Skip to main content

Doing bus rescans to discover new LUNs

We discovered a serious issue when rescanning the SCSI bus on (Oracle/Red Hat) Linux to discover newly added LUNs and I thought I'd mention it here.

Our (old) Dell PowerEdge 2950 come with virtual media to allow mounting virtual floppy images (for driver disks, e.g.) and virtual CDs (great for ISOs). Dell adds these devices to the USB SCSI bus and therefore they show up as /dev/sda and /dev/sdb. I.e. before any regular (boot) media. While it makes sense, it creates a problem for us when adding new SAN LUNs to an existing Oracle database server, because we need to add some ASM disks.

When doing the bus rescan
echo "- - -" > /sys/class/scsi_host/host3/scan
the virtual media get enumerated again as sda and sdb, which throws off existing device mapping. Our boot LUN which was sdb is now sdd and that used to be an ASM disk. After initialization with ASM, my boot LUN gets wiped and things get ugly. No root device, read-only access and upon reboot grub fails because boot devices are lost. Enter panic mode!

I can think of two three possible work-arounds:
1) rescan only the FC SCSI busses i.e. echo "1 - -" or something
2) Use Qlogic's HBA utilities to rescan and hopefully prevent the Dell virtual media from appearing
3) Using udev to prevent disk IDs from getting "lost" as udev IDs are unique, always, by design.

I'll let you know what happens... BTW, we are using OEL 4u5 x86_64 on our servers.
Labels:

Comments

Post a Comment

Popular posts from this blog

Preventing PuTTY timeouts

Just found a great tip to prevent timeouts of PuTTY sessions. I'm fine with timeouts by the host, but in our case the firewall kills sessions after 30 minutes of inactivity... When using PuTTY to ssh to your Linux/Unix servers, be sure to use the feature to send NULL packets to prevent a timeout. I've set it to once every 900 seconds, i.e. 15 minutes... See screenshot on the right.
1 comment
Read more

Tuning the nscd name cache daemon

I've been playing a bit with the nscd now and want to share some tips related to tuning the nscd.conf file. To see how the DNS cache is doing, use nscd -g. nscd configuration: 0 server debug level 26m 57s server runtime 5 current number of threads 32 maximum number of threads 0 number of times clients had to wait yes paranoia mode enabled 3600 restart internal passwd cache: no cache is enabled [other zero output removed] group cache: no cache is enabled [other zero output removed] hosts cache: yes cache is enabled yes cache is persistent yes cache is shared 211 suggested size 216064 total data pool size 1144 used data pool size 3600 seconds time to live for positive entries 20 seconds time to live for negative entries 66254 cache hi...
2 comments
Read more

Dell Linux - OMSA Hardware Monitoring

Just getting started using Dell's OpenManage Server Administrator (OMSA) on our Oracle Linux platform. There are some confusing instructions going around so it's not immediately clear what to do, hence my blogging here. :) There is a site on Dell - Hardware Monitoring , as well as a wiki with instruction on how to setup their OMSA tooling using yum or up2date. [update]My first update for their instructions: be sure your server has Internet access, as most servers will use a proxy or so. use export http_proxy=http://yourproxy.example.com:port to configure it just for the session, and setup up2date to use an HTTP proxy by editing the settings in /etc/sysconfig/rhn/up2date .
2 comments
Read more