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block: Assume sparse support for block devices

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There is no non destructive readonly ioctl to query block device
discard or write zeroes capabilities. BLKZEROOUT is guaranteed to
succeed via kernel software fallback. BLKDISCARD may fail at runtime
with EOPNOTSUPP on devices that lack trim support, but the error
propagates to the guest as VIRTIO_BLK_S_IOERR and well behaved
guests handle it gracefully.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
This commit is contained in:
Anatol Belski
2026-02-24 22:13:46 +01:00
committed by Rob Bradford
parent e067c76880
commit 6404d2d513
1 changed files with 13 additions and 28 deletions
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41
block/src/lib.rs
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@@ -846,34 +846,19 @@ fn probe_file_sparse_support(fd: libc::c_int) -> bool {
supported
}
/// Probe sparse support for a block device using ioctls.
fn probe_block_device_sparse_support(fd: libc::c_int) -> bool {
ioctl_io_nr!(BLKDISCARD, 0x12, 119);
ioctl_io_nr!(BLKZEROOUT, 0x12, 127);
let range: [u64; 2] = [0, 0];
// SAFETY: FFI call with valid fd and valid range buffer
let punch_hole = unsafe { ioctl(fd, BLKDISCARD() as _, &range) } == 0;
if !punch_hole {
let err = io::Error::last_os_error();
debug!("Block device BLKDISCARD probe returned: {err}");
}
// SAFETY: FFI call with valid fd and valid range buffer
let zero_range = unsafe { ioctl(fd, BLKZEROOUT() as _, &range) } == 0;
if !zero_range {
let err = io::Error::last_os_error();
debug!("Block device BLKZEROOUT probe returned: {err}");
}
let supported = punch_hole || zero_range;
info!(
"Probed block device sparse support: punch_hole={punch_hole}, zero_range={zero_range} => {supported}"
);
supported
/// Probe sparse support for a block device.
///
/// Block devices always report sparse support. `BLKZEROOUT` is guaranteed to
/// succeed as the kernel provides a software fallback writing explicit zeros
/// when the hardware lacks a native write zeroes command. `BLKDISCARD` may fail
/// at runtime with `EOPNOTSUPP` on devices without trim or discard support, but
/// Linux guests handle this gracefully by ceasing discard requests.
///
/// There is no non destructive read only ioctl to query block device discard
/// or write zeroes capabilities.
fn probe_block_device_sparse_support(_fd: libc::c_int) -> bool {
info!("Block device: assuming sparse support");
true
}
/// Preallocate disk space for a disk image file.