XFS Deep Dive: Layout, Inode Management, and Read/Write Operations

1. XFS Layout

The XFS layout follows the official XFS layout documentation. Each allocation group (AG) starts with a superblock; the primary superblock of AG0 stores all AG information, while secondary superblocks are used only when the primary is damaged. The second sector (AGF) manages free space using two B+ trees indexed by block number and block count. The third sector (AGI) stores inode information, allocating inodes in chunks of 64. The fourth sector (AGFL) holds a free‑list of block pointers reserved for internal use.

2. XFS Operations

XFS defines three operation sets: iops for inode metadata (size, timestamps, etc.), fops for file data read/write at the page‑cache level, and aops for direct disk‑level I/O. These are illustrated through

open

,

read

, and

write

calls.

3. Inode Operations and File Creation

Inode operations handle inode attribute updates, creation, lookup, linking, and file creation. The directory inode uses

dir_inode->i_ops = xfs_dir_ci_inode_operations / xfs_dir_inode_operations

(found in

fs/xfs/xfs_iops.c

) to set ACLs and assign the iops/fops/aops sets.

File creation checks quota availability, reserves blocks and log space, then allocates an inode via the following steps:

xfs_trans_alloc

: reserve blocks and log space for a transaction.

xfs_trans_reserve_quota

: reserve quota based on user/group/project limits.

xfs_dir_ialloc

: allocate the inode.

xfs_trans_commit

: commit the transaction to the log.

Disk‑level inode allocation involves reading the AGI structure with

xfs_ialloc_read_agi

, locating the inode B+‑tree root, and using

xfs_ialloc_ag_alloc

to allocate a new inode, handling both contiguous and spare allocations via

xfs_inobt_insert

and

xfs_inobt_insert_sprec

.

4. XFS Write Operations

Memory‑level writes use

struct file_operations xfs_file_operations

, while disk‑level writes use

struct address_space_operations xfs_address_space_operations

. Data is first written to the page cache; when dirty pages accumulate, they are flushed to disk via

sync/fsync

or background writeback. XFS supports three write paths: buffer I/O (with page cache), direct I/O (bypassing cache), and DAX write (for persistent memory devices).

XFS manages file extents with B+‑tree structures; each extent records logical file offset, physical block address, length, and status. The mapping is built by

xfs_iread_extents

and

xfs_iext_lookup_extent

, and actual I/O is performed by the iomap module via

iomap_write_actor

.

5. XFS Read Operations

Read operations start with

xfs_file_iomap_begin

, which obtains the iomap for the current file position. The physical block address is calculated as

iomap->addr + pos - iomap->offset

.

6. Xlog Mechanism

XFS uses a journaling log (xlog) to ensure consistency; log records are written before data blocks are updated.

7. Block Layer

The block layer handles I/O submission and completion. After a device finishes an I/O, a hardware interrupt triggers

do_IRQ

, which may raise a soft IRQ to process the request completion (e.g.,

blk_mq_requeue_work

).

8. XFS Tools

Common XFS utilities include:

mkfs.xfs

: format a device with XFS.

xfs_fsr

: defragment a mounted XFS filesystem.

xfs_bmap

: display block mapping of a file.

xfs_info

: show filesystem information.

xfs_admin

: modify filesystem parameters (requires unmounted filesystem).

xfs_copy

: parallel copy of an XFS filesystem.

xfs_metadump

/

xfs_mdrestore

: dump and restore filesystem metadata.

xfs_db

: interactive debugging tool for XFS structures.

These commands allow administrators to create, inspect, maintain, and troubleshoot XFS filesystems.