Interpreting DM-Multipath output in Linux

[satish]

Dear Erman,

Recently,i was introduced to multipathing in our production environment and had not heard about this earlier.After some digging,i think i am starting to get a handle on how the concept works in theory but having trouble if i look into the box and need your support

from multipath -11,Got below output

PRD-DATA19 (360002ac0000000000000007e0001c9fc) dm-46 3PARdata,VV
size=100G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
  |- 2:0:0:53 sdar 66:176  active ready running
  |- 1:0:1:53 sdff 130:16  active ready running
  |- 2:0:1:53 sdcy 70:96   active ready running
  `- 1:0:0:53 sdhm 133:192 active ready running


From theory,When you have multiple paths to a LUN, Linux creates a SCSI device for each path. This means that a single LUN might appear as/dev/sdar,/dev/sdff,/dev/sdcy,/dev/sdhm if there are four paths to it. To make it easy to keep track of the LUNs, DM-Multipath creates a single device in /dev/mapper/ for each LUN that includes all the paths.
For example, /dev/mapper/360002ac0000000000000007e0001c9fc is the multipath device that is created on top of /dev/sdar,/dev/sdff,/dev/sdcy,/dev/sdhm

1)Are the below scsi devices physical? OR just the names assigned by Linux?
sdar,sdff,sdcy,sdhm

2)Upto my knowledge,all these four cane be called as paths to a storage lun(360002ac0000000000000007e0001c9fc)?

3)Can we identify what the scsi device that is mapped to this lun(360002ac0000000000000007e0001c9fc)

4)In above,case i have 100GB lun assigned?please correct me if am wrong

Thank you

[ErmanArslansOracleBlog]

sdhm , sdcy , sdff  and sdar are the block device names.
They all point to the same device.. So you can think that they are identical.
"They are pointing to the same  device."
Multipath devices are like the logical devices, they are on top of sd devices.
They are orchestrating the I/O to the sd disks..
sd disks are pointing to the same devices, but they are pointing to the same device using different paths.
(through Different HBAs, different cables or different SAN switches)
So, multipath and multipath logical devices operate on /dev/sd devices, they are managing the I/O going through these sd devices.
They are managing it, it terms of load balancing and fault tolerancy.

I hope you get the idea.

[satish]

Dear Erman,

Thanks for the update.Now i have got some idea

1)How these block device names sdhm , sdcy , sdff  and sdar are assigned?
2)As said,They all point to the same device.so in our case they all pointing to 360002ac0000000000000007e0001c9fc.please correct me if we are wrong

Thank you,
satish

[ErmanArslansOracleBlog]

1) OS gives the names. sd standars for scsi disk. for example: sda1 means the first partition of scsi disk named a. It is in the form of sdlp, where l is aletter denoting the physical drive, and p is a number denoting the partition on that physical drive.
read this -> http://man7.org/linux/man-pages/man4/sd.4.html

so, in your case, you have 4 letters(rather than 3).. sdhm..
This is probably because you have lots of scsi devices , which are in use.. So OS needs to create the sd device names using 4 digits.

2) Yes, they point to the same device. You can see it by checking the scsi wwids of those disk's (/dev/sd*)

[satish]

It is now clear to us.Thanks for all the support we get through this blog.