[PATCH RFC 00/10] large atomic writes for xfs with CoW

John Garry
2025-02-04T12:02:09Z

Currently atomic write support for xfs is limited to writing a single
block as we have no way to guarantee alignment and that the write covers
a single extent.

This series introduces a method to issue atomic writes via a software
emulated method.

The software emulated method is used as a fallback for when attempting to
issue an atomic write over misaligned or multiple extents.

The basic idea of this CoW method is to alloc a range in the CoW fork,
write the data, and atomically update the mapping.

This is an RFC as I am not confident on the reflink changes at all. In
addition, I guess that the last change to provide a hint to the allocator
will not be liked.

Initial mysql performance testing has shown this method to perform ok.
However, there we are only using 16K atomic writes (and 4K block size),
so typically - and thankfully - this software fallback method won't be
used often.

For other FSes which want large atomics writes and don't support CoW, I
think that they can follow the example in [0].

Based on v6.14-rc1.

[0] https://lore.kernel.org/linux-xfs/20250102140411.14617-1-john.g.garry@oracle.com/

John Garry (10):
xfs: Switch atomic write size check in xfs_file_write_iter()
xfs: Refactor xfs_reflink_end_cow_extent()
iomap: Support CoW-based atomic writes
xfs: Make xfs_find_trim_cow_extent() public
xfs: Reflink CoW-based atomic write support
xfs: iomap CoW-based atomic write support
xfs: Add xfs_file_dio_write_atomic()
xfs: Commit CoW-based atomic writes atomically
xfs: Update atomic write max size
xfs: Allow block allocator to take an alignment hint

fs/iomap/direct-io.c | 23 ++++--
fs/xfs/libxfs/xfs_bmap.c | 7 +-
fs/xfs/libxfs/xfs_bmap.h | 6 +-
fs/xfs/xfs_file.c | 68 +++++++++++++++---
fs/xfs/xfs_iomap.c | 70 +++++++++++++++++-
fs/xfs/xfs_iops.c | 25 ++++++-
fs/xfs/xfs_iops.h | 3 +
fs/xfs/xfs_mount.c | 28 ++++++++
fs/xfs/xfs_mount.h | 1 +
fs/xfs/xfs_reflink.c | 149 +++++++++++++++++++++++++++++----------
fs/xfs/xfs_reflink.h | 7 +-
include/linux/iomap.h | 9 +++
12 files changed, 333 insertions(+), 63 deletions(-)

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2.31.1