The way the Linux SD driver works is it reserves 16 minor 'slots' per block device, the first minor represents the whole disk, and the remaining 15 are reserved for partitions. Knowing this offset one can write a little python code to create an index table that aids debugging, mapping minor numbers back to block device names. This of course presumes that names are being recycled.
# python -c "print zip(range(0,443,16), map(chr, range(97, 123)))"
[(0, 'a'), (16, 'b'), (32, 'c'), (48, 'd'), (64, 'e'), (80, 'f'), (96, 'g'), (112, 'h'), (128, 'i'), (144, 'j'), (160, 'k'), (176, 'l'), (192, 'm'), (208, 'n'), (224, 'o'), (240, 'p'), (256, 'q'), (272, 'r'), (288, 's'), (304, 't'), (320, 'u'), (336, 'v'), (352, 'w'), (368, 'x'), (384, 'y'), (400, 'z')]
So sdb would be 8,16 and sdm would be 8,192, sdm1 is 8,193. Now the next time you see something awful like this:
Jul 10 15:08:25 | checker failed path 8:144 in map mpath2
Jul 10 15:08:25 | libdevmapper: ioctl/libdm-iface.c(1740): dm message mpath2 NF fail_path 8:144
[16384]
You can say with confidence that this is device sdj.
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