I am re-writing from scratch the "manual" mode of the Slint installer, which until now uses "setup" and associated scripts from the Slackware installer with some adaptations, and plan to provide it for Slint 15 which I hope to release one month or so after Slackware 15.

In this manual (or expert) mode I will allow (not propose) to set up separate partitions for /home, /var, /usr/local, /opt and /tmp and propose for these (if not yet populated, but in case of /tmp) to format them if not yet done using one of the file systems btrfs, ext4, f2fs, or xfs.

In a further (not the next) version of the installer I also consider making btrfs the default in "auto" mode (instead of ext4 generally, or f2fs for some flash devices) as already do recent versions of Fedora and OpenSUSE.

In anticipation, my question is: I know that it is technically possible to convert an ext4 file system to btrfs, but if, say, user has chosen separate partitions for the directories listed below, will it be possible after having converting them to btrfs if needed, to convert the content of these directories to btrfs subvolumes?

Any clue, caveat and information appreciated, especially from folks having related experience.

From developer perspective I would not touch *converting* from one filesystem to another during installation because the risk of unrecoverable data due from unmanaged event(s).

Context: I was even paranoid during ext3 -> ext4

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Thanks for giving me an occasion to clarify: I certainly do not intend to make this a feature of the installer. What I want is to be able to provide sound good information and advice to users interested to do that themselves.

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I've just read latest kernel log... there are some 20 btrfs bugfixes...

And even some ext4 fixes ffs!

I use ext4 on mechanical drives because ext4 can keep a bad-sectors list and avoids using them. I use btrfs on my SDD drives because error bits are remapped by firmware. I got bit on btrfs when a laptop drive started throwing bad sectors. Had to reformat to ext4 and restore a recent backup. I create the EFI and slackware partitions using usbboot.img on a thumb drive, then tell setup to use existing partitions. It is a simple matter to re-organize the filesystem after install once the system is up and running. Why confuse folks with a more elaborate setup fraught with possible input errors?

This doesn't answer my request for advice. That you do not need or want to use btrfs does not imply that no one does. In case you do not know the distinctive features of btrfs, have a look here.

tl;dr: to "how-to?", "don't" is not an useful answer.

I just started using btrfs for my data partition (with multiple disks as the backing store), so am by no means an expert.

However, to answer your question: I'm pretty sure no. It is possible to have multiple disks be combined into 1 btrfs filesystem that then has multiple subvolumes, but I don't think you can transition the way you describe. Most (all?) of what I know is from these articles:

https://btrfs.wiki.kernel.org/index.php/Getting_started
https://btrfs.wiki.kernel.org/index....ide#Subvolumes
https://btrfs.wiki.kernel.org/index....ltiple_Devices

I actually haven't read this one, but Arch usually has really good documentation:
https://wiki.archlinux.org/title/Btrfs#Subvolumes

... however, if there is enough disk space on one of the disks maybe you could make it work. Create subvolumes on just that disk, copy everything to it, and then add the remaining disks to that btrfs file system:
https://btrfs.wiki.kernel.org/index....ng_new_devices
But that's basically the "restore from backup" method, which I don't think is what you're after.

When dealing with multiple disks be aware of the problem with RAID on SMR drives:
https://raid.wiki.kernel.org/index.php/Timeout_Mismatch
This one bit me. I tried having RAID5 on top of my multiple disks, and had occasional corruption. Finally found the above article, reconfigured to btrfs sitting on top of the raw disks (no RAID, just btrfs utilizing all the combined disk space) and haven't had problems since.

I think your question is worthy of the mailing list:
https://btrfs.wiki.kernel.org/index....s_mailing_list

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Thanks for the hints and the links, drumz. More reading ahead.