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Special Forums Hardware Filesystems, Disks and Memory Faster way: SAN hd to SAN hd copying Post 303007674 by rbatte1 on Monday 20th of November 2017 05:57:48 AM
Old 11-20-2017
When you say 'a faster way', are you trying to move the data from one SAN to another, so that you can detach the original and you need to make the offline time as short as possible?

If your disk is LVM, then you might be better to proceed like this:-
  • Get the new LUN assigned and re-scan to pick it up if necessary
  • Make the whole new LUN into an LVM Physical Volume with pvcreate
  • Add the PV into the volume group containing the data you want to move
  • Alter the logical volume holding your filesystem/data to have two copies, specifying the new PV as the target. This will take a long time, but the applications remain available because the data is still accessible.
  • Verify that the logical volume has no errors reported.
  • Alter the logical volume to have just a single copy, specifying the old SAN device to be removed
  • Repeat the mirror/drop for as many logical volumes as you need to move to completely free the old PV
  • Remove the old PV from the volume group
  • Detach the old LUN from the OS
  • Detach the old LUN from the SAN side

Using the tools in the OS rather than trying to do the work yourself should be a lot neater. The logical volume is moved, but the filesystem has no idea about it, so all the files remain exactly as they were and your applications, backups or whatever can happily carry on.

I prefer to add a mirror to the new device rather than just execute a move for large logical volumes just in case there are any issues in the process. You don't destroy the original so you can always drop the new copy and start again.


Have I missed the point and waffled on about something irrelevant? My apologies if I have, but I hope that this helps. If you are looking to make another copy to be used separately, then we might be able to use LVM to do that too, but that would be a copy at a fixed point in time that would then go out of date with respect to the original as either is changed (you suggest that there are many concurrent processes working on it)


Anyway, I hope that this helps. Please feel free to correct me if I'm way off target.


Kind regards,
Robin
 

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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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