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Andrea Mazzoleni 2026-08-25 22:27:53 +02:00
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@ -12,6 +12,78 @@ All done ?
=============
* Use statx() instead of lstat() when scanning directories.
Besides providing the normal stat information, statx() exposes metadata that
is useful to correctly identify filesystem, mount, and subvolume boundaries
that cannot be detected reliably with st_dev alone.
The scanner should identify the data root using three values:
```
st_dev
stx_mnt_id
stx_subvol
```
and avoid descending into a directory whenever any of these identifiers is
different from the corresponding identifier of the data root.
st_dev remains the primary and always available check. It detects different
filesystems and also already detects nested Btrfs subvolumes, because Btrfs
reports a different st_dev for different subvolumes.
Use STATX_MNT_ID/stx_mnt_id to additionally detect different mount instances,
including bind mounts of the same filesystem. Such mounts can have the same
st_dev and therefore cannot be detected by the current st_dev comparison
alone.
Use STATX_SUBVOL/stx_subvol to detect nested Bcachefs subvolumes. Bcachefs
subvolumes can share the same st_dev and mount ID while having different
subvolume IDs. This additional subvolume check is not required for Btrfs,
where st_dev already distinguishes nested subvolumes.
Do not make STATX_SUBVOL support depend only on the kernel headers available
at build time. A SnapRAID binary built on an older system should still be
able to use stx_subvol when later executed on a newer kernel/filesystem that
supports it. STATX_SUBVOL was added in Linux 6.10 by consuming previously
reserved space in struct statx, so its ABI can be handled compatibly even
when compiling against older headers.
Provide compatibility definitions for missing STATX_SUBVOL/stx_subvol ABI
declarations where necessary, ideally isolated in the portability layer
instead of exposing header-version conditionals to the scanner. If support
for build environments predating the libc statx() wrapper is required,
consider implementing the portability wrapper using the raw statx syscall so
that availability is determined by the runtime kernel rather than by the
libc version used to build SnapRAID.
Internally, missing optional identifiers may be represented as 0:
```
mnt_id = 0 if STATX_MNT_ID is unavailable
subvol_id = 0 if STATX_SUBVOL is unavailable
```
The comparison can then remain simple:
```
if (st_dev != root_st_dev ||
mnt_id != root_mnt_id ||
subvol_id != root_subvol_id)
/* do not descend */
```
This works because st_dev is always compared as well, while the optional
identifiers are expected to be available consistently for objects on the
same filesystem during a scan. On systems where an optional statx field is
unavailable, both the root and its descendants will normally receive 0 for
that identifier, naturally falling back to the existing st_dev-only
behavior.
Note that 0 is an internal SnapRAID value meaning "identifier unavailable";
it must not be interpreted as a kernel guarantee that mount ID 0 or
subvolume ID 0 can never be valid. The design relies only on unavailable
optional information comparing equal and therefore not introducing an
additional boundary check.
* Add a new 'restore' command, similar to 'fix', that recovers a whole disk
using the same multithreaded approach as sync/scrub.