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

[Mark Harmstone <mark@harmstone.com>]

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.

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