Btrfs RAID 5/6 Sub-Page Support Readied For Linux 5.19

[zilexa]

Since there are a few BTRFS experts or even developers here, I have a question I could not find an answer to not even in the mailinglists:

Using BTRFS without raid (single device dup meta) and without quotas because I have no need for quotas on my homeserver:

How can I properly figure out disk space usage per subvolume and snapshots? What tool really works to give me a simple overview?

[waxhead]

Since there are a few BTRFS experts or even developers here, I have a question I could not find an answer to not even in the mailinglists:
Using BTRFS without raid (single device dup meta) and without quotas because I have no need for quotas on my homeserver:
How can I properly figure out disk space usage per subvolume and snapshots? What tool really works to give me a simple overview?

First of all I am no expert, but an eager BTRFS "evangelist" and user.

Since you are asking about a simple overview, quite honestly I believe the simplest estimate you will get is by using "du -h -s /path/to/subvolume/"
Other than that I am afraid I can't give you a good answer.

I believe (note: I don't know) that the extent tree v2 will help somewhat here, but when you think about it. Especially when you consider that subvolumes at some point the future may allow for different storage profiles in combinations with reflink, snapshots etc - finding out the real disk usage is rather sketchy.

Sorry for not being more helpful.

[zilexa]

Since you are asking about a simple overview, quite honestly I believe the simplest estimate you will get is by using "du -h -s /path/to/subvolume/"

No, that's why i asked.

Quoting someone else:

As a general rule, classic du is not reliable with BTRFS, and quite often can give seemingly nonsensical results like this. This ultimately arises from the fact that stat() (the system call that du uses to see how much space each file is using on-disk) doesn't know or care about reflinks, snapshots, transparent compression, or pretty much anything else that BTRFS provides beyond the standard POSIX semantics.

The impact of this is that du will count any blocks shared between files via reflinks (which includes any blocks that have been deduplicated, blocks shared because of files being copied using the clone ioctl, and any that are part of snapshots) once fore each file that holds a reference to that block, and therefore du can show apparent disk usage far greater than actual space usage.

All these discussions about RAID.. while for most people especially selfhosters/home users RAID doesn't make sense. Backups do.
BTRFS is a great alternative for ext4 for personal use, also as homeuser who doesn't want to spend twice the money on storage just for duplication (raid is not a backup).
Much more important is the lack of insight in space used. That's a much more basic requirement.

[waxhead]

No, that's why i asked.

Quoting someone else:

I am aware of that but for a simple home user with simple usage du is good enough in most cases. For the home user you are usually mostly interested in how much free space you have and how much space things consume if you are copying it somewhere else.

All these discussions about RAID.. while for most people especially selfhosters/home users RAID doesn't make sense. Backups do.
BTRFS is a great alternative for ext4 for personal use, also as homeuser who doesn't want to spend twice the money on storage just for duplication (raid is not a backup).
Much more important is the lack of insight in space used. That's a much more basic requirement.

BTRFS "RAID" is not regular raid and it does make sense also for the home user. Backups are almost worthless if you have no guarantee that your files are correct. Not many bother to run checksums on their files. I did and found several corrupted files over the years. For data that you value BTRFS with redundant storage profile is great and worth the extra cost of storage. Backups cost money to and I would not trust any backup that was not on btrfs or zfs. E.g. a filesystem that actually checks that your data is sane.

[reza]

Think of your storage device as a loaf of bread, and butter (-fs) as the data. BTRFS can spawn several loafs of bread (storage devices) of different sizes:

This is an awesome analogy and explanation. Thank you!