lvm2 raid vs. btrfs raid (current status) - flexible raid?

[Zucca]

I'm not sure if this is the right category. If not, I hope a mod moves this to a right place.

So. To my knowledge lvm2 has built-in raid functionality.
Currently I have everything but /boot on one big btrfs storage pool. I use raid1 for the pool since raid6 (and raid5 with it) is buggy/broken. I used to have it on raid6, which was ideal for my 6xSSD setup.

Since the raid5/6 flaw was found from btrfs I've had few millisecond thoughts on switching to lvm2+xfs. But I've never actually found any info on how good the raid functionality on lvm2 is compared to btrfs.

Things I'm curious to know. Does lvm2 have...

• stable raid6 functionality?
• the flexibility to accept many different sized disks in raid mode and still manage to spread the data without leaving too much of unused space (btrfs does this pretty well)?
• other features btrfs has, like compression, deduplication..?

I need the flexibility, because I replace only 1 to 3 of the a the time. Usually 2 at the time. For example I still have two 128GB SSDs in the cage and propably my next space expansion will throw those two to another PCs and they get replaced with possibly 300-600GB ones. So my setup will have many different sized drives attached.

And finally:

• No. I don't want to mdraid+lvm2. I've been there. Not very flexible in terms of "just plug a new drive and balance" -flexible.
• Hardware solutions like drobo are nice, but most of them use propietary or non-standard raid levels, and in case of the hw failure... no no no and NO. Despite I already have six slotted HDD cage for my drives.
• This topic is about lvm2 and btrfs raid so... Let's keep in topic (mostly at least). Thank you?

I'm curious to know if any of you use lvm2 internal raid and how it is working for you.

[frostschutz]

this might be an interesting read: http://unix.stackexchange.com/a/182503/30851

[Zucca]

Looking pretty bad for lvm-raid. :\
Although that post is from about a year ago. I wonder how much is changed..?
One thing on that article made clear that it's fairly easy to have many logical volumes (partitions inside lvm) with different raid levels. With btrfs you must create a new btrfs filesystem (storage pool) for each different raid level you need. But what I've read, btrfs devs are going to implement raid levels per subvolume (that is inside the same btrfs pool).

I've also read that zfs would be fine, but it cannot use different sized drives too efficiently.
_________________
..: Zucca :..
Gentoo IRC channels reside on Libera.Chat.
--

[frostschutz]

Nothing much changed. Given a choice between lvm raid and mdadm raid, I'd always go for mdadm. It's flexible enough. You can combine raid levels, and different sized disks, if you are creative enough. RAID-1 for /boot partition across all disks, RAID-5 for the smallest common across all disks, RAID 1 for the two disks that are larger, no raid for the single disk that is the largest as a backup partition, or whatever.

And then LVM on top of that (or btrfs, if you prefer) and you no longer worry about RAID because it's in the background for everything.

Different RAID level per LV is a huge chaos... RAID is never about logical volumes. It's about physical volumes. When a physical disk fails you have to know what to replace and what that means for your RAIDs: you get this at a glance in /proc/mdstat. In LVM you tend to not even know which LV is stored where...

But you don't want it, so if it's not a server, and doesn't have to keep running on hardware failure, how about not using raid at all?

Backups are so much more important than any kind of RAID and it's not like you get any performance improvements either.

[Zucca]

I have backups.
The current btrfs setup on all disks just makes my life easier when I replace drives.
I just hope that the raid6 feature of btrfs matures to a stable level some day. But based on the development speed* in the past, I might need to wait for a long time.


*) I know it's slow for a reason.
_________________
..: Zucca :..
Gentoo IRC channels reside on Libera.Chat.
--

[takhisis_astrafall]

Hello,

I'm thinking about move from a hardware raid5 to mdam+lvm2

What do you mean by : "No. I don't want to mdraid+lvm2. I've been there. Not very flexible in terms of "just plug a new drive and balance" -flexible." ?

Regards

[Zucca]