this post was submitted on 02 Jan 2024

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Help on BTRFS setup (programming.dev)

submitted 6 months ago by [email protected] to c/[email protected]

17 comments fedilink hide all child comments

I'm attempting a new install. I want to use btrfs with swapfile.

Do I need to disable compression on my swap subvolume?

Is there anything else I should keep in mind for fstab if I want to, say, not keep track of my Downloads folder when snapshotting?

Here is my fstab:

LABEL=arch@btrfs        /               btrfs           rw,relatime,discard=async,space_cache=v2,subvolid=256,subvol=>

LABEL=arch@btrfs        /home           btrfs           rw,relatime,discard=async,space_cache=v2,subvolid=257,subvol=>

LABEL=arch@btrfs        /var/cache/pacman/pkg   btrfs           rw,relatime,discard=async,space_cache=v2,subvolid=259>

LABEL=arch@btrfs        /var/log        btrfs           rw,relatime,discard=async,space_cache=v2,subvolid=258,subvol=>

LABEL=arch@btrfs        /.snapshots     btrfs           rw,relatime,discard=async,space_cache=v2,subvolid=260,subvol=>

LABEL=arch@btrfs        /swap           btrfs           rw,relatime,discard=async,space_cache=v2,subvolid=263,subvol=>

LABEL=efi@fat32         /efi            vfat            rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=asci>

/swap/swapfile          none            swap            defaults        0 0

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[–] [email protected] 5 points 6 months ago (1 children)

Yeah it's supported. It's listed in the docs for btrfs and arch.

[–] [email protected] 1 points 6 months ago (1 children)

I'd highly recommend setting up a swap partition instead.

[–] [email protected] 2 points 6 months ago (2 children)

Why?

[–] [email protected] 3 points 6 months ago* (last edited 6 months ago) (2 children)

As swap is recommended just in case all RAM is maxed it's better to have a swap partition as swap files have certain limitations when in combined use with BTRFS:

"subvolume - cannot be snapshotted if it contains any active swapfiles"
has a chance to fragment
has issues with hibernation (that I've personally encountered multiple times)

here's the link for more info: https://man.archlinux.org/man/btrfs.5#SWAPFILE_SUPPORT

after switching to a swap partition with BTRFS I've experienced no issues for a user configuration

[–] [email protected] 2 points 6 months ago* (last edited 6 months ago) (1 children)

I don't see how swap has much chance to fragment. A swapfile has to be fully allocated up front and cannot be CoW. If it's allocated well in the first place, it will stay that way.

The swap code doesn't really do I/O through the filesystem. AIUI, it locks the file, gets the disk block #s from the FS, and after that it accesses those blocks directly.

[–] [email protected] 1 points 6 months ago

From the BTRFS manual:

"Nodatacow implies nodatasum, and disables compression."

"Datasum implies datacow"

Based on these notes I'd assume that since swapfile disables COW that it also disables checksumming which is where the risk of fragmentation occurs

I could be wrong tho

[–] [email protected] 2 points 6 months ago (1 children)

"subvolume - cannot be snapshotted if it contains any active swapfiles"

Make a subvolume only for the swapfile.

has a chance to fragment

This is true for all files. Is it a bigger problem for swap?

has issues with hibernation (that I've personally encountered multiple times)

This one I can't refute. How long ago did you have these issues?

[–] [email protected] 3 points 6 months ago* (last edited 6 months ago) (1 children)

Make a subvolume only for the swapfile.

doesn't this kinda defeat the purpose/benefits of using a swapfile?

This is true for all files. Is it a bigger problem for swap?

specificly swapfiles yes, for swap partitions nope

How long ago did you have these issues?

Dec 2022, was still using and testing with swapfiles then and said fuck it as it caused too much problems.

I can't rule out user error till I retest and strictly "follow the guide to the T" as I made modifications while following the same Arch guide for swapfile with BTRFS

edit:
also for clarification, I'm still not sure which one is optimal/best as I initially thought that using swapfile was forward thinking for the future, I'm using and recommending swap partitions as it seems to be the easiest to implement once and use continuously without any problems atm.

[–] [email protected] 2 points 6 months ago (1 children)

The reason I use a swap file is so that I can have only one partition backed by LUKS disk encryption, rather than having to screw around with lvm which comes with its own performance overhead and all. I've personally never had issues hibernating to.it, but given how much buggy uefi firmware is out there I'm not surprised to hear that other have issues

[–] [email protected] 1 points 6 months ago

If you need to set up a special dedicated subvolume, might aswell set up a partition instead; it's just simpler.

With a swapfile you also can't do multi-device setups which is a limitation I personally couldn't live with.