Should I use a CoW filesystem on my PC if I only wanted snapshot capabilities ? Post: 303044797

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Top Forums UNIX for Beginners Questions & Answers Should I use a CoW filesystem on my PC if I only wanted snapshot capabilities ? Post 303044797 by stomp on Wednesday 4th of March 2020 05:42:58 AM

03-04-2020

Registered User

Another comment:

A con against zfs is the inability to remove VDEVs. A VDEV is a subpart of a volume.

Example:

Say you have a data volume consisting of a single disk(=vdev, 1 TB). You decide to replace your vdev of a single disk with a raid-1 vdev(1 TB), since you want add redundancy to be safe in case of a disk crash. That's possible. Over the years, you add another 2 vdevs(2x2TB,2x4TB) with raid-1 arrays. So you then have 3 vdevs making up your volume consisting of 2 disks each with an overall capacity of 7 TB.

You now decide you want to increase your storage again and simultaneously reorganize your 3 x raid1(6 disks=>7 TB usable) to 1 x raidz2(5x6 TB =>18 TB usable) to be able to cope with more simulateous disk crashes(2 disk crashes without data loss here) and at the same time reduce the number of active disks(6->5).

With zfs this is only possible by reformatting, since device removal is not fully supported by now. So you have to copy all the data, which must be done offline. ZFS top level device removal is in development at the moment, but i expect some years to pass until even raidz vdevs can be removed.

With LVM you can just add the new underlying disks and remove the old disks. No problem. All is possible to be done online. Btrfs can do that to and is even flexible to do more advanced migrations.

And Here are some experience reports about btrfs and zfs from users:

ZFS Vs BTRFS : linux

Some not to long gone data loss stories about btrfs are also there. I assume the cause may be lacking knowledge about file system operation. But of course that's only a suspicion.

Last edited by stomp; 03-04-2020 at 06:52 AM..

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xfs_freeze

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)