Bug 1572314
| Summary: | lvconvert man page regarding s|--snapshot to "reverse" a prior split is misleading | ||
|---|---|---|---|
| Product: | Red Hat Enterprise Linux 7 | Reporter: | Corey Marthaler <cmarthal> |
| Component: | lvm2 | Assignee: | LVM and device-mapper development team <lvm-team> |
| lvm2 sub component: | Snapshots | QA Contact: | cluster-qe <cluster-qe> |
| Status: | CLOSED DUPLICATE | Docs Contact: | |
| Severity: | low | ||
| Priority: | unspecified | CC: | agk, heinzm, jbrassow, mcsontos, msnitzer, prajnoha, prockai, zkabelac |
| Version: | 7.5 | ||
| Target Milestone: | rc | ||
| Target Release: | --- | ||
| Hardware: | x86_64 | ||
| OS: | Linux | ||
| Whiteboard: | |||
| Fixed In Version: | Doc Type: | If docs needed, set a value | |
| Doc Text: | Story Points: | --- | |
| Clone Of: | Environment: | ||
| Last Closed: | 2018-04-27 14:27:40 UTC | Type: | Bug |
| Regression: | --- | Mount Type: | --- |
| Documentation: | --- | CRM: | |
| Verified Versions: | Category: | --- | |
| oVirt Team: | --- | RHEL 7.3 requirements from Atomic Host: | |
| Cloudforms Team: | --- | Target Upstream Version: | |
| Embargoed: | |||
Man looks somewhat confusing. By default conversion 'makes' really a new snapshot. If user wants to 'prevent' zeroing of COW LV option -Zn has to be specified. -- This behavior is consistent across all conversion - i.e. converting LV to thin-pool data/metadata make it look like just lvcreated Same with cachepool. If the old data are supposed to be preserved - user needs to know what he is doing and has to specify '-Zn'. I guess some enhancement of man page making this more clear is needed. ...and is a duplicate of Bug 1557401. Corey, feel free to close one or the other, whatever is more convenient for you. *** This bug has been marked as a duplicate of bug 1557401 *** |
Description of problem: I may be reading this incorrectly, however, when I read "reverse", to me it implies that it reinstates the prior COW state before the --splitsnapshot was ever executed. The reality is that using 'lvconvert -s' to "combine a former COW snapshot LV with a former origin" is no different than just creating a new snapshot at that point in time, and that any COW data that existed on the split snap before a convert back to snapshot is lost. Actually, once a snap is split, what value does it have? How would one use that split off COW? lvconvert --splitsnapshot Separates a COW snapshot from its origin LV. The LV that is split off contains the chunks that differ from the origin LV along with metadata describing them. This LV can be wiped and then destroyed with lvremove. -s|--snapshot Combine a former COW snapshot LV with a former origin LV to reverse a previous --splitsnapshot command. # Existing origin volume [root@host-087 ~]# lvs -a -o +devices LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices origin snapper -wi-ao---- 4.00g /dev/sdb1(0) [root@host-087 ~]# df -h Filesystem Size Used Avail Use% Mounted on /dev/mapper/snapper-origin 4.0G 39M 3.7G 2% /mnt/origin ## Write 100 files to origin [root@host-087 ~]# /usr/tests/sts-rhel7.5/bin/checkit -w /mnt/origin -f /tmp/origin -n 100 ## Verify 100 files on origin [root@host-087 ~]# /usr/tests/sts-rhel7.5/bin/checkit -w /mnt/origin -f /tmp/origin -v ## Create snapshot [root@host-087 ~]# lvcreate -s /dev/snapper/origin -c 128 -n splitoff -L 2G Logical volume "splitoff" created. [root@host-087 ~]# lvs -a -o +devices LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices origin snapper owi-aos--- 4.00g /dev/sdb1(0) splitoff snapper swi-a-s--- 2.00g origin 0.00 /dev/sdb1(1024) [root@host-087 ~]# mount /dev/snapper/splitoff /mnt/snap [root@host-087 ~]# df -h Filesystem Size Used Avail Use% Mounted on /dev/mapper/snapper-origin 4.0G 37M 3.8G 1% /mnt/origin /dev/mapper/snapper-splitoff 4.0G 37M 3.8G 1% /mnt/snap ## Verify 100 files on snapshot [root@host-087 ~]# /usr/tests/sts-rhel7.5/bin/checkit -w /mnt/snap -f /tmp/origin -v [root@host-087 ~]# umount /mnt/snap ## --splitsnapshot [root@host-087 ~]# lvconvert --yes --splitsnapshot snapper/splitoff Logical Volume snapper/splitoff split from its origin. ## Just a simple COW volume now, nolonger mountable [root@host-087 ~]# mount /dev/snapper/splitoff /mnt/snap mount: unknown filesystem type 'DM_snapshot_cow' root@host-087 ~]# ls -l /mnt/origin/ | wc -l 102 ## Remove the 100 files from the origin volume [root@host-087 ~]# rm -rf /mnt/origin/* [root@host-087 ~]# ls -l /mnt/origin/ | wc -l 1 [root@host-087 ~]# umount /mnt/origin ## No longer merge-able since it was split [root@host-087 ~]# lvconvert --merge snapper/splitoff --yes snapper/splitoff is not read-only - refusing to merge. ## However the "is not read-only" is just a red herring since changing it to ro doesn't matter [root@host-087 ~]# lvs -a -o +devices LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices origin snapper -wi-a----- 4.00g /dev/sdb1(0) splitoff snapper -wi-a----- 2.00g /dev/sdb1(1024) [root@host-087 ~]# lvchange -pr snapper/splitoff --yes Logical volume snapper/splitoff changed. ## See, still can't merge back even if RO until "converted" back to snap to "reverse" split. [root@host-087 ~]# lvconvert --merge snapper/splitoff --yes Unable to merge non-mirror image snapper/splitoff. ## Convert back to snap, but it appears this doesn't "reverse" anything. This only just created a new snap at this point in time, no different than if I have just used lvcreate to create a new one [root@host-087 ~]# lvconvert --yes -s snapper/origin snapper/splitoff WARNING: Converting logical volume snapper/splitoff to snapshot exception store. THIS WILL DESTROY CONTENT OF LOGICAL VOLUME (filesystem etc.) WARNING: snapper/splitoff not zeroed. Logical volume snapper/splitoff converted to snapshot. ## Merge back, if this was real reverse, then shouldn't that restore the files like if I have never merged the snap to begin with? [root@host-087 ~]# lvconvert --merge snapper/splitoff --yes Merging of volume snapper/splitoff started. snapper/origin: Merged: 100.00% [root@host-087 ~]# lvs -a -o +devices LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices origin snapper -wi-a----- 4.00g /dev/sdb1(0) ## Remount, the files are gone. Really, once a snap is split, it's basically worthless unless I'm missing something? [root@host-087 ~]# mount /dev/snapper/origin /mnt/origin/ [root@host-087 ~]# ls -l /mnt/origin/ | wc -l 1 Version-Release number of selected component (if applicable): 3.10.0-862.el7.x86_64 lvm2-2.02.177-4.el7 BUILT: Fri Feb 16 13:22:31 CET 2018 lvm2-libs-2.02.177-4.el7 BUILT: Fri Feb 16 13:22:31 CET 2018 lvm2-cluster-2.02.177-4.el7 BUILT: Fri Feb 16 13:22:31 CET 2018 lvm2-lockd-2.02.177-4.el7 BUILT: Fri Feb 16 13:22:31 CET 2018 lvm2-python-boom-0.8.5-4.el7 BUILT: Fri Feb 16 13:37:10 CET 2018 cmirror-2.02.177-4.el7 BUILT: Fri Feb 16 13:22:31 CET 2018 device-mapper-1.02.146-4.el7 BUILT: Fri Feb 16 13:22:31 CET 2018 device-mapper-libs-1.02.146-4.el7 BUILT: Fri Feb 16 13:22:31 CET 2018 device-mapper-event-1.02.146-4.el7 BUILT: Fri Feb 16 13:22:31 CET 2018 device-mapper-event-libs-1.02.146-4.el7 BUILT: Fri Feb 16 13:22:31 CET 2018 device-mapper-persistent-data-0.7.3-3.el7 BUILT: Tue Nov 14 12:07:18 CET 2017