much sense.

ext2 (which is now backed by the ext4 code) supports "discard", even though the documentation doesn't say this. It is possible that some other fs types don't support discard, but all the recent ones do. And even if there is a filesystem that doesn't do discard, and somebody is using our zram devices with it, it's probably better if the mount fails than using the fs without discard. So even though this is a small compatibility break, I don't expect this to be a problem in practice.

I tested this with ext2, and it's clear that the blocks don't get deallocated when files are removed. With swap, it seems that the discarding happens even without the option, and none of the docs I could see mention "discard" for swap. It seems we don't need to do the same for swap.

nabijaczleweli commented 3 years ago

This is quite crude, why not have an options = device option defaulting to discard instead? That'd also discard swaps, which seems desirable. I'll have a PR in a few.

keszybz commented 3 years ago

Yeah, in the end I didn't mention this, but I assume that #90 will be merged that'll provide this. This was supposed to be a simple measure to put in place until we have something fancier.

I'll have a PR in a few.

Let's continue the discussion there.

keszybz commented 3 years ago

Replaced by #103.