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

[Ming Zhang <mingz@ele.uri.edu>]

On Tue, 2006-04-25 at 16:13 -0600, Ty! Boyack wrote:
> The first scenario you give is most likey the one to occur.  I'm 
> thinking the server is active, has active volumes which use the iSCSI 
> array as PVs, and may or may not have applications accessing it at the 
> time of the failure.  I'm glad to hear that the other volumes should be 
> accessable (assuming we don't stripe across the devices).  It also makes 
> sense that the user will get a r/w error or i/o error. 

maybe u can consider a raid5 on top of these iscsi disks, if u app are
very unhappy or buggy to see these r/w errors.



> 
> I'm still wondering if the disk comes back if the LV will be available 
> again, or if LVM will mark it as failed, and assume that all blocks on 

it will not automatically available i think, but with that "echo ...", u
might be able to see it again.


> it are invalid now and forevermore.  Or is this a function of the 
> filesystem?  I'll be building a test case and will certainly have fun 
> breaking it like this.  I'm glad to know of the ability to test it as 
> Jonathan pointed out that will be a simpler test bed.

keep us updated. thanks.


> 
> Good info - thanks folks!
> 
> -Ty!
> 
> 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!
> >>>>
> >>>>        
> >>>>
> >>>_______________________________________________
> >>>linux-lvm mailing list
> >>>linux-lvm@redhat.com
> >>>https://www.redhat.com/mailman/listinfo/linux-lvm
> >>>read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> >>>
> >>>      
> >>>
> >>_______________________________________________
> >>linux-lvm mailing list
> >>linux-lvm@redhat.com
> >>https://www.redhat.com/mailman/listinfo/linux-lvm
> >>read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> >>    
> >>
> >
> >_______________________________________________
> >linux-lvm mailing list
> >linux-lvm@redhat.com
> >https://www.redhat.com/mailman/listinfo/linux-lvm
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> >  
> >
> 
>