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freeze_super(9) [centos man page]

FREEZE_SUPER(9) 						   The Linux VFS						   FREEZE_SUPER(9)

freeze_super - lock the filesystem and force it into a consistent state SYNOPSIS
int freeze_super(struct super_block * sb); ARGUMENTS
sb the super to lock DESCRIPTION
Syncs the super to make sure the filesystem is consistent and calls the fs's freeze_fs. Subsequent calls to this without first thawing the fs will return -EBUSY. During this function, sb->s_writers.frozen goes through these values: SB_UNFROZEN File system is normal, all writes progress as usual. SB_FREEZE_WRITE The file system is in the process of being frozen. New writes should be blocked, though page faults are still allowed. We wait for all writes to complete and then proceed to the next stage. SB_FREEZE_PAGEFAULT Freezing continues. Now also page faults are blocked but internal fs threads can still modify the filesystem (although they should not dirty new pages or inodes), writeback can run etc. After waiting for all running page faults we sync the filesystem which will clean all dirty pages and inodes (no new dirty pages or inodes can be created when sync is running). SB_FREEZE_FS The file system is frozen. Now all internal sources of fs modification are blocked (e.g. XFS preallocation truncation on inode reclaim). This is usually implemented by blocking new transactions for filesystems that have them and need this additional guard. After all internal writers are finished we call ->freeze_fs to finish filesystem freezing. Then we transition to SB_FREEZE_COMPLETE state. This state is mostly auxiliary for filesystems to verify they do not modify frozen fs. sb->s_writers.frozen is protected by sb->s_umount. COPYRIGHT
Kernel Hackers Manual 3.10 June 2014 FREEZE_SUPER(9)

Check Out this Related Man Page

xfs_freeze(8)                                                 System Manager's Manual                                                xfs_freeze(8)

xfs_freeze - suspend access to an XFS filesystem SYNOPSIS
xfs_freeze -f | -u mount-point DESCRIPTION
xfs_freeze suspends and resumes access to an XFS filesystem (see xfs(5)). xfs_freeze halts new access to the filesystem and creates a stable image on disk. xfs_freeze is intended to be used with volume managers and hardware RAID devices that support the creation of snapshots. The mount-point argument is the pathname of the directory where the filesystem is mounted. The filesystem must be mounted to be frozen (see mount(8)). The -f flag requests the specified XFS filesystem to be frozen from new modifications. When this is selected, all ongoing transactions in the filesystem are allowed to complete, new write system calls are halted, other calls which modify the filesystem are halted, and all dirty data, metadata, and log information are written to disk. Any process attempting to write to the frozen filesystem will block waiting for the filesystem to be unfrozen. Note that even after freezing, the on-disk filesystem can contain information on files that are still in the process of unlinking. These files will not be unlinked until the filesystem is unfrozen or a clean mount of the snapshot is complete. The -u flag is used to un-freeze the filesystem and allow operations to continue. Any filesystem modifications that were blocked by the freeze are unblocked and allowed to complete. One of -f or -u must be supplied to xfs_freeze. NOTES
A copy of a frozen XFS filesystem will usually have the same universally unique identifier (UUID) as the original, and thus may be pre- vented from being mounted. The XFS nouuid mount option can be used to circumvent this issue. In Linux kernel version 2.6.29, the interface which XFS uses to freeze and unfreeze was elevated to the VFS, so that this tool can now be used on many other Linux filesystems. SEE ALSO
xfs(5), lvm(8), mount(8). xfs_freeze(8)

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