Module glommio::io

glommio::io provides data structures targeted towards File I/O.

File I/O in Glommio comes in two kinds: Buffered and Direct I/O.

Ideally an application would pick one of them according to its needs and not mix both. However if you do want to mix both, it is recommended that you do not do so in the same device: Kernel settings like I/O schedulers and merge settings that are beneficial to one of them can be detrimental to the others.

If you absolutely must use both in the same device, avoid issuing both Direct and Buffered I/O in the same file: at this point you are just trying to drive Linux crazy.

Buffered I/O

Buffered I/O will use the operating system page cache. It is ideal for simpler applications that don't want to deal with caching policies and have I/O performance as a maybe important, but definitely not crucial part of their performance story.

Disadvantages of Buffered I/O:

• Hard to know when resources are really use, which make controlled processes almost impossible (the time of write to device is detached from the file write time)
• More copies than necessary, as the data has to be copied from the device to the page cache, from the page cache to the internal file buffers, and in abstract linear implementations like AsyncWriteExt and AsyncReadExt from user-provided buffers to the file internal buffers.
• Advanced features for io_uring like Non-interrupt mode, registered files, registered buffers, will not work with Buffered I/O
• Read amplification for small random reads, as the OS is bounded by the page size (usually 4kB), even though modern NVMe devices are perfectly capable of issuing 512-byte I/O.

The main structure to deal with Buffered I/O is

BufferedFile is targeted at random Direct I/O. Reads from and writes to it expect a position.

Direct I/O

Direct I/O will not use the Operating System page cache and will always touch the device directly. That will always work very well for stream-based workloads (scanning a file much larger than memory, writing a buffer that will not be read from in the near future, etc) but will require a user-provided cache for good random performance.

There are advantages to using a user-provided cache: Files usually contain serialized objects and every read have to deserialize them. A user-provided cache can cache the parsed objects, among others. Still, not all applications can or want to deal with that complexity.

Disadvantages of Direct I/O:

• I/O needs to be aligned. Both the buffers and the file positions need specific alignments. The DmaBuffer should hide most of that complexity, but you may still end up with heavy read amplification if you are not careful.
• Without a user-provided cache, random performance can be bad.

There are two main structs that deal with File Direct I/O:

DmaFile is targeted at random Direct I/O. Reads from and writes to it expect a position.

DmaStreamWriter and DmaStreamReader perform sequential I/O and their interface is a lot closer to other mainstream rust interfaces in std::fs.

However, despite being sequential, I/O for the two Stream structs are parallel: DmaStreamWriter exposes a setting for write-behind, meaning that it will keep accepting writes to its internal buffers even with older writes are still in-flight. In turn, DmaStreamReader exposes a setting for read-ahead meaning it will initiate I/O for positions you will read into the future sooner.

Structs

BufferedFile	Constructs a file that is backed by the operating system page cache
Directory	A directory representation where asynchronous operations can be issued
DmaFile	Constructs a file that can issue DMA operations. All access uses Direct I/O, and all operations including open and close are asynchronous.
DmaStreamReader	Provides linear access to a DmaFile. The DmaFile is a convenient way to manage a file through Direct I/O, but its interface is conductive to random access, as a position must always be specified.
DmaStreamReaderBuilder	Builds a DmaStreamReader, allowing linear access to a Direct I/O DmaFile
DmaStreamWriter	Provides linear access to a DmaFile. The DmaFile is a convenient way to manage a file through Direct I/O, but its interface is conductive to random access, as a position must always be specified.
DmaStreamWriterBuilder	Builds a DmaStreamWriter, allowing linear access to a Direct I/O DmaFile
ReadResult	ReadResult encapsulates a buffer, returned by read operations like get_buffer_aligned and read_dma
StreamReader	Provides linear read access to a BufferedFile.
StreamReaderBuilder	Builds a StreamReader, allowing linear read access to a BufferedFile
StreamWriter	Provides linear write access to a BufferedFile.
StreamWriterBuilder	Builds a StreamWriter, allowing linear write access to a BufferedFile

Functions

remove	remove an existing file given its name
rename	rename an existing file.
stdin	Allows asynchronous read access to the standard input

Type Definitions

DmaBuffer

A buffer that can be used with DmaFile.