[linux-lvm] Re: LVM + Multipathing

Tue Feb 20 17:13:11 UTC 2007

Using device-mapper-multipath to create a device alias (/dev/mapper/<...>)
which you would then use as a PV in your LVM setup. Device-mapper-multipath
can indeed handle multi-pathingsuch that a failure of one one of the path
would not lead to any IO issues and LVm would just keep on ticking.

             "Matt P"                                                      
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                                       Re: [linux-lvm] Re: LVM +           
             20.02.2007 18:03          Multipathing                        

             Please respond to                                             
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Your problem might be related to this bug
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=213921

On 2/20/07, Luca Berra <bluca at comedia.it> wrote:
> On Tue, Feb 20, 2007 at 06:00:25PM +0530, Ritesh Raj Sarraf wrote:
> >Luca Berra wrote:
> >
> >> On Sat, Feb 17, 2007 at 09:52:27PM +0530, Ritesh Raj Sarraf wrote:
> >>>Hi,
> >>>
> >>>I have a question regarding the combination of LVM + Multipathing in a
SAN
> >>>environment.
> >>>
> >>>I have a LUN mapped to a host using iSCSI. I have two two paths to the
LUN.
> >>>This setup, using multipathing, provides me failover functionality.
> >>>
> >>>My question is how does LVM react to path failures ?
> >>>In a failover environment, multipathing can take upto 120 seconds to
detect
> >>>that the active path to the LUN is no more available and then switch
to the
> >>>secondary path. During this 120 seconds, how does LVM react ? Does it
really
> >>>sense that the device is offlined ? Or that information is never
passed to LVM
> >>>and it just allows I/O to happen leaving that responsibility to the
> >>>multipathing layer ?
> >>
> >> i believe LVM does not react at all.
> >>
> >
> >I brought this question because I'm seeing Filesystem READONLY issues
when doing
> >a takeover/giveback.
> >
> >My understanding is that a filesystem usually goes read-only, only when
it
> >senses errors in the drive. Now I believe LVM might the culprit because
beneath
> >the filesystem and above the block device, LVM is the only layer.
> to clarify my point above:
> i meant that LVM (actually device-mapper) does not do anything special.
> it just passes the IO requests from the fs layer to the
> underlying block device and if the underlyng block device returns an IO
> error then it is passed back to the fs. which will cause ext2 to remount
> the filesystem readonly.
>
> L.
>
> --
> Luca Berra -- bluca at comedia.it
>         Communication Media & Services S.r.l.
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