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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
Old 03-04-2020
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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voldiskadm(8)						      System Manager's Manual						     voldiskadm(8)

NAME
voldiskadm - Menu interface for LSM disk administration SYNOPSIS
/usr/sbin/voldiskadm DESCRIPTION
The voldiskadm script is an interactive tool that presents a menu of possible operations to the user. When an operation is selected, the script guides the user through the necessary steps, and prompts for data needed to complete the operation. The voldiskadm interface is intended mainly for beginning users and for those who prefer a simple method for doing common operations. The interface uses query-based prompts to gather input, with defaults supplied when possible. Context-sensitive help is available at every prompt by typing ?. Also, a list option can be used to get information on available target disks for an operation. For operations that require a device name, one or more names can be specified using a space-separated list. Names in the list can have the form dskn or rdskn (for an entire disk) or dsknp or rdsknp (for a specific disk partition). Disk names relate directly to device names in the /dev/disk and /dev/rdisk directories. The file, /etc/vol/disks.exclude, may be used to exclude disks from use by voldiskadm. Each line of the file specifies the name of a disk to exclude (for example, dsk5). The voldiskadm menu includes the following options: Add or initialize one or more disks This option prompts for one or more disk device addresses. The specified disks can be added to an existing disk group, added to a new disk group, added to a disk group as spares, or initialized but not added to a disk group (reserved for use as replacement disks). After specifying the disks, the user is prompted for a disk group (rootdg by default) and a disk name. If no name is specified, a default disk name is assigned (diskn for disks in the rootdg disk group and diskgroupn for disks in other disk groups). The disks are then checked to ensure that there is no information already on them. If there is, the user is given the option of encapsulating the disks. Encapsulate one or more disks This option prompts for one or more disk addresses. It then calls volencap to encapsulate the specified partitions. Remove a disk This option prompts for a disk, by disk media name. The disk is checked to ensure that no subdisks reside on the disk. If the disk is in use, the operation fails with a recommendation to first move all volumes off the disk. If this disk is the last disk in a disk group, the user is prompted for whether the disk group should be removed from the system, as well. The operation proceeds by calling voldg rmdisk to remove the disk from its disk group. If this is the last disk in its disk group, voldg deport is used, instead, to remove the disk group from use. Remove a disk for replacement This option prompts for a disk, by disk media name. The disk is checked for volumes that would lose all mirrors as a result of the operation. If such volumes exist, they are listed and the user is prompted to ensure that the operation should proceed. The operation proceeds by calling voldg -k rmdisk to dissociate the media record from the physical disk. If some formatted disks are under LSM control but not assigned to a disk group, the user is prompted for whether one of these disks should be used as a replacement. Replace a failed or removed disk This option prompts for a disk media name. The named media record must be dissociated from a disk. If the media record is not in the removed state, unused disks are scanned for matching disk IDs. If a disk with a matching disk ID is found, the user is prompted for whether that disk should be reattached. If a matching disk is not used, the user is prompted for a new disk, by device name. If the named replacement disk has a valid disk header, but is not allocated to a disk group, the user is prompted for whether the disk should be reinitialized. If the named replacement disk is listed as allocated to a disk group or to another host, the user is prompted to ensure that the operation should proceed. If the device is to be initialized, a new disk label is written to the disk to reflect its private and public regions. Given an initialized disk, the operation proceeds by replacing the disk in a disk group with voldg -k adddisk. Mirror volumes on a disk This option prompts for a disk, by disk media name. It then prompts for a destination disk within the same disk group, also by disk media name. Specifying no destination disks indicates that any disk is suitable. The operation proceeds by calling volmirror to mirror the volumes. Mirroring volumes from the boot disk will produce a disk that can be used as an alternate boot disk. This is done by calling the volrootmir command. Move volumes from a disk This option prompts for a disk, by disk media name. It then prompts for a possible list of destination disks, also by disk media name. Specifying no destination disks indicates that any disk is suitable. The operation proceeds by calling volevac to move sub- disks off the disk. Enable access to (import) a disk group This option prompts for a disk, by device address. The operation proceeds by calling voldg import to import the disk group stored on that disk. Disable access to (deport) a disk group This option prompts for a disk group name. The prompt display lists alternate disk groups and the disks (media name and access name) that they contain. The operation proceeds by calling voldg deport. Enable (online) a disk device This option prompts for a disk device. The prompt display allows for a display of disks on the system. The operation only func- tions for disks currently in an offline state. It then proceeds to make the disk accessible. Disable (offline) a disk device This option prompts for a disk device. The prompt display allows for a display of disks on the system. The operation only func- tions for disks currently in an online state but not part of any disk group. It then proceeds to mark the disk as offline such that the Logical Storage Manager makes no further attempt at accessing the disk. Mark a disk as a spare for a disk group This option sets up a disk to be used as a spare device for its disk group. A spare disk can be used to automatically replace a disk that has failed. No space can be used on a disk that is marked as a spare. Turn off the spare flag for a disk This option removes a disk from those that can be used as a spare and returns its space to the general pool of available space. Recover plexes and volumes after replacement This operation performs plex attachment, RAID-5 subdisk recovery, and resynchronize operations for the named volumes, or for volumes residing on the named disks (media name). If no media name or volume operands are specified, the operation applies to all volumes (or to all volumes in the specified disk group). ERRORS
See the voldiskadd(8) reference page for a description of errors related to the initialization operation. FILES
A list of disks to exclude from use by voldiskadm. SEE ALSO
disklabel(8), volintro(8), voldg(8), voldisk(8), voldiskadd(8), voldisksetup(8), volrootmir(8) voldiskadm(8)
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