After @daftaupe convinced me to give #btrfs a try, I just learned how to mount multiple #subvolume|s of a #LUKS #encrypted devices through #pam_mount:
@daftaupe @martin
Seems that things change, but according to the FAQ, there are still some issues with this matter (know how much Space left and use it)
https://btrfs.wiki.kernel.org/index.php/FAQ#Why_is_free_space_so_complicated.3F
Il still think that mixing réplication (raid) and file system is a very bad idea.
@martin @hoper There's nothing magical :) it's basically having 2 btrfs partitionned drives, making a snapshot of a subvolume and sending that read-only snapshot from one to the other disk. That prevent us from the use a of a tool like rsync as there's an option to send only the increment when you already have subvolumes on the other drive. Based on : https://btrfs.wiki.kernel.org/index.php/Incremental_Backup
@martin
In m'y mind, a snapshot is not a clone. It's just a sort of "diff" of two devices. Something that you can't separate from it's source.
So I dont understand how you can have a filesystem somewhere, and a snapshot of it elsewhere that can be usefull. A copy, yes. Easy. But a snapshot ? Guess there is plenty of things I dont understand about btrfs.
Maybe the first reason for me to not like it :o)
@martin @daftaupe
A "systemd like solution" LOL. Exactly ! As long at it's work as intended, perfect thing. But when you want to change, add or remove a thing you can't because it's far too complex tu use and really understand.
Mdadm and lvm2, both are doing a only thing but they did it well. Can't make differents raid level per file, yes. But at raid level you can do anything (reshaping, and so on)
@fink @daftaupe @martin
This break the "layer" model. i have 3 layer : raid, volume management, and fs (Can Swap the two first). If one of them is upgraded or replace by another, no problem. This also respect the unix philosophy : do a single thing, but do it well. Computing had always used layer model (network...). Because it's good and reliable. And for me, keeping my data safe is the most important thing.
@hoper @fink @martin Talking about data safety I wonder why Synology is recommending btrfs (they probably want to take the risk of putting their customers data in danger), https://www.synology.com/en-us/dsm/Btrfs. I think you might be interested about the paragraph related to data corruption. By the way which fs are you using that provides the same features ?
@daftaupe @fink @martin
Just checked this link, thanks.
Metadata miroring is a FS feature. For exemple ext4 make many copy of the superbloc. Guess that one day ext(5?) will offer more. In fact, I don't feel the need to have multiple copy of metadata because :
hardware problems (badblock) can't be an issue anymore with raidX
Logical problem (silent corruption) is also an issue already solved by mdadm with raid6. (callded "self healing" on synology link)
For last feature, snapshots >LVM.
By the way, talking about links, what about his one ?
https://btrfs.wiki.kernel.org/index.php/RAID56
"The parity RAID code has multiple serious data-loss bugs in it. It should not be used for anything other than testing purposes."
Or this one :
https://www.phoronix.com/scan.php?page=news_item&px=Btrfs-For-Linux-4.16
Saying that very serious bug about raid have been corrected in linux 4.16. (Sorry, I'm still using a 4.9 kernel, and I think this is not so old...)
It's good to see people making news things, and other in a hurry to test it...
Me ? I will wait :)
@fink @daftaupe @martin
So yes, btrfs have more functionality. Some of them can be really great for OS drive (said it before). Allow rollback after an upgrade, snapshot per file and so on. But for the data... I don't need more functionality, I need something 100% stable, repairable on worst scénario, and wich make me feel secure. Because I can tell exactly how my files are stored, exactly where on the drives etc.
@fink @daftaupe @martin
Last thing about the layer model. Sometimes you don't need all layers. For example you don't always need a file system. Think about swap, or about oracle or other database software that will work better if you just give them raw devices, because they have their own integrated "file system". Can you make a "raw volume" (without any fs or metadata or whatever on it) with btrfs ?
@martin @daftaupe
Tried btrfs à long time ago. Snapshots Can be great for OS Drive. But for data storage nothing Can Beat mdadm+lvm2. Big problem with btrfs : you can't know how much space is still available. (At the Time i checked it at least.)