When working with Xen virtual machines in production environments, filesystem operations can sometimes have unexpected consequences. A particularly tricky situation occurs when the root filesystem gets locked in read-only mode, especially with errors like:

[Mon Jul  7 14:59:06 2014] EXT4-fs error (device xvda): ext4_remount:4558: Abort forced by user

The error typically occurs when the ext4 filesystem encounters inconsistencies and the kernel enforces read-only mode for safety. Common triggers include:

• Improper device mapper operations
• Filesystem corruption
• Underlying storage issues
• Previous unclean unmount

Beyond the standard mount -o remount,rw / command, try these methods:

# Method 1: Force remount with sync option
mount -o remount,rw,sync /dev/xvda /

# Method 2: Using explicit device specification
mount -o remount,rw /dev/xvda /

# Method 3: Combined with filesystem check
fsck -nf /dev/xvda
mount -o remount,rw /dev/xvda /

If standard methods fail, consider these advanced techniques:

# Check for filesystem errors first
e2fsck -f /dev/xvda

# Attempt journal replay
tune2fs -f -O ^has_journal /dev/xvda
tune2fs -j /dev/xvda

# Alternative mount syntax
mount -n -o remount,rw /

For Xen VMs, additional steps might be needed:

# Check Xen block device status
xenstore-ls device/vbd

# Verify frontend/backend connection
xenstore-read /local/domain/0/backend/vbd/<domain-id>/<device-id>/params

When immediate remount isn't possible, temporary solutions include:

# Create overlay filesystem
mkdir /tmp/rwroot
mount -t tmpfs tmpfs /tmp/rwroot
mkdir -p /tmp/rwroot/{upper,work}
mount -t overlay overlay -o lowerdir=/,upperdir=/tmp/rwroot/upper,workdir=/tmp/rwroot/work /mnt/rw

• Always test device mapper operations in staging
• Implement proper LVM snapshots instead of direct RO mounts
• Monitor filesystem health with smartd and other tools
• Configure proper Xen disk cache modes (writethrough instead of writeback)

While attempting to create a filesystem copy by mounting an external device mapper as read-only, I inadvertently triggered a cascading effect that made my Xen VM's root filesystem (/) mount as read-only. The mount status shows:

/dev/xvda on / type ext4 (ro,relatime,errors=remount-ro,user_xattr,barrier=1,data=ordered)

Typical solutions like these don't work:

# Attempt 1: Basic remount
mount -o rw,remount /

# Attempt 2: Non-recursive remount  
mount -n -o remount,rw /

# Attempt 3: Force filesystem check
fsck -f /dev/xvda

The kernel logs reveal EXT4's protective mechanism kicking in:

[timestamp] EXT4-fs error (device xvda): ext4_remount:4558: Abort forced by user

When Ext4 encounters potential corruption (or forced RO state), it implements several safeguards:

• Journal verification locks
• Superblock protection flags
• Mount option persistence

Here's the complete procedure that worked for my Xen VM environment:

# Step 1: Unmount dependent filesystems
umount /proc
umount /sys
umount /dev/pts

# Step 2: Drop to single-user mode
telinit 1

# Step 3: Force remount with explicit device
mount -o remount,rw /dev/xvda /

# Step 4: Verify journal status
tune2fs -l /dev/xvda | grep -i journal

# Step 5: Reinitialize mount points
mount -a

If the standard method fails, try manipulating the virtual block device:

# Xen-specific block device refresh
echo 1 > /sys/block/xvda/device/rescan

# Force filesystem unfreeze
dmsetup suspend /dev/xvda
dmsetup resume /dev/xvda

To avoid recurrence:

• Always use -o ro,noload when mounting for backup
• Implement LVM snapshots instead of direct mounts
• Set up monitoring for unexpected RO transitions