Remount read-only device as read-write on xen guest

mountread-onlyubuntu-10.04xen

I have a xen guest machine running ubuntu 10.04 with a disk device marked as read only.

Now if while the xen guest is running I mark the device as read-write how can I tell the guest it can now write to it?

The obvious mount -o rw doesn't work it still says it's write protected. Restarting udev doesn't do anything, I also tried hdparm -r 0 /dev/xvdd then remounting which appears to work until the device is unmounted and remounted which then causes all written data to disappear.

Anyone know how to achieve this without rebooting the guest?

Best Answer

Try using the remount option with the mount command as in mount -o remount,rw /dev/xvdd. An example using ro can be found in /etc/init.d/umountroot.

Some virtual file systems write their changes to a separate files allowing multiple virtual servers to use one file as the base file system. Persistence options for the file system alter what happens when the virtual server is restarted.

Related Solutions

Linux – USB forwarding from dom0 to domU

I use Xen with USB passthrough to my domU on Centos5.

So far I can tell: Forward the complete USB-stuff or nothing. Do not try to forward only some OHCI/UHCI devices.

So here it runs more or less (haven't started a stress test yet):

Disconnect all USB devices
Unload the USB devices from dom0 via pciback. Unload UHCI/OHCI and EHCI devices. (Write some kind of init.d script for your needs). Details about pciback are here under point 1
Restart xend
lsusb should output nothing on dom0
edit you domU configuration file with the pci = [ '0000:yy:xx.0'] stuff
Start your domU
domU should report USB devices via lspci (other pciids) and lsusb

My dom0 is booted with kernel-parameter noirqdebug (grub).

Xen DomU – Xen DomU Root Filesystems Becoming Read-Only on iSCSI Virtual IP Failover

I eventually solved this by using the following advice and settings from the open-iscsi documentation:

8.2 iSCSI settings for iSCSI root
---------------------------------

When accessing the root parition directly through a iSCSI disk, the
iSCSI timers should be set so that iSCSI layer has several chances to try to
re-establish a session and so that commands are not quickly requeued to
the SCSI layer. Basically you want the opposite of when using dm-multipath.

For this setup, you can turn off iSCSI pings by setting:

node.conn[0].timeo.noop_out_interval = 0
node.conn[0].timeo.noop_out_timeout = 0

And you can turn the replacement_timer to a very long value:

node.session.timeo.replacement_timeout = 86400

After setting up the connection to each LUN as described above, the failover works like a charm, even if it takes several minutes to happen.

Best Answer

Related Solutions

Linux – USB forwarding from dom0 to domU

Xen DomU – Xen DomU Root Filesystems Becoming Read-Only on iSCSI Virtual IP Failover

Related Topic