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.
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 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.
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!
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--
-===========================-
Ty! Boyack
NREL Unix Network Manager
ty@nrel.colostate.edu
(970) 491-1186
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