[NTLUG:Discuss] syslog message ReiserFS: dm-3 <--what's dm-3??

[Richard Geoffrion]

Ok...so I've finished the message...and I've figured out SOME of what I 
was asking.  I don't know how to do it without jumping through the hoops 
I did, but for now, the immediate concern is quelled.   So now....the 
rest of this post becomes academic.  What is a dm-3 why is it used 
instead of like..oh..the device name or the LVM name??   How do I 
correlate the two (dm-x --vs-- /dev/X)?  What is that 'dm-3' thing 
called...so I know what I need to google for?  I don't even know WHAT! I 
need to google for..so if you can point me in the right direction..then 
thanks..

-----------------------

Quick background.

Slackware 10, two motherboard controlled SATA drives as system 
disks(/dev/sda, /dev/sdb),  two 3ware raid controller with one 6-disk 
raid-5 array as /dev/sdc on one card and four single  hot-pull drives  
configured as /dev/sd[d,e,f, and g.]

LVM is configured on /dev/sd[d-g] with each disk being its own seperate 
Physical Volume(pv), Volume Group(vg), and Logical Volume(lv).

So I get this message in syslog.
Jul 25 02:53:25 gwibkp kernel: ReiserFS: dm-3: warning: zam-7001: io 
error in reiserfs_find_entry


OK..so what the heck is dm-3?  I'm  sure it's a device...but how / where 
does it get assigned and how do I coorelate it with either my /dev/$VG/ 
$LV  --or-- /dev/sdd-g  devices??

A snippet of  `cat /proc/diskstats`  shows:
  253    1 dm-1 0 0 0 0 0 0 0 0 0 0 0
  253    2 dm-2 0 0 0 0 0 0 0 0 0 0 0
  253    0 dm-0 0 0 0 0 0 0 0 0 0 0 0
  253    3 dm-3 0 0 0 0 0 0 0 0 0 0 0

...but that doesn't help me too much..

So here I am hoping that describing this would help the answer 
surface..but I still don't know.  What I DO know is that during the 
composition of this message I umount'ed two LVs that turned out to be  
dm-0 and dm-3.   I removed the dm-0 device from the hot pull tray and 
remounted the LV that had previously been referenced as dm-3.  When it 
mounted, I noticed that it now shows up in syslog as dm-0.  After a bit 
more hair pulling.. I ran `pvdisplay` and that DID do a good job of 
correlating /dev/sdg with the physical disk that I KNOW I umounted, left 
in and then re-mounted. So I have identified /dev/sdg as my 
failing/fsck-needing  identity-crisis drive.

Running the 3ware command also showed me in what order the system 
assigned /dev 's to my drives...  Since sda, sdb, and sdc are on the 
motherboard and other 3ware controller respectively...  the following 
information from the `tw_cli info c3` command shows me that   u0 = /sdd, 
u1=sde..etc..    So here is my confusion....  u3 WAS dm-3 when I started 
this whole process. (I didn't realize it at the time..but through my 
troubleshooting steps--I now know.)  After umounting the devices 
attached to u3 and u0, removing u0 all together..and then remounting u3 
-- well after doing all of that, u3 became dm-0 in the syslog kernel 
messages.  I'm now running reiserfsck on /$VG/$LV ( u3 ) and the kernel 
messages are referring to dm-0.   So.. I still don'tk now how to 
correlate the dm-X devices with my actual physical disks!!!!   I also 
don't understand why the 3ware driver won't stop seeing a drive that is 
NO LONGER IN THE @#$%@^@$ system!    Controller 3-Port 2 / U0 is NO 
LONGER IN THE BAY..but it's still listed.


Unit  UnitType  Status         %Cmpl  Stripe  Size(GB)  Cache  AVerify  
OvrECC
------------------------------------------------------------------------------
u0    SINGLE    OK             -      -       232.82    ON     OFF      
-       
u1    SINGLE    OK             -      -       232.82    ON     OFF      
-       
u2    SINGLE    OK             -      -       232.82    ON     OFF      
-       
u3    SINGLE    OK             -      -       232.82    ON     OFF      
-       

Port   Status           Unit   Size        Blocks        Serial
---------------------------------------------------------------
p0     OK               u1     233.76 GB   490234752     Y62A1H6E     
p1     OK               u2     233.76 GB   490234752     Y6304J2E     
p2     OK               u0     233.76 GB   490234752     Y6304J0E     
p3     OK               u3     233.76 GB   490234752     Y62Z8WGE     
p4     NOT-PRESENT      -      -           -             -
p5     NOT-PRESENT      -      -           -             -
p6     NOT-PRESENT      -      -           -             -
p7     NOT-PRESENT      -      -           -             -