Struct AsyncFileBackend 

pub struct AsyncFileBackend { /* private fields */ }

Direct-I/O file-backed BlockBackend for the high-IOPS path.

Differs from SyncFileBackend structurally: lockless positioned I/O. Uses pread(2) / pwrite(2) via std::os::unix::fs::FileExt::{read_at, write_at}, so multiple threads can issue concurrent operations against the same fd without serialising through a Mutex<File>. The 100 K IOPS budget in 71 § 3 needs the lockless path — a mutex-serialised engine caps at the latency of one pread per op (≈ 30 μs ⇒ 33 K IOPS ceiling on a single thread).

F_NOCACHE is not set here because squib-virtio carries #![forbid(unsafe_code)] (I-CRATE-2): the fcntl(F_NOCACHE) call lives in squib-host::block_io::set_f_nocache, which can wrap the fd before it’s handed to this constructor (the operator-supplied path is opened once at boot, so the small upfront cost is fine). The host-side cache pressure without F_NOCACHE is small for the Lambda workload profile; the perf-tuning lane benchmark (71 § 3) gates the regression once the bench harness lights up.

Functionally interchangeable with SyncFileBackend; constructed via Self::open and dropped in via the same Arc<dyn BlockBackend> the device frontend takes today.

Implementations

impl AsyncFileBackend

pub fn open(path: &Path, read_only: bool) -> Result<Self>

Open path for lockless positioned I/O.

The fd is shared across every thread that calls into BlockBackend; concurrent positioned I/O is safe via pread/pwrite.

Errors

std::io::Error if the file cannot be opened.

pub fn from_file(file: File, read_only: bool) -> Result<Self>

Wrap an already-open File. Used by squib-host after applying F_NOCACHE via the unsafe fcntl boundary: the operator constructs the fd, hands it to us, and we own the read/write side.

Errors

std::io::Error if the file’s metadata can’t be queried.

Trait Implementations

impl BlockBackend for AsyncFileBackend

fn size_bytes(&self) -> u64

Total size of the backing storage in bytes.

fn read_at(&self, byte_offset: u64, buf: &mut [u8]) -> Result<()>

Read buf.len() bytes starting at byte_offset.

fn write_at(&self, byte_offset: u64, buf: &[u8]) -> Result<()>

Write buf.len() bytes starting at byte_offset.

fn flush(&self) -> Result<()>

Synchronous flush (fsync).

fn read_only(&self) -> bool

true if the backing storage is opened read-only.

impl Debug for AsyncFileBackend

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.