Understanding the Linux Kernel, 3rd Edition

The sync( ), fsync( ), and fdatasync( ) System Calls

In this section, we examine briefly the three system calls available to user applications to flush dirty buffers to disk:

sync( )

Allows a process to flush all dirty buffers to disk

fsync( )

Allows a process to flush all blocks that belong to a specific open file to disk

fdatasync( )

Very similar to fsync( ), but doesn’t flush the inode block of the file

The sync ( ) System Call

The service routine sys_sync( ) of the sync( ) system call invokes a series of auxiliary functions:

    wakeup_bdflush(0);
    sync_inodes(0);
    sync_supers( );
    sync_filesystems(0);
    sync_filesystems(1);
    sync_inodes(1);

As described in the previous section, wakeup_bdflush( ) starts a pdflush kernel thread, which flushes to disk all dirty pages contained in the page cache.

The sync_inodes( ) function scans the list of superblocks looking for dirty inodes to be flushed; it acts on a wait parameter that specifies whether it must wait until flushing has been performed or not. The function scans the superblocks of all currently mounted filesystems; for each superblock containing dirty inodes, sync_inodes( ) first invokes sync_sb_inodes( ) to flush the corresponding dirty pages (we described this function earlier in the section "Looking for Dirty Pages To Be Flushed“), then invokes sync_blockdev( ) to explicitly flush the dirty buffer pages owned by the block device that includes the superblock. This is done because the write_inode superblock method of many disk-based filesystems ...