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FS#61399 - [linux-lts] btrfs mount with 4.19 3x slower than 4.14

Attached to Project: Arch Linux
Opened by desbma (desbma) - Monday, 14 January 2019, 12:23 GMT
Task Type Bug Report
Category Packages: Core
Status Unconfirmed
Assigned To No-one
Architecture All
Severity Low
Priority Normal
Reported Version
Due in Version Undecided
Due Date Undecided
Percent Complete 0%
Votes 1
Private No


I have a /data BTRFS partition that is very slow to mount with kernel linux-lts-4.19.14-1: about ~18s.

The partition is mounted on boot by systemd, but it is reproducible by uounting and then mounting it again.

Reverting back to linux-lts-4.14.90-1 it mounts in ~6s (which I already consider slow).

The only mount options I have in fstab are: noatime,flushoncommit

The partition is of ~4TB and about 90% full. It is backed by a LUKS block device, but my other LUKS partitions with ext4 above mount in an instant.
dmesg does not show any warning or error.

I tried:
* running a balance with: sudo btrfs balance start -dusage=50 -musage=50 /data
* removing the flushoncommit mount option

None of that made the mount faster.

Any idea?
This task depends upon

Comment by desbma (desbma) - Sunday, 03 February 2019, 00:52 GMT
I managed to go back to 4.14 mount speed by disabling blk_mq with boot parameter 'scsi_mod.use_blk_mq=0'