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RE: [dm-devel] Device Access Question

Title: Device Access Question
I suggest you use the dm-multipath mapped device names instead of the target device ones.  Otherwise, you can run into problems caused by the fact that the cached partition table for each of the possibly multiple target paths are not kept in synch with the partition table of the multipath mapped device.
In this case, fdisk is failing because it attempts to read the partition table (MBR block 0) of the disk and /dev/sdq is the passive path to the disk.  This is correct behavior for Linux dm-multipath since it does not attempt to provide multipathing for accesses to the target device paths, e.g., /dev/sdq.
Also, be careful because this behavior can be intermittent since this operation would have succeeded if this logical block was resident in the page cache, which could happen if /dev/sdq was an active path to the logical unit just a little while beforehand.

From: dm-devel-bounces redhat com [mailto:dm-devel-bounces redhat com] On Behalf Of Herr, Manuel
Sent: Thursday, September 28, 2006 6:03 AM
To: dm-devel redhat com
Subject: [dm-devel] Device Access Question

Is it correct that if an LUN is seen over 2 Paths,
1 active 1 standby i cannot fdisk the standby device?

#multipath -l
[size=11 GB][features="1 queue_if_no_path"][hwhandler="1 emc"]
\_ round-robin 0 [active]
 \_ 3:0:0:7 sdi 8:128 [active][ready]
\_ round-robin 0 [enabled]
 \_ 3:0:1:7 sdq 65:0  [active][ready]

#fdisk -l /dev/sdi
Disk /dev/sdi: 11.8 GB, 11811160064 bytes
255 heads, 63 sectors/track, 1435 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdi1               1        1435    11526606   83  Linux

#fdisk -l /dev/sdq
--> creates in /var/log/messages
Sep 28 12:01:22 saptf2 kernel: end_request: I/O error, dev sdq, sector 0
Sep 28 12:01:22 saptf2 kernel: Buffer I/O error on device sdq, logical block 0

Regards Manuel Herr

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