[linux-lvm] lvm2 *TEMPORARY* PV failure - what happens?

[Ming Zhang]

thx.

any idea why partially activated VGs can only allow read only access to
LVs on even good PVs. I think as long we do not do LVM level change like
create new/resize/delete LVs, the write access to LVs data should be
allowed right?

ming


On Tue, 2006-04-25 at 16:34 -0500, Jonathan E Brassow wrote:
> y, sounds right.  It's pretty much what I get.
> 
>   brassow
> 
> On Apr 25, 2006, at 3:39 PM, Ming Zhang wrote:
> 
> > assume 2 scenarios
> >
> > 1) this PV is under use when it is disconnected temporarily. then
> > eventually will return r/w errors to applications. but other LVs are
> > still accessible.
> >
> > 2) system is off and boot up again. for this system will complain PV
> > with UUID ... is not found. so the only way is to partially activate 
> > VG.
> >
> > am i correct here?
> >
> > ming
> >
> >
> >
> > On Tue, 2006-04-25 at 15:21 -0500, Jonathan E Brassow wrote:
> >> It is simple to play with this type of scenario by doing:
> >>
> >> echo offline > /sys/block/<sd dev>/device/state
> >>
> >> and later
> >>
> >> echo running > /sys/block/<sd dev>/device/state
> >>
> >> I know this doesn't answer your question directly.
> >>
> >>   brassow
> >>
> >>
> >> On Apr 25, 2006, at 2:57 PM, Ming Zhang wrote:
> >>
> >>> my 2c. fix me if i am wrong
> >>>
> >>> either activate the VG partially, and then all LVs on other PVs are
> >>> still accessible. I remember these LVs will only have RO access. 
> >>> Though
> >>> I have no idea why.
> >>>
> >>> use dm-zero to generate a fake PVs and add to VG, then allow VG to
> >>> activate and access those LV. But i do not know if you access a LV 
> >>> that
> >>> is partially or fully on this PV, what will happen.
> >>>
> >>> Ming
> >>>
> >>>
> >>> On Tue, 2006-04-25 at 13:08 -0600, Ty! Boyack wrote:
> >>>> I've been intrigued by the discussion of what happens when a PV 
> >>>> fails,
> >>>> and have begun to wonder what would happen in the case of a 
> >>>> transient
> >>>> failure of a PV.
> >>>>
> >>>> The design I'm thinking of is a SAN environment with several
> >>>> multi-terabyte iSCSI arrays as PVs, being grouped together into a
> >>>> single
> >>>> VG, and then carving LVs out of that.  We plan on using the CLVM 
> >>>> tools
> >>>> to fit into a clustered environment.
> >>>>
> >>>> The arrays themselves are robust (RAID 5/6, redundant power 
> >>>> supplies,
> >>>> etc.) and I grant that if we lose the actual array (for example, if
> >>>> multiple disks fail), then we are in the situation of a true and
> >>>> possibly total failure of the PV and loss of it's data blocks.
> >>>>
> >>>> But there is always the possiblity that we could lose the CPU, 
> >>>> memory,
> >>>> bus, etc. in the iSCSI controller portion of the array, which will
> >>>> cause
> >>>> downtime, but no true loss of data.  Or someone may hit the wrong
> >>>> power
> >>>> switch and just reboot the thing, taking it offline for a short 
> >>>> time.
> >>>> Yes, that someone would probably be me.  Shame on me.
> >>>>
> >>>> The key point is that the iSCSI disk will come back in a few
> >>>> minutes/hours/days depending on the failure type, and all blocks 
> >>>> will
> >>>> be
> >>>> intact when it comes back up.  I suppose the analagous situation 
> >>>> would
> >>>> be using LVM on a group of hot swap drives and pulling one of the
> >>>> disks,
> >>>> waiting a while, and then re-inserting it.
> >>>>
> >>>> Can someone please walk me through the resulting steps that would
> >>>> happen
> >>>> within LVM2 (or a GFS filesystem on top of that LV) in this 
> >>>> situation?
> >>>>
> >>>> Thanks,
> >>>>
> >>>> -Ty!
> >>>>
> >>>
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> >>>
> >>
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> >
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> >
>