thebestvibedev/tromcho.net
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Disable use_atomic_flag by default

Browse Source
This commit is contained in:
Vitaliy Filippov
2025-12-22 01:52:07 +03:00
parent 21cf5c8815
commit 8ec7faa675
6 changed files with 117 additions and 44 deletions
Download Patch File Download Diff File Expand all files Collapse all files
docs/config/osd.en.md
+16 -9
View File
@@ -682,7 +682,10 @@ with replicated pools and reach the best possible write performance.
Default value is auto-detected during OSD initialization from
`/sys/block/xx/queue/atomic_write_max_bytes` or assumed to be 4096 bytes
because all known disks support 4 KB atomic writes.
because all known disks support 4 KB atomic writes. Auto-detection is only used for
NVMe disks because SAS disks require the explicit WRITE ATOMIC command which requires
RWF_ATOMIC (see below [#use_atomic_flag]) but that flag works incorrectly in current
Linux versions.
You can also check if your NVMe drives support atomic writes by running
the command `nvme id-ctrl /dev/nvme0n1 | grep awupf`. If the reported value,
@@ -697,12 +700,16 @@ reducing Write Amplification and improving write performance up to 2 times.
- Type: boolean
This option controls whether the Vitastor OSD uses RWF_ATOMIC write flag with atomic
writes. This flag is only supported on Linux kernel since 6.11. Atomic writes are
generally only safe to use with this flag because it tells the kernel to never fragment
write requests and also to check the write against the actual atomic write capabilities
of the device.
This option controls whether Vitastor OSDs use RWF_ATOMIC write flag with atomic writes.
This flag is supported since Linux 6.11 and adds some safety to atomic writes - the kernel
guarantees to not fragment write requests with it and also to check them against the actual
device atomic write capabilities.
This option is enabled by default when atomic_write_size is set to a value larger than 4 KB.
You can disable it if you're sure that your disks support atomic writes and you want to
bypass the Linux atomic write checks.
However, the option is disabled by default because the flag is currently UNUSABLE - Linux
incorrectly requires writes with that flag to be of power-of-2 length and length-aligned.
I.e., for example, 12 KB writes and not-8-KB aligned 8 KB writes are forbidden by the kernel,
even though the NVMe specification allows them.
For NVMe disks with `scheduler=none` writes aren't fragmented anyway so it's not a big deal.
However, you can rebuild your kernel with [this patch](../../patches/linux-fix-atomic-write-checks.diff)
and turn this option on. It will make your atomic writes a bit safer.