Dfyjkt

Few days ago we have encountered an unexpected error where one of the mounted drive on our RedHat linux machine became Read-Only. The issue was cause by the network outage in the datacenter.

Now I need to see if I can reproduce the same behavior where drive will be re-mounted as Read-Only while application is running.

I tried to remounted it was read-only but that didn't work because there are files that are opened (logs being written).

Is there a way to temporary cause the read-only if I have root access to the machine (but no access to the hypervisor).

That volume is mounted via /etc/fstab. Here is the record:

UUID=abfe2bbb-a8b6-4ae0-b8da-727cc788838f / ext4 defaults 1 1
UUID=8c828be6-bf54-4fe6-b68a-eec863d80133 /opt/sunapp ext4 rw 0 2

Here are the output of few commands that shows details about our mounted drive. I can add more details as needed.

Output of fdisk -l

Disk /dev/vda: 268.4 GB, 268435456000 bytes, 524288000 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x0008ba5f

 Device Boot Start End Blocks Id System
/dev/vda1 * 2048 524287966 262142959+ 83 Linux

Disk /dev/vdb: 42.9 GB, 42949672960 bytes, 83886080 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Output of lsblk command:

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
vda 253:0 0 80G 0 disk 
└─vda1 253:1 0 80G 0 part /
vdb 253:16 0 250G 0 disk /opt/sunup

Output of blkid command:

/dev/vda1: UUID="abfe2bbb-a8b6-4ae0-b8da-727cc788838f" TYPE="ext4" 
/dev/sr0: UUID="2017-11-13-13-33-07-00" LABEL="config-2" TYPE="iso9660" 
/dev/vdb: UUID="8c828be6-bf54-4fe6-b68a-eec863d80133" TYPE="ext4" 

Output of parted -l command:

Warning: Unable to open /dev/sr0 read-write (Read-only file system). /dev/sr0
has been opened read-only.
Error: /dev/sr0: unrecognised disk label
Model: QEMU QEMU DVD-ROM (scsi) 
Disk /dev/sr0: 461kB
Sector size (logical/physical): 2048B/2048B
Partition Table: unknown
Disk Flags: 

Model: Virtio Block Device (virtblk)
Disk /dev/vda: 268GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags: 

Number Start End Size Type File system Flags
 1 1049kB 268GB 268GB primary ext4 boot


Model: Virtio Block Device (virtblk)
Disk /dev/vdb: 42.9GB
Sector size (logical/physical): 512B/512B
Partition Table: loop
Disk Flags: 

Number Start End Size File system Flags
 1 0.00B 42.9GB 42.9GB ext4

Yes, you can do it. But the method proposed here may cause data loss, so use it only for testing.

Supposing you have /dev/vdb mounted as /opt/sunapp, do this:

1. First, unmount it. You may need to shut down any applications using it first.


2. 
   

   Configure a loop device to mirror the contents of /dev/vdb:

   
   
   

   losetup /dev/loop0 /dev/vdb
   

   
   


3. 
   

   Then, mount /dev/loop0 instead of /dev/vdb:

   
   
   

   mount /dev/loop0 /opt/sunapp -o rw,errors=remount-ro
   

   
   


4. 
   

   Now, you can run your application. When it is time to make /opt/sunapp read-only, use this command:

   
   
   

   blockdev --setro /dev/vdb
   

   
   
   

   After that, attempts to write to /dev/loop0 will result in I/O errors. As soon as file system driver detects this, it will remount the file system as read-only.

   
   

To restore everything back, you will need to unmount /opt/sunapp, detach the loop device, and make /dev/vdb writable again:

umount /opt/sunapp
losetup -d /dev/loop0
blockdev --setrw /dev/vdb

When I had some issues like corrupted disks, I had used ntfsfix.
Please see if these commands, solve the problem.

sudo ntfsfix /dev/vda
sudo ntfsfix /dev/vdb