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LVM has recently added several new device types based on dm-raid, thinp, thin snap, (and soon) dm-cache. These are in addition to linear, stripe, mirror. We also have crypt, multipath, and md outside LVM.
We need to specify which combinations of these device types are allowed to be stacked, and in which order. Additional factors, like external origins, metadata, and log devices must be considered.
Code may be required to disallow unsupported combinations.
Instructions for how to properly create these stacked devices needs to be included in the LVM documentation.
Full testing of the allowed combinations needs to be included in the QE plan.
This request was resolved in Red Hat Enterprise Linux 7.0.
Contact your manager or support representative in case you have further questions about the request.
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This request was NOT resolved in Red Hat Enterprise Linux 7.0.
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Comment 6Jonathan Earl Brassow
2015-04-09 22:51:12 UTC
This is - to a degree - a documentation issue. Perhaps there are some checks that could be done during creation to prevent inappropriate stacking.
Comment 8Jonathan Earl Brassow
2016-01-19 00:15:24 UTC
Marian has made some good progress on this and produced a document that is ready to be reviewed by the doc team and then posted in the knowledge base. CC'ing slevine on this bug so he's aware.
The document has been published. Awaiting more comments.
What next?
Shall we link it from Documentation?
I will add link from lvm site.
Comment 11Jonathan Earl Brassow
2016-08-25 15:10:28 UTC
Document is present on our customer portal:
https://access.redhat.com/articles/2106521
We should also take care to update this document as things change in the future (e.g. for dm-integrity).
Marking as verified after document revision and testing the stacking as shown in this matrix https://access.redhat.com/articles/2106521#matrix
Adding attachment with more detailed results of testing.
X/Y X/Linear X/Stripe X/Mirror
Pool Meta/Y OK – verified OK – verified -
Pool Data/Y OK – verified OK – verified -
Ext.Ori for Thin S./Y OK – verified OK – verified OK – verified
Ori. for Old S./Y OK – verified OK – verified OK – verified
Cache Data/Y OK – verified OK – verified -
Cache Meta/Y OK – verified OK – verified -
Cache Origin/Y OK – verified OK – verified -
-------
X/RAIDn X/Thin Volume X/Old Snapshot X/Cache Volume
Preferred - - -
OK – verified - - OK!– verified
OK – verified OK – verified - -
OK – verified OK – verified - -
OK!– verified - - - -
OK!– verified - - - -
OK – verified MAYBE - - TODO
NOTES:
Old snap of mirror is not actually recommended (type raid1 is).
Command warns user when legacy mirror is used explicitly:
# lvs -a -o lv_name,lv_attr,pool_lv,origin,devices
LV Attr Devices
...
mirror_lv mwi-a-m--- mirror_lv_mimage_0(0),mirror_lv_mimage_1(0),mirror_lv_mimage_2(0)
[mirror_lv_mimage_0] Iwi-aom--- /dev/sda(0)
[mirror_lv_mimage_1] Iwi-aom--- /dev/sdb(0)
[mirror_lv_mimage_2] Iwi-aom--- /dev/sdc(0)
[mirror_lv_mlog] lwi-aom--- /dev/sdj(0)
# lvcreate -s vg/mirror_lv -L 1G
Using default stripesize 64.00 KiB.
WARNING: Snapshots of mirrors can deadlock under rare device failures.
WARNING: Consider using the raid1 mirror type to avoid this.
WARNING: See global/mirror_segtype_default in lvm.conf.
mirror_lv must be active exclusively to create snapshot
Perhaps it would be worth mentioning in the new documentation, just for completeness?
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://rhn.redhat.com/errata/RHBA-2016-1445.html