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Boot Solaris with a RO root filesystem

Topics: 
OpenSolaris

I just finished a very interesting case of a coredumping TSM client on Solaris. After investigation of the core dump, it seemed that the TSM client barfed over an erroneous inode. Some more diagnosis revealed indeed filesystem corruption, unfortunately on the root file system. Normally, one would boot from CDROM or issue a netboot, to correct the corruption, but it turned out the Jumpstart config of the host was really foobarred. I neither did have the time to correct the Jumpstart server config, or walk over to the data center to insert a Solaris DVD.

At times like that, I resort to little tricks in the bootsequence of Solaris : if you boot with boot -a -s, you can specify the location of the startup files. If you enter a /dev/null for the /etc/system file, the host will continue to boot, but with a read-only filesystem :

Rebooting with command: boot -a -s
Boot device: /pci@8,600000/SUNW,qlc@4/fp@0,0/disk@0,0:b File and args: -a -s
Enter filename [kernel/sparcv9/unix]:
Enter default directory for modules [/platform/SUNW,Sun-Fire-280R/kernel /platform/sun4u/kernel /kernel /usr/kernel]:
=> Name of system file [etc/system]: /dev/null
SunOS Release 5.10 Version Generic_118833-24 64-bit
Copyright 1983-2006 Sun Microsystems, Inc. All rights reserved.
Use is subject to license terms.
root filesystem type [ufs]:
Enter physical name of root device
[/pci@8,600000/SUNW,qlc@4/fp@0,0/disk@w500000e0155145d1,0:b]:
Booting to milestone "milestone/single-user:default".
Hostname: qwerty
SUNW,eri0 : 100 Mbps full duplex link up
Requesting System Maintenance Mode
SINGLE USER MODE

Root password for system maintenance (control-d to bypass):
single-user privilege assigned to /dev/console.
Entering System Maintenance Mode

After a few rounds of fsck's, the root filesystem turned out to be corrected, and only 2 files seemed to be impacted by the file system check. As the TSM client worked again, I could easily restore those from the backup.