Struct glommio::io::DmaFile

pub struct DmaFile { /* private fields */ }

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 DmaFile::write_at and DmaFile::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 nonaligned 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)

Warning: If the file has been opened with `append`, then the position will get ignored and the buffer will be written at the current end of file. See the [man page] for `O_APPEND`

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 write_rc_at(&self, buf: Rc<DmaBuffer>, pos: u64) -> Result<usize, ()>

Equivalent to DmaFile::write_at except that the caller retains non-mutable ownership of the underlying buffer. This can be useful if you want to asynchronously process a page concurrently with writing it.

Examples

use futures::join;
use glommio::{
    io::{DmaBuffer, DmaFile},
    timer::sleep,
    LocalExecutor,
};
use std::rc::Rc;

fn populate(buf: &mut DmaBuffer) {
    buf.as_bytes_mut()[0..5].copy_from_slice(b"hello");
}

async fn transform(buf: &[u8]) -> Vec<u8> {
    // Dummy implementation that just returns a copy of what was written.
    sleep(std::time::Duration::from_millis(100)).await;
    buf.iter()
        .map(|a| if *a == 0 { 0 } else { *a + 1 })
        .collect()
}

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

    let mut buf = file.alloc_dma_buffer(4096);
    // Write some data into the buffer.
    populate(&mut buf);

    // Seal the buffer by moving ownership to a non-threaded reference
    // counter on the heap.
    let buf = Rc::new(buf);

    let (written, transformed) = join!(
        async { file.write_rc_at(buf.clone(), 0).await.unwrap() },
        transform(buf.as_bytes())
    );
    assert_eq!(written, 4096);
    file.close().await.unwrap();

    // transformed AND buf can still be used even though the buffer got
    // written. Note that there may be performance issues if buf is large
    // and you remain hanging onto it.
});

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 Self::read_at_aligned instead

pub fn read_many<V, S>(
    self: &Rc<DmaFile>,
    iovs: S,
    buffer_limit: MergedBufferLimit,
    read_amp_limit: ReadAmplificationLimit
) -> ReadManyResult<V, impl Stream<Item = (ScheduledSource, ReadManyArgs<V>)>>where
    V: IoVec + Unpin,
    S: Stream<Item = V> + Unpin,

Submit many reads and process the results in a stream-like fashion via a ReadManyResult.

This API will optimistically coalesce and deduplicate IO requests such that two overlapping or adjacent reads will result in a single IO request. This is transparent for the consumer, you will still receive individual ReadResults corresponding to what you asked for.

The first argument is a stream of IoVec. The last two arguments control how aggressive the IO coalescing should be:

• buffer_limit controls how large a merged IO request can get;
• read_amp_limit controls how much read amplification is acceptable.

It is not necessary to respect the O_DIRECT alignment of the file, and this API will internally align the reads appropriately.

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 deallocate(&self, offset: u64, size: u64) -> Result<(), ()>

Erases a range from the file without changing the size. Check the man page for fallocate for a list of the supported filesystems. Partial blocks are zeroed while whole blocks are simply unmapped from the file. The reported file size (file_size) is unchanged but the allocated file size may if you’ve erased whole filesystem blocks (allocated_file_size)

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

pre-allocates space in the filesystem to hold a file at least as big as the size argument. No existing data in the range [0, size) is modified. If keep_size is false, then anything in [current file length, size) will report zeroed blocks until overwritten and the file size reported will be size. If keep_size is true then the existing file size is unchanged.

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 1 MiB, that means that only 1 out of 4 256 KiB 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.

Note: this syscall might be issued in a background thread depending on the system’s capabilities.

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

Rename this file.

Note: this syscall might be issued in a background thread depending on the system’s capabilities.

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.

Note: this syscall might be issued in a background thread depending on the system’s capabilities.

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

Returns the size of a file, in bytes

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

Returns the size of the filesystem cluster, in bytes

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

Closes this DMA file.

pub fn path(&self) -> Option<Ref<'_, Path>>

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

pub async fn close_rc(self: Rc<DmaFile>) -> Result<CloseResult, ()>

Convenience method that closes a DmaFile wrapped inside an Rc.

Returns CloseResult to indicate which operation was performed.

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.