= iomap =

'''iomap''' grew out of need to provide '''modern''' block mapping abstraction for filesystems to read/write for three different IO access methods:

  * Direct IO
  * Buffered IO
  * DAX

Block mapping provides a mapping between data cached in memory, in the page cache, and the location on persistent storage where that data lives. [[https://lwn.net/Articles/930173/|LWN has an incredible review of the old buffer-heads block-mapping and why they are inefficient]], since the the inception of Linux. Since buffer-heads work on a 512-byte block based paradigm, it creates an overhead for modern storage media which no longer necessarily works only on 512-blocks. This document strives to provide a template for LSFMM for what will hopefully eventually become upstream Linux kernel documentation for '''iomap''' and guidance for developers on converting a filesystem over from buffer-heads to '''iomap'''.

<<TableOfContents(8)>>

== A modern block abstraction ==

Instead of assuming a granularity of storage media 512-blocks at time, '''iomap''' allows filesystems to query storage media for data using block ranges. Since block mapping are provided for a block ranges for cache data in memory, in the page cache, naturally this implies operations on block ranges will also deal with multipage operations in the page cache. Folios are used to help provide multipage operations in memory.

== struct iomap_ops ==

A filesystem is encouraged to provide struct iomap_ops for beginning an IO operation and ending an IO operation on a block range, and so the `struct iomap_ops` data structure has `iomap_begin()` and `iomap_end()` callbacks.

Experience in adopting '''iomap''' on XFS has has shown that the filesystem implementation of these operations can be simplified considerably if one `struct iomap_ops` is provided per major filesystem IO operation:

  * read
  * direct writes
  * DAX writes
  * buffered writes
  * xattr - FIEMAP_FLAG_XATTR
  * seek

In terms of simplicity, you may convert a filesystem in this order instead:

For example:

  * `struct iomap_ops` xfs_'''read'''_iomap_ops` iomap: lift the xfs writeback code to iomap
  * `struct iomap_ops` xfs_'''direct_write'''_iomap_ops
  * `struct iomap_ops` xfs_'''dax_write'''_iomap_ops
  * `struct iomap_ops` xfs_'''buffered_write'''_iomap_ops - xfs: split out a new set of read-only iomap ops
  * `struct iomap_ops` xfs_'''xattr'''_iomap_ops - xfs: fix SEEK_DATA for speculative COW fork preallocation
  * `struct iomap_ops` xfs_'''seek'''_iomap_ops - iomap: move the iomap_dio_rw ->end_io callback into a structure

== struct iomap_dio_ops ==

Used for Direct-IO. These will call `iomap_dio_write()`.

  * `struct iomap_dio_ops` xfs_'''dio_write'''_ops->end_io() - iomap: add a filesystem hook for direct I/O bio submission
  * `struct iomap_dio_ops` xfs_'''dio_write'''_ops->submit_io() - xfs: split the iomap ops for buffered vs direct writes

== struct iomap_writeback_ops ==

  * `struct iomap_writeback_ops` xfs_'''writeback'''_ops - xfs: support CoW in fsdax mode

== Converting filesystems from buffer-head to iomap guide ==
=== One op at at time ===

You may try to convert a filesystem IO operation at time, for instance this order reflects the order in which XFS started converting over to iomap:

  * xattr
  * seek
  * direct writes
  * buffered writes
  * read
  * DAX writes

== Defining a simple filesystem ==

The easiest