[dm-devel] Re: Root device multipathed host freeze with the latest upstream multipath-tools package

[Kiyoshi Ueda]

Hi Martin,

On Wed, 23 Jan 2008 16:28:16 +0530, Martin George wrote:
> Kiyoshi Ueda wrote:
> > Hi Martin,
> > 
> > Thank you for your testing.
> > Please see my comments below.
> > 
> > On Tue, 22 Jan 2008 21:56:13 +0530, "Sarraf, Ritesh" wrote:
> >  > Hi Kiyoshi,
> >  > 
> >  > I took the latest upstream multipath-tools package (Jan 15, 2008) and
> >  > installed it on my RHEL 5.1 host to verify the libprio fix. To simulate
> >  > the FCP path faults, I ran your script (as attached in the mail) which
> >  > alternately offlined/onlined the corresponding SCSI paths of the root dm
> >  > device in the syfs. Listing my observations below:
> >  > 
> >  > 1) The freeze was still reproducible. On checking the sysrq dumps (as
> >  > attached), I could see it was the script itself i.e. test.sh which seems
> >  > to have stalled on the exec () system call perhaps waiting for inode
> >  > write out for updated access time (the script resides on my root dm
> >  > device itself). As suggested by you in the bugzilla, I remounted the
> >  > root device using the noatime option and then reran the script - I have
> >  > not hit the freeze yet. Is this the expected behavior?
> > 
> > As for your script, it is the expected behavior.
> > I found that you added some sleep commands to my original script
> > posted by the following email.
> > 
> >     http://marc.info/?l=dm-devel&m=119465024621783&w=2 
> > <http://marc.info/?l=dm-devel&m=119465024621783&w=2>
> > 
> > sleep is not shell build-in command, so need to access the root device.
> > I guess that is the reason of the freeze.
> 
> So does that mean you should never access the root partition in such a 
> scenario? What about utilities like syslogd which may access the root to 
> log messages? There could be many such utilities for that matter which 
> accesses the root and all would have to be stopped.

No.
Generally you can access the root.
But you can't in your single-threaded test script.

On your testing scenario, only your script would online/offline paths
for the root like this:
    while true; do
        <offline all paths>
        sleep
        <online all paths>
        sleep
    done

So if your script accesses to the root after it offlines all paths,
it is freezed and nobody will online the paths.
So you must avoid your script to be freezed.
Other utilities accessing to the root don't matter.

I guess that your script is freezed at the sleep after the offline.

Thanks,
Kiyoshi Ueda