multipath md devices
Hi,
We have multipath devices created on SAN Luns. Say md0 is created on /dev/sdj and /dev/sde, the latter being the alternate path for /dev/sdj.
I've noticed the following:
1) Without much IO to the md device, and I pull out the cable to say /dev/sdj, the /proc/mdstat still shows both devices. /proc/mdstat won't get updated unless I start some considerable IO to the md device. Even mdadm scan/query o/p shows both the paths, which is not true. As we start IO, /proc/mdstat reflects that one of the devices, /dev/sdj in this case, has failed. Thereafter mdadm outputs would be correct too.
The entries (link down) in syslog and dmesg are almost instantaneous when the cable is pulled out. This makes it very difficult to monitor multipath devices, as we cannot rely on /proc/mdstat to read.
2) Another situation: Device md0 is active, with healthy multipaths /dev/sdj and /dev/sde, under reasonable IO activity. If the cable to /dev/sdj is yanked out, md0 remains still active, thanks to the alternate path, sde. However, it fails to go back and re-construct the spare path allocation even after the fibre link is restored. Here, if I pull the cable out for sde even after 30 minutes, the machine ends up failing to write to /dev/md0 as it does not care whether /dev/sdj is back online, unless I failed, removed and add /dev/sdj manually from the mdadm command line. If something is hard mounted on /dev/md0, it may end up in a system crash.
To conclude, if one path goes off, and comes back after a while, and then the second path goes off, md0 cannot be read, unless someone manually did fail, remove and add the first device which came back online, before the second path goes off.
Any help towards this will be much appreciated.
Thanks,
--AM.
Quoted 14 lines · Anu Matthew
Hi,
We have multipath devices created on SAN Luns. Say md0 is created on
/dev/sdjand/dev/sde, the latter being the alternate path for/dev/sdj.I've noticed the following:
1) Without much IO to the md device, and I pull out the cable to say
/dev/sdj, the/proc/mdstatstill shows both devices./proc/mdstatwon't get updated unless I start some considerable IO to the md device. Even mdadm scan/query o/p shows both the paths, which is not true. As we start IO,/proc/mdstatreflects that one of the devices,/dev/sdjin this case, has failed. Thereafter mdadm outputs would be correct too.
Quoted 3 lines
The entries (link down) in syslog and dmesg are almost instantaneous when the cable is pulled out. This makes it very difficult to monitor multipath devices, as we cannot rely on
/proc/mdstatto read.
/proc/mdstat will be correct once the first physical read/write on the yanked path fails.
Quoted 11 lines
2) Another situation: Device md0 is active, with healthy multipaths
/dev/sdjand/dev/sde, under reasonable IO activity. If the cable to/dev/sdjis yanked out, md0 remains still active, thanks to the alternate path, sde. However, it fails to go back and re-construct the spare path allocation even after the fibre link is restored. Here, if I pull the cable out for sde even after 30 minutes, the machine ends up failing to write to/dev/md0as it does not care whether/dev/sdjis back online, unless I failed, removed and add/dev/sdjmanually from the mdadm command line. If something is hard mounted on/dev/md0, it may end up in a system crash.
Quoted 4 lines
To conclude, if one path goes off, and comes back after a while, and then the second path goes off, md0 cannot be read, unless someone manually did fail, remove and add the first device which came back online, before the second path goes off.
Yeah, IBM wrote a little app to help with that. We stuffed it into the mdadm package we ship since that seemed the most appropriate place for it. It's called mdmpd and that's it's job basically. Very simple app, but doesn't run on upstream kernels at the moment (it wants the md event interface which hasn't yet been submitted upstream by Neil).
Quoted 9 lines
Any help towards this will be much appreciated.
Thanks,
--AM.
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Signature · 4 lines
Doug Ledford <[email protected]> 919-754-3700 x44233 Red Hat, Inc. 1801 Varsity Dr. Raleigh, NC 27606
// Yeah, IBM wrote a little app to help with that. We stuffed it into the mdadm package we ship since that seemed the most appropriate place for it. It's called mdmpd and that's it's job basically. Very simple app, but doesn't run on upstream kernels at the moment (it wants the md event interface which hasn't yet been submitted upstream by Neil). //
Doug, thanks a lot, we appreciate your response.
One of our boxes with the multipath problem (sic) runs on RHEL ES3.0 with active RHN subscription and is at 2.4.21-15.ELsmp, and has mdadm-1.5.0-3, with mdmpd running. Yet it crashed last week as the paths went away from a SAN switch f/w upgrade, as they took off the paths one 4 hours after another, but never both the paths at the same time.
I saw your post on Bugzilla, dated 2004-06-07 about the md Event Interface Patch. Do we still have to throw that patch in, manually? Or does 2.4.21-20.ELsmp have it already? I tried to read up the Change Log and File lists, could gather nothing much.
Thanks,
--Anu Matthew
Quoted 5 lines · Doug Ledford
Yeah, IBM wrote a little app to help with that. We stuffed it into the mdadm package we ship since that seemed the most appropriate place for it. It's called mdmpd and that's it's job basically. Very simple app, but doesn't run on upstream kernels at the moment (it wants the md event interface which hasn't yet been submitted upstream by Neil).
iirc the it has been submitted and refused
http://marc.theaimsgroup.com/?t=109417961300007&r=1&w=2&n=8
Signature · 6 lines
Luca Berra -- [email protected] Communication Media & Services S.r.l. /"\ \ / ASCII RIBBON CAMPAIGN X AGAINST HTML MAIL / \
Quoted 10 lines · On Thursday September 23, [email protected] wrote:
Quoted 5 lines · Doug Ledford
Yeah, IBM wrote a little app to help with that. We stuffed it into the mdadm package we ship since that seemed the most appropriate place for it. It's called mdmpd and that's it's job basically. Very simple app, but doesn't run on upstream kernels at the moment (it wants the md event interface which hasn't yet been submitted upstream by Neil).
iirc the it has been submitted and refused
http://marc.theaimsgroup.com/?t=109417961300007&r=1&w=2&n=8
Well..... it's still alive in -mm, and where there's life, there's hope (and need of vitals, as the old Gaffer usually added:-).
NeilBrown
Quoted 8 lines · Neil Brown
Quoted 7 lines · On Thursday September 23, [email protected] wrote:
Quoted 5 lines · Doug Ledford
Yeah, IBM wrote a little app to help with that. We stuffed it into the mdadm package we ship since that seemed the most appropriate place for it. It's called mdmpd and that's it's job basically. Very simple app, but doesn't run on upstream kernels at the moment (it wants the md event interface which hasn't yet been submitted upstream by Neil).
Quoted 6 lines
Quoted 3 lines
iirc the it has been submitted and refused
http://marc.theaimsgroup.com/?t=109417961300007&r=1&w=2&n=8Well..... it's still alive in -mm, and where there's life, there's hope (and need of vitals, as the old Gaffer usually added:-).
I didn't know that. You know, I resubbed to linux-kernel about 2 days after this conversation ended, so I read the archives yesterday and wanted to write some responses to what was in there today. Now, whether I'll have the time to get to it today or not is one thing, but I am planning on responding and it's nice to know that it's not *totally* dead already.
Signature · 4 lines
Doug Ledford <[email protected]> 919-754-3700 x44233 Red Hat, Inc. 1801 Varsity Dr. Raleigh, NC 27606