filesystems/ext4/atomic_writes.rst

10 Atomic (untorn) block writes ensure that either the entire write is committed
17    EXT4 supports atomic write operations with a single filesystem block since
18    v6.13. In this the atomic write unit minimum and maximum sizes are both set
20    e.g. doing atomic write of 16KB with 16KB filesystem blocksize on 64KB
25    using a feature known as bigalloc. The atomic write unit's minimum and
27    based on the underlying device’s supported atomic write unit limits.
44 NOTE: EXT4 does not support software or COW based atomic write, which means
56 following constraints. The minimum atomic write size is the larger of the fs
57 block size and the minimum hardware atomic write unit; and the maximum atomic
58 write size is smaller of the bigalloc cluster size and the maximum hardware
59 atomic write unit.  Bigalloc ensures that all allocations are aligned to the
83 a system crash or unexpected power loss during the write operation, the affected
87 To enforce this guarantee, we ensure that the write target is backed by
90 completion path (typically in ->end_io()). If a write is allowed to proceed over
92 mid-write, the system could observe partially updated regions after reboot, i.e.
94 were never marked written. This violates the atomicity and/or torn write
103 actual write I/O. If the commit fails, the whole I/O must be aborted to prevent
105 Only after this step, the actual data write operation is performed by the iomap.
153 Creating Filesystems with Atomic Write Support
156 First check the atomic write units supported by block device.
188 The write must be aligned to the filesystem's block size and not exceed the
189 filesystem's maximum atomic write unit size.
195  * ``stx_atomic_write_unit_min``: Minimum size of an atomic write request.
196  * ``stx_atomic_write_unit_max``: Maximum size of an atomic write request.
198    separate memory buffers that can be gathered into a write operation
209 The underlying storage device must support atomic write operations.
213 * ``/sys/block/<device>/queue/atomic_write_unit_min`` - Minimum atomic write size
214 * ``/sys/block/<device>/queue/atomic_write_unit_max`` - Maximum atomic write size