Recently, I ran into an issue where all links under /dev/disk/by-id were missing. The way that devices show up under /dev/disk/by-id is when the scsi_id command queries the devices for VPD (SCSI inquiry Vital Product Data) and when the disk returns the information it can create the appropriate link. There are two ways of checking for that data, to do so run the following:

[root@rac2 ~]# scsi_id -p 0x80 -g -s /block/sda
[root@rac2 ~]# scsi_id -p 0x83 -g -s /block/sda
  • 0x83 is Device Identification Vital Product Data and 0x80 is Unit Serial Number

as you can see, nothing is returned. I did some research and I found that after ESX 4.0, a SCSI inquiry will not be answered within the VM. There is actually a VMware KB that talks about it, VMware KB The Case of VMware and the missing SCSI ID. Now to fix the issue we can set the following in the vmx file:

disk.EnableUUID = "TRUE"

and then it will correctly answer to the SCSI inquiry. I made the above change to a Linux VM, and here is what I saw:

[root@rac1 ~]# scsi_id -p 0x80 -g -s /block/sda
SVMware Virtual disk 6000c2907159c1057f111400f66d4642
[root@rac1 ~]# scsi_id -p 0x83 -g -s /block/sda
36000c2907159c1057f111400f66d4642

Now they are returning the VPD information. And also looking under /dev/disk/by-id, I see the following:

[root@rac1 ~]# ls -l /dev/disk/by-id/*
lrwxrwxrwx 1 root root 9 Mar 25 13:55 /dev/disk/by-id/ata-VMware_Virtual_IDE_CDROM_Drive_10000000000000000001 -> ../../hdc
lrwxrwxrwx 1 root root 9 Mar 25 13:55 /dev/disk/by-id/scsi-36000c2907159c1057f111400f66d4642 -> ../../sda
lrwxrwxrwx 1 root root 10 Mar 25 13:55 /dev/disk/by-id/scsi-36000c2907159c1057f111400f66d4642-part1 -> ../../sda1
lrwxrwxrwx 1 root root 10 Mar 25 13:55 /dev/disk/by-id/scsi-36000c2907159c1057f111400f66d4642-part2 -> ../../sda2
lrwxrwxrwx 1 root root 9 Mar 25 13:55 /dev/disk/by-id/scsi-36000c2993b74b940e6952b381d6be0d5 -> ../../sdb
lrwxrwxrwx 1 root root 10 Mar 25 13:55 /dev/disk/by-id/scsi-36000c2993b74b940e6952b381d6be0d5-part1 -> ../../sdb1
[root@rac1 ~]#

As a side note, under different distros the scsi_id command might be under different locations and might accept other flags. For Fedora 16, it looks like this:

[root@fed ~]$ /lib/udev/scsi_id -p 0x83 -g -d /dev/sda
35000c50024837909

and on Ubuntu:

root@ub: ~$ /lib/udev/scsi_id -p 0x83 -g -d /dev/sda
35000c86787963

and on RHEL

[root@rac1 ~]# /sbin/scsi_id -p 0x83 -g -s /block/sdb
36000c2993b74b940e6952b381d6be0d5

Published by Karim Elatov

26 March 2012

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