Re: [PATCH] btrfs: don't force DIO writes to be serialized

[David Sterba <dsterba@suse.cz>]

On Thu, Apr 23, 2026 at 11:04:37AM +0100, Mark Harmstone wrote:
> On 22/04/2026 9.57 pm, David Sterba wrote:
> > On Wed, Apr 22, 2026 at 03:03:35PM +0100, Mark Harmstone wrote:
> >> Before btrfs switched to the new mount API in 2023, we were setting
> >> SB_NOSEC in btrfs_mount_root(). This flag tells the VFS that the
> >> filesystem may have files which don't have security xattrs, enabling it
> >> to do some optimizations.
> >>
> >> Unfortunately this was missed in the transition, meaning that IS_NOSEC
> >> will always return false for a btrfs inode. This means that
> >> btrfs_direct_write() calls will always get the inode lock exclusively,
> >> meaning that DIO writes to the same file will be serialized.
> >>
> >> On my machine, this one-line change results in a ~59% improvement in DIO
> >> throughput:
> > 
> > That's quite an improvement. What's the actual fio script you've used?
> > Also the DIO depends on the block group profile wrt the buffered
> > fallback so that would be good to know too.
> 
> It is. There's a big dropoff in DIO write performance in 6.8 that we 
> never recovered from. I'm going to look into some sort of automated 
> performance so this kind of thing can't happen casually.

As you've used fio for that, please add the test case to the
fstests/tests/perf. Though the 'pref' tests haven't caught up I'd like
to at least collect them in our btrfs/fstests repository.

> This was on a VM with 8 cores and 8GB of RAM, with a real NVMe exposed 
> through PCI passthrough. The figures for XFS and ext4 in comparison are 
> both about ~3GB/s.
> 
> # cat go
> #!/bin/bash
> mkfs.btrfs -f /dev/nvme0n1
> mount /dev/nvme0n1 /mnt/test
> mkdir /mnt/test/nocow
> chattr +C /mnt/test/nocow
> fio /root/test.fio
> 
> # cat /root/test.fio
> [global]
> rw=randwrite
> ioengine=io_uring
> iodepth=64
> size=1g
> direct=1
> startdelay=20
> force_async=4
> ramp_time=5
> runtime=60
> group_reporting=1
> numjobs=32
> time_based
> disk_util=0
> clat_percentiles=0
> disable_lat=1
> disable_clat=1
> disable_slat=1
> filename=/mnt/test/nocow/fiofile
> [test]
> name=test
> bs=4k
> stonewall

Thanks.