XFS supports atomic writes - or untorn writes - based on two different
methods:
- HW offload in the disk
- FS method based on out-of-place writes
The value reported in stx_atomic_write_unit_max will be the max size of the
FS-based method.
The max atomic write unit size of the FS-based atomic writes will
typically be much larger than what is capable from the HW offload. However,
FS-based atomic writes will also be typically much slower.
Advertise this HW offload size limit to the user in a new statx member,
stx_atomic_write_unit_max_opt.
We want STATX_WRITE_ATOMIC to get this new member in addition to the
already-existing members, so mention that a value of 0 means that
stx_atomic_write_unit_max holds this optimised limit.
Linux will zero unused statx members, so stx_atomic_write_unit_max_opt
will always hold 0 for older kernel versions which do not support
this FS-based atomic write method (for XFS).
Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
Differences to RFC (v1):
- general rewrite
- mention that linux zeroes unused statx fields
diff --git a/man/man2/statx.2 b/man/man2/statx.2
index ef7dbbcf9..29400d055 100644
--- a/man/man2/statx.2
+++ b/man/man2/statx.2
@@ -74,6 +74,9 @@ struct statx {
\&
/* File offset alignment for direct I/O reads */
__u32 stx_dio_read_offset_align;
+\&
+ /* Direct I/O atomic write max opt limit */
+ __u32 stx_atomic_write_unit_max_opt;
};
.EE
.in
@@ -266,7 +269,8 @@ STATX_SUBVOL Want stx_subvol
(since Linux 6.10; support varies by filesystem)
STATX_WRITE_ATOMIC Want stx_atomic_write_unit_min,
stx_atomic_write_unit_max,
- and stx_atomic_write_segments_max.
+ stx_atomic_write_segments_max,
+ and stx_atomic_write_unit_max_opt.
(since Linux 6.11; support varies by filesystem)
STATX_DIO_READ_ALIGN Want stx_dio_read_offset_align.
(since Linux 6.14; support varies by filesystem)
@@ -514,6 +518,20 @@ is supported on block devices since Linux 6.11.
The support on regular files varies by filesystem;
it is supported by xfs and ext4 since Linux 6.13.
.TP
+.I stx_atomic_write_unit_max_opt
+The maximum size (in bytes) which is optimised for writes issued with
+torn-write protection.
+If non-zero, this value will not exceed the value in
+.I stx_atomic_write_unit_max
+and will not be less than the value in
+.I stx_atomic_write_unit_min.
+A value of zero indicates that
+.I stx_atomic_write_unit_max
+is the optimised limit.
+Slower writes may be experienced when the size of the write exceeds
+.I stx_atomic_write_unit_max_opt
+(when non-zero).
+.TP
.I stx_atomic_write_segments_max
The maximum number of elements in an array of vectors
for a write with torn-write protection enabled.
--
2.31.1
