Horrible problems while mounting BTRFS RAID1 array (on a single HDD)

Question

Since I made my 2TB HDD (Samsung M9T which I bought precisely to make it a BTRFS RAID1 array for data security) into 2 x 1TB btrfs RAID1 array, I keep having horrible mounting problems.
There are two subvolumes on that array:
@data
@sys_backup
fstab entries for them look like so:
UUID=a0555768-a37f-4bd8-95e5-66ebd6a09c75 /mnt/Disk_D btrfs device=/dev/mapper/2TB-1TB_RAID1a,device=/dev/mapper/2TB-1TB_RAID1b,rw,noatime,compress=lzo,autodefrag,space_cache,nofail,commit=180,subvol=@data 0 0
UUID=a0555768-a37f-4bd8-95e5-66ebd6a09c75 /mnt/arch_backup btrfs device=/dev/mapper/2TB-1TB_RAID1a,device=/dev/mapper/2TB-1TB_RAID1b,rw,noatime,compress=zlib,autodefrag,space_cache,nofail,x-systemd.automount,commit=250,subvol=@sys_backup 0 0
Now, I rarely access @sys_backup, but I have never found it not being mounted. @data on the other hand mounts hardly ever.
Most of the time, after boot, then I try to access /mnt/Disk_D mountpoint, it either
A - turns out empty, and running “mount” command confirms there's nothing mounted there. Trying to mount it manually renders absolutely no effect. Not even an error message.
or
B – every program used to access that mountpoint (terminal, file manager, conky displaying free space or anything else) freezes and I have to kill it. Even tryping “/mnt/D” and pressing TAB for autocompletion crashes the terminal.
I then need to reboot anywhere from 1 to 15 more times to get it mount properly.
I had this problem since I started using RAID1 3 months ago. I had it on kernels 3.17, 3.18 and 3.19.
Other things that made no difference include:
➤ adding/removing “device=” option
➤ adding/removing x-systemd.automount option
➤ changing UUIDs o /dev/mapper/... entries
➤ completing scrub on that partition
➤ running dry run btrfs check (no errors indicated)
This is horrible. Is there any solution to it, or is it just the current state of BTRFS? And if it is the later, then what other than btrfs Raid1 solution could be implemented?
Last edited by Lockheed (2015-02-01 07:27:35)

Answer 1

firecat53 wrote:There's nothing in that linked article about doing a RAID 1 on a single disk.
The whole discussion I linked is about doing a RAID 1 on a single disk. And the article linked from that discussion is only to prove the point of btrfs self-healing.
firecat53 wrote:If you think about it, you are going to absolutely kill your read/write times by having the drive head trying to keep up a RAID 1 mirror on two partitions of the same drive.
The read performance is unchanged. The write performance is 50% of the standard partition, but since it is a data drive, that is not a serious issue. Certainly less serious than data security.
firecat53 wrote:My guess is that's why you are having issues.
Maybe so, but I don't see a reason to assume it and even it is so, then I don't think those issues should be there.
firecat53 wrote:You're going to be much better off buying two 1TB drives and using those in RAID 1!!  Or just a single 2TB btrfs drive....with a UPS and another drive as a backup drive.
I can't do that because it is a laptop SSD with system + HDD with data.
firecat53 wrote:I've been using btrfs on my laptop SSD and on my server for almost a year now with only one problem (corruption from a power loss to the server before I got the UPS).
I have been using btrfs on my laptop SSD + HDD and on my RPi server since over a year and btrfs check was detecting minor issues every few months.
I now run SSD (non-raid) + HDD (2 partitions in RAID1) on laptop, and SD (2 partitions in RAID1) + HDD (2 partitions in RAID1) on RPi (which - by the way, has no such problems with mounting neither SD nor HDD in Raid1) and I have not yet had any error with btrfs check.