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Old 06-12-2014, 05:45 AM   #1
Rohit_4739
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Redhat DM multipathing question


Hi All,

I have few questions about RHEL DM multipating,so can someone help me understand few things about it, i will start by putting multipath -ll output.

Code:
360060e8006d16e000000d16e000035c1 dm-10 HITACHI,OPEN-V
[size=64G][features=1 queue_if_no_path][hwhandler=0][rw]
\_ round-robin 0 [prio=4][active]
 \_ 3:0:0:4  sdac 65:192 [active][ready]
 \_ 3:0:1:4  sdao 66:128 [active][ready]
 \_ 2:0:0:4  sde  8:64   [active][ready]
 \_ 2:0:1:4  sdq  65:0   [active][ready]
What i want to know from this is
a) What is dm-12 here? Is it the multipath device created by Multipathd for the SAN LUN (with wwid 360060e8006d16e000000d16e000035c1)which is seen by OS?
I understand this is a 4 patehd device and there are 4 paths for every LUN with 2 paths on each HBA. So then
b) What does sdac,sdao,sde and sdq mean here beacuse we already have a dm-12 as multipath device of 64G. Further if i look for these devices with fdisk, each one shows as individual disk of 64G each including the dm device itself.

Please do correct me if there is something wrong with my understanding, becasue i am not sure how exactly things are working in background. As far as i know is that SAN allocates a LUN to server and maps it to the server via 4 paths for redundancy so that if 1 path goes down device is still up for I/O. And these device labels sdac,sdao,sde and sdq are created by OS only and SAN has no knowledge of these. So why OS needs to create these labels when it already knows that the device is coming via dm setup and has four paths.

Further more, ultimately when we are to use these LUNs for any purpose we are referring the sdac, sdao and sde,sdq; so imagine a situation if i try to create 4 different file systems from these devices; what will happen in that case. Here is the fdisk output also for the same

Code:
Disk /dev/dm-12: 68.7 GB, 68720394240 bytes
255 heads, 63 sectors/track, 8354 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

      Device Boot      Start         End      Blocks   Id  System
/dev/dm-12p1               1        8354    67103473+  83  Linux


Disk /dev/sdac: 68.7 GB, 68720394240 bytes
255 heads, 63 sectors/track, 8354 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

    Device Boot      Start         End      Blocks   Id  System
/dev/sdac1               1        8354    67103473+  83  Linux


Disk /dev/sdao: 68.7 GB, 68720394240 bytes
255 heads, 63 sectors/track, 8354 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

    Device Boot      Start         End      Blocks   Id  System
/dev/sdao1               1        8354    67103473+  83  Linux


Disk /dev/sde: 68.7 GB, 68720394240 bytes
255 heads, 63 sectors/track, 8354 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sde1               1        8354    67103473+  83  Linux


Disk /dev/sdq: 68.7 GB, 68720394240 bytes
255 heads, 63 sectors/track, 8354 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdq1               1        8354    67103473+  83  Linux
Please spare me and correct me if i am understaindg something really wrong here.
 
Old 06-12-2014, 09:42 AM   #2
AlucardZero
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dm-10 (you wrote 12 but the output you pasted says 10) is the multipath device. sdac, ao, e, and q are the same disk, but the system sees it four times, one for each for path. You will want to deal with 360060e8006d16e000000d16e000035c1 and ignore sdac/ao/e/q and dm-10. If you perform an operation on an individual path, the change will show up on all 5 views, but you will be bypassing multipath's load balancing (if any) and redundancy.

Last edited by AlucardZero; 06-12-2014 at 03:43 PM.
 
Old 06-12-2014, 11:44 AM   #3
sgrlscz
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Actually, you should not use the dm-* files. Per the Red Hat documentation - "Any devices of the form /dev/dm-n are for internal use only and should never be used." - See 2.1. Multipath Device Identifiers - https://access.redhat.com/site/docum...h_devices.html

You would use /dev/mapper/360060e8006d16e000000d16e000035c1. If you configure "user_friendly_names", you'd get something like /dev/mapper/mpatha.
 
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