Red Hat Bugzilla – Bug 863703
Inconsistencies with flags of thin snapshots and origins
Last modified: 2014-11-26 02:34:29 EST
I have noticed inconsistencies in the flags with regard to thin volumes. The first column of the flags is 'o' when a normal snapshot is taken of a thin volume but is 'V' when a thin snap is used. I haven't checked to see if 'O' is used for merging with thin snapshots. The other issue I have run into is that a thin snapshot doesn't put an 's' into column 1 or column 7. This makes parsing more difficult since a thin volume and thin snap look the same in the flags. It seems that one of the columns could be changed to an 's' without losing any information. The first column also is used for merging 'S' so that would probably be the best one. ~# lvs LV VG Attr LSize Pool Origin Data% Pool_One MyGroup twi-a-tz- 1.00g 9.96 fat_snap1 MyGroup swi-a-s-- 112.00m thin_vol2 0.00 thin_vol2 MyGroup owi-a-tz- 2.00g Pool_One 4.98 ~# lvs LV VG Attr LSize Pool Origin Data% Pool_One MyGroup twi-a-tz- 1.00g 9.96 thin_snap1 MyGroup Vwi-a-tz- 2.00g Pool_One thin_vol2 4.98 thin_vol2 MyGroup Vwi-a-tz- 2.00g Pool_One 4.98 LVM version: 2.02.98(2)-git (2012-08-24) Library version: 1.02.77-git (2012-08-24) Driver version: 4.22.0
It's worth to note here user should not be mistaking old-style COW snapshots with new 'thin' snapshots - they are not the same in terms of reported data and behave differently in many areas. Thus letter 's' should probably remain dedicated for old-style snaps. You cannot merge thin-snaps and you cannot get the 'real' size it takes. The fact that thin volume is snapshot of some other volume is rather 'syntax sugar' here - since such volume is fully autonomous thin volume - you can have it online/offline as you need (unlike with old snaps).
We are likely not going to change the behavior here. Column 7 is target type - and target type really is (t)thin. (s)napshot target segtype is dedicated to old-style snaps. (lvcreate --type snapshot | lvcreate --type thin) Old-snaps and thin-snaps cannot be treated equally and also the lvm2 requires different treatment for those types. If you need to check if 'thin' volume is still a snapshot - check presence of "Orig" field.
Also, we've added separate fields for each lv_attr field recently in lvm2 v2.02.108 as well as additional fields that help with identification of the LV type in v2.02.110 (lv_layout and lv_role fields). These are much more readable than the lv_attr bits that provide condensed view only.