Quorums / VGDAs
I'm trying to find information on the availability factors of LVM. As I understand it, a quorum is required for a volume group to be available. I have some questions in this regard:
1) What defines a quorum - over 50% of the PVs in the VG, or over 50% of the VGDAs?
2) How many VGDAs are stored on each physical volume. From AIX experience, this varied depending on the number of PVs in the VG.
3) If quorum is lost on an active VG, is the entire VG automatically varied off?
4) Can a VG without quorum be forcefully varied on?
5) Can quorum checking be disabled?
Thanks,
Anthony
Quoted 6 lines · Anthony Johnson
I'm trying to find information on the availability factors of LVM. As I understand it, a quorum is required for a volume group to be available. I have some questions in this regard:
1) What defines a quorum - over 50% of the PVs in the VG, or over 50% of the VGDAs?
VG quorum is given in case all of its PVs are available.
Quoted 2 lines
2) How many VGDAs are stored on each physical volume. From AIX experience, this varied depending on the number of PVs in the VG.
In the LVM1 metadata there's allways 1 VGDA on each PV which has internal redundancy.
With the new LVM2 formats you can have as many VGDA copies as you like.
Quoted 2 lines
3) If quorum is lost on an active VG, is the entire VG automatically varied off?
No, it will stay accessable with the exception of the dead PV(s).
Quoted 1 line
4) Can a VG without quorum be forcefully varied on?
With LVM1.1 (see CVS; WARNING: his is interim development code!) and with final LVM2 later this year, you can activate a VG which has lost quorum.
Quoted 1 line
5) Can quorum checking be disabled?
No, but with the above SW you can activate the VG in case there's at least 1 PV still accessable.
Quoted 9 lines
Thanks,
Anthony_______________________________________________
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Signature · 15 lines
Regards,
Heinz -- The LVM Guy --
*** Software bugs are stupid.
Nevertheless it needs not so stupid people to solve them ***
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Heinz Mauelshagen Sistina Software Inc.
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56242 Marienrachdorf
Germany
[email protected] +49 2626 141200
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I'm having some trouble understanding these answers. Perhaps I am misunderstanding the purpose of a "quorum" on Linux. On AIX, quorums ensure that at least one copy of the latest VGDA is alsays avaialble by ensuring there are always a quorum (over half) of the VGDA copies available. The VGDA is stored on each PV (and sometimes 2 copies are stored on each PV). If you lose quorum, you lose the VG (to ensure we don't varyon with an old copy).
You imply that a "quorum" requires ALL of the PVs be avialable in Linux. The dictionary definition of a "quorum", however, is more than half. If a VG in Linux can be varied on with only one good copy of a VGDA (in perhaps a 3-PV VG), then how do you ensure that you are always looking at and updating the most recent copy? If, for instance, I have a 4-disk VG, and I lose 2 disks, then shutdown and varon the VG with the alternate 2 disks (the previously missing ones), I will be seeing only the older VGDA. Is the VGDA also stored in the /etc/lvmtab.d? If so, can you varyon if this VGDA was removed?
I guess the big question is: How does LVM for Linux ensure that we are always using the most recent VGDA data (in case we're looking at active PVs that were previously missing). I have to do a lecture to an AIX group on the differences between LVM on AIX and Linux, and quorums were always a big issue for AIX.
Thanks,
Anthony
Quoted 21 lines
Quoted 2 lines
1) What defines a quorum - over 50% of the PVs in the VG, or over 50% of the VGDAs?
VG quorum is given in case all of its PVs are available.
Quoted 2 lines
2) How many VGDAs are stored on each physical volume. From AIX experience, this varied depending on the number of PVs in the VG.
In the LVM1 metadata there's allways 1 VGDA on each PV which has internal redundancy.
With the new LVM2 formats you can have as many VGDA copies as you like.Quoted 2 lines
3) If quorum is lost on an active VG, is the entire VG automatically varied off?
No, it will stay accessable with the exception of the dead PV(s).
Quoted 1 line
4) Can a VG without quorum be forcefully varied on?
With LVM1.1 (see CVS; WARNING: his is interim development code!) and with final LVM2 later this year, you can activate a VG which has lost quorum.
Quoted 3 lines · Anthony Johnson
Heinz,
I'm having some trouble understanding these answers. Perhaps I am misunderstanding the purpose of a "quorum" on Linux.
Well,
I used the term "quorum" as we use it with LVM1. There's piles of different "quorum" definitions for various things out there, I am afraid ;-)
Quoted 9 lines
On AIX, quorums
ensure that at least one copy of the latest VGDA is alsays avaialble by ensuring there are always a quorum (over half) of the VGDA copies available. The VGDA is stored on each PV (and sometimes 2 copies are stored on each PV). If you lose quorum, you lose the VG (to ensure we don't varyon with an old copy).You imply that a "quorum" requires ALL of the PVs be avialable in Linux. The dictionary definition of a "quorum", however, is more than half.
Well, this is the problem with definitions ;-)
In the LVM1 format case the VGDA is assembled using the metadata of *all* PVs and thats why quorum for *LVM1* means that 100% of the PVs are needed.
With LVM2 you will have the choice to put VGDA metadata on as many PVs as you like (for eg.: just on 3 out of 50). This is to avoid metadata update delays as much as possible.
In this example you don't need access to all PVs in order to achive quorum. But if you manage to kill those 3 at once, you need a valid VGDA backup.
OTOH this goes beyond the AIX approach giving you VG quorum even with much less than 50% of the PVs available!
Quoted 6 lines
If a VG in Linux can be varied on with only one good copy of a VGDA (in perhaps a 3-PV VG), then how do you ensure that you are always looking at and updating the most recent copy? If, for instance, I have a 4-disk VG, and I lose 2 disks, then shutdown and varon the VG with the alternate 2 disks (the previously missing ones), I will be seeing only the older VGDA. Is the VGDA also stored in the
/etc/lvmtab.d?
Yes, with LVM1 there is a working copy of the VGDA of each VG.
Quoted 2 lines
If so, can
you varyon if this VGDA was removed?
That working copy is recreated every time you run vgscan.
In general an activated VG *without* quorum is read-only in case of the LVM1 ondisk format so your example describes a don't care scenario. With the LVM2 metadata format (which we call format2) there are sequence numbers kept with each copy which define the recent one.
Hope this makes things a little cleare :)
Regards,
Heinz -- The LVM Guy --
Quoted 29 lines
I guess the big question is: How does LVM for Linux ensure that we are always using the most recent VGDA data (in case we're looking at active PVs that were previously missing). I have to do a lecture to an AIX group on the differences between LVM on AIX and Linux, and quorums were always a big issue for AIX.
Thanks,
AnthonyQuoted 19 lines
Quoted 2 lines
1) What defines a quorum - over 50% of the PVs in the VG, or over 50% of the VGDAs?
VG quorum is given in case all of its PVs are available.
Quoted 2 lines
2) How many VGDAs are stored on each physical volume. From AIX experience, this varied depending on the number of PVs in the VG.
In the LVM1 metadata there's allways 1 VGDA on each PV which has internal redundancy.
With the new LVM2 formats you can have as many VGDA copies as you like.Quoted 2 lines
3) If quorum is lost on an active VG, is the entire VG automatically varied off?
No, it will stay accessable with the exception of the dead PV(s).
Quoted 1 line
4) Can a VG without quorum be forcefully varied on?
Quoted 8 lines
Quoted 2 lines
With LVM1.1 (see CVS; WARNING: his is interim development code!) and with final LVM2 later this year, you can activate a VG which has lost quorum.
_______________________________________________
linux-lvm mailing list
[email protected]
http://lists.sistina.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://www.sistina.com/lvm/Pages/howto.html
*** Software bugs are stupid.
Nevertheless it needs not so stupid people to solve them ***
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Heinz Mauelshagen Sistina Software Inc.
Senior Consultant/Developer Am Sonnenhang 11
56242 Marienrachdorf Germany
[email protected] +49 2626 141200
FAX 924446
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-