[linux-lvm] Re: LVM + Multipathing

[Luca Berra]

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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