ZFS: Create a snapshot of a filesystem (tech-recipes)


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ZFS: Create a snapshot of a filesystem (tech-recipes)

Of the many cool features of the ZFS filesystems, one of the coolest is taking snapshots of a live filesystem. This provides a read-only point-in-time copy of the whole filesystem. While this sounds slow and expensive in disk usage, ZFS makes snapshots efficient in time and space.

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Test Your Knowledge in Computers #493
Difficulty: Medium
When a program executes, each command executes one after the other, top-to-bottom. This is known as inconsequential control flow.
True or False?
xfs_freeze(8)						      System Manager's Manual						     xfs_freeze(8)

NAME
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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