Hello all, I'm having some trouble with btrfs on a laptop, possibly due to qgroups. Specifically, some file system activities (e.g., snapshot creation, baloo_file_extractor from KDE Plasma) cause the system to hang for up to about 40 minutes, maybe more. It always causes (most of) my desktop to hang, (although I can usually navigate between pre-existing Konsole tabs) and prevents new programs from starting. I've seen the system load go up to >30 before the laptop suddenly resumes normal operation. I've been seeing this since Linux 4.7, maybe already 4.6.
Now, I thought that maybe this was (indirectly) due to an overly full file system (~90% full), so I deleted some things I didn't need to get it up to 15% free. (For the record, I also tried mounting with ssd_spread.) After that, I ran a balance with -dusage=50, which started out promising, but then went back to the "bad" behaviour. *But* it seemed better than before overall, so I started a balance with -musage=10, then -musage=50. That turned out to be a mistake. Since I had to transport the laptop, and couldn't wait for "balance cancel" to return (IIUC it only returns after the next block (group?) is freed), I forced the laptop off. After I next turned on the laptop, the balance resumed, causing bootup to fail, after which I remembered about the skip_balance mount option, which I tried in a rescue shell from an initramfs. But wait, that failed, too! Specifically, the stack trace I get whenever I try it includes as one of the last lines: "RIP [<ffffffff8131226f>] qgroup_fix_relocated_data_extents+0x1f/0x2a8" (I can take photos of the full stack trace if requested.) So then I ran "btrfs qgroup show /sysroot/", which showed many quota groups, much to my surprise. On the upside, at least now I discovered the likely reason for the performance problems. (I actually think I know why I'm seeing qgroups: at one point I was trying out various snapshot/backup tools for btrfs, and one (I forgot which) unconditionally activated quota support, which infuriated me, but I promptly deactivated it, or so I thought. Is quota support automatically enabled when qgroups are discovered, or did I perhaps not disable quota support properly?) Since I couldn't use skip_balance, and logically can't destroy qgroups on a read-only file system, I decided to wait for a regular mount to finish. That has been running since Tuesday, and I am slowly growing impatient. Thus I arrive at my question(s): is there anything else I can try, short of reformatting and restoring from backup? Can I use btrfs-check here, or any other tool? Or...? Also, should I be able to avoid reformatting: how do I properly disable quota support? (BTW, searching for qgroup_fix_relocated_data_extents turned up the ML thread "[PATCH] Btrfs: fix endless loop in balancing block groups", could that be related?) The laptop is currently running Gentoo with Linux 4.8.10 and btrfs-progs 4.8.4. Greetings -- Marc Joliet -- "People who think they know everything really annoy those of us who know we don't" - Bjarne Stroustrup
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