[pbs-devel] [PATCH] fix: use fragmented block size for space calculation

[Gabriel Goller <g.goller@proxmox.com>]

We currently calculate the size of a datastore using `statfs64`, which
returns the number of blocks in the fs and the two block sizes:
fragemented block size(f_frsize) and block size (f_bsize). To calculate
eg the total space in a datastore we use total_blocks * f_bsize, which
is not always correct.

`f_frsize` is the minimum unit of allocation on the filesystem (in
bytes) and in 99% of the cases equal to `f_bsize`, but in some cases
it differs. For example some filesystems allow smaller blocks for small
files, in case f_frsize < f_bsize. In that case, f_frsize * total_blocks
returns (mostly) the correct result (ceph also did some weird stuff, which is
now being fixed though [0][1]). `statvfs` also documents this as the
recommended way ('fsblkcnt_t f_blocks;   /* Size of fs in f_frsize units */')[2].

This patch aligns the the behavior with the libc utilities (also used by
`df`) [3].

Motivation: [4] (Forum post)

[0]: https://tracker.ceph.com/issues/3793
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=92a49fb0f79f3300e6e50ddf56238e70678e4202
[2]: https://www.man7.org/linux/man-pages/man3/statvfs.3.html
[3]: https://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/fsusage.c#n147
[4]: https://forum.proxmox.com/threads/pbs-3-1-2-wrong-datastore-information-sshfs.139875/#post-626959

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
---
 proxmox-sys/src/fs/mod.rs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/proxmox-sys/src/fs/mod.rs b/proxmox-sys/src/fs/mod.rs
index 8fb677c..44c3da5 100644
--- a/proxmox-sys/src/fs/mod.rs
+++ b/proxmox-sys/src/fs/mod.rs
@@ -121,12 +121,16 @@ pub fn fs_info<P: ?Sized + nix::NixPath>(path: &P) -> nix::Result<FileSystemInfo
     let res = path.with_nix_path(|cstr| unsafe { libc::statfs64(cstr.as_ptr(), &mut stat) })?;
     nix::errno::Errno::result(res)?;
 
-    let bsize = stat.f_bsize as u64;
+    let block_size = if stat.f_frsize == 0 {
+        stat.f_bsize as u64
+    } else {
+        stat.f_frsize as u64
+    };
 
     Ok(FileSystemInformation {
-        total: stat.f_blocks * bsize,
-        used: (stat.f_blocks - stat.f_bfree) * bsize,
-        available: stat.f_bavail * bsize,
+        total: stat.f_blocks * block_size,
+        used: (stat.f_blocks - stat.f_bfree) * block_size,
+        available: stat.f_bavail * block_size,
         total_inodes: stat.f_files,
         free_inodes: stat.f_ffree,
         fs_type: stat.f_type,
-- 
2.43.0