Struct glommio::io::DmaFile

pub struct DmaFile { /* fields omitted */ }

An asynchronously accessed Direct Memory Access (DMA) file.

All access uses Direct I/O, and all operations including open and close are asynchronous (with some exceptions noted). Reads from and writes to this struct must come and go through the DmaBuffer type, which will buffer them in memory; on calling write_at and read_at, the buffers will be passed to the OS to asynchronously write directly to the file on disk, bypassing page caches.

See the module-level documentation for more details and examples.

Implementations

impl DmaFile

pub fn align_up(&self, v: u64) -> u64

align a value up to the minimum alignment needed to access this file

pub fn align_down(&self, v: u64) -> u64

align a value down to the minimum alignment needed to access this file

impl DmaFile

pub fn is_same(&self, other: &DmaFile) -> bool

Returns true if the DmaFiles represent the same file on the underlying device.

Files are considered to be the same if they live in the same file system and have the same Linux inode. Note that based on this rule a symlink is not considered to be the same file.

Files will be considered to be the same if:

• A file is opened multiple times (different file descriptors, but same file!)
• they are hard links.

Examples

use glommio::{io::DmaFile, LocalExecutor};
use std::os::unix::io::AsRawFd;

let ex = LocalExecutor::default();
ex.run(async {
    let mut wfile = DmaFile::create("myfile.txt").await.unwrap();
    let mut rfile = DmaFile::open("myfile.txt").await.unwrap();
    // Different objects (OS file descriptors), so they will be different...
    assert_ne!(wfile.as_raw_fd(), rfile.as_raw_fd());
    // However they represent the same object.
    assert!(wfile.is_same(&rfile));
    wfile.close().await;
    rfile.close().await;
});

pub fn alloc_dma_buffer(&self, size: usize) -> DmaBuffer

Allocates a buffer that is suitable for using to write to this file.

pub async fn create<P: AsRef<Path>>(path: P) -> Result<DmaFile, ()>

Similar to create() in the standard library, but returns a DMA file

pub async fn open<P: AsRef<Path>>(path: P) -> Result<DmaFile, ()>

Similar to open() in the standard library, but returns a DMA file

pub async fn write_at(&self, buf: DmaBuffer, pos: u64) -> Result<usize, ()>

Write the buffer in buf to a specific position in the file.

It is expected that the buffer and the position be properly aligned for Direct I/O. In most platforms that means 4096 bytes. There is no write_at_aligned, since a non aligned write would require a read-modify-write.

Buffers should be allocated through alloc_dma_buffer, which guarantees proper alignment, but alignment on position is still up to the user.

This method acquires ownership of the buffer so the buffer can be kept alive while the kernel has it.

Note that it is legal to return fewer bytes than the buffer size. That is the situation, for example, when the device runs out of space (See the man page for write(2) for details)

Examples

use glommio::{io::DmaFile, LocalExecutor};

let ex = LocalExecutor::default();
ex.run(async {
    let file = DmaFile::create("test.txt").await.unwrap();

    let mut buf = file.alloc_dma_buffer(4096);
    let res = file.write_at(buf, 0).await.unwrap();
    assert!(res <= 4096);
    file.close().await.unwrap();
});

pub async fn read_at_aligned(
    &self,
    pos: u64,
    size: usize
) -> Result<ReadResult, ()>

Reads from a specific position in the file and returns the buffer.

The position must be aligned to for Direct I/O. In most platforms that means 512 bytes.

pub async fn read_at(&self, pos: u64, size: usize) -> Result<ReadResult, ()>

Reads into buffer in buf from a specific position in the file.

It is not necessary to respect the O_DIRECT alignment of the file, and this API will internally convert the positions and sizes to match, at a cost.

If you can guarantee proper alignment, prefer read_at_aligned instead

pub async fn fdatasync(&self) -> Result<(), ()>

Issues fdatasync for the underlying file, instructing the OS to flush all writes to the device, providing durability even if the system crashes or is rebooted.

As this is a DMA file, the OS will not be caching this file; however, there may be caches on the drive itself.

pub async fn pre_allocate(&self, size: u64) -> Result<(), ()>

pre-allocates space in the filesystem to hold a file at least as big as the size argument.

pub async fn hint_extent_size(&self, size: usize) -> Result<i32>

Hint to the OS the size of increase of this file, to allow more efficient allocation of blocks.

Allocating blocks at the filesystem level turns asynchronous writes into threaded synchronous writes, as we need to first find the blocks to host the file.

If the extent is larger, that means many blocks are allocated at a time. For instance, if the extent size is 1MB, that means that only 1 out of 4 256kB writes will be turned synchronous. Combined with diligent use of fallocate we can greatly minimize context switches.

It is important not to set the extent size too big. Writes can fail otherwise if the extent can’t be allocated.

pub async fn truncate(&self, size: u64) -> Result<(), ()>

Truncates a file to the specified size.

Warning: synchronous operation, will block the reactor

pub async fn rename<P: AsRef<Path>>(&mut self, new_path: P) -> Result<(), ()>

rename this file.

Warning: synchronous operation, will block the reactor

pub async fn remove(&self) -> Result<(), ()>

remove this file.

The file does not have to be closed to be removed. Removing removes the name from the filesystem but the file will still be accessible for as long as it is open.

Warning: synchronous operation, will block the reactor

pub async fn file_size(&self) -> Result<u64, ()>

Returns the size of a file, in bytes

pub async fn close(self) -> Result<(), ()>

Closes this DMA file.

pub fn path(&self) -> Option<&Path>

Returns an Option containing the path associated with this open directory, or None if there isn’t one.

Trait Implementations

impl AsRawFd for DmaFile

fn as_raw_fd(&self) -> RawFd

Extracts the raw file descriptor.

impl Debug for DmaFile

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

Formats the value using the given formatter.