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

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.


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.


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?



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