Although this is a relatively old question, the answer is still the same. You have a virtual machine (running on a physical host) and some sort of storage (either shared storage – a FC SAN, iSCSI storage, an NFS share – or local storage).
With virtualisation, many virtual machines try to access the same physical resources at the same time. Due to physical limitations (number of read/write operations – IOPS; throughput; latency) there might be a problem to satisfy all storage requests of all physical machines at the same time. What usually happens: you will be able to see "SCSI retries" and failed SCSI operations in the operating systems of your virtual machines. If you get too many errors/retries in a certain period of time, the kernel will set the mounted filesystems read-only in order to prevent damage to the filesystem.
To cut the long story short: Your physical storage is not "powerful" enough. There are too many processes (virtual machines) accessing the storage system at the same time, your virtual machines do not get the response from the storage fast enough, and the filesystem goes read-only.
There are not terribly many things you can do. The obvious solution is better/additional storage. You can also modify the parameters for SCSI timeouts in the Linux kernel. Details are described, e.g., in:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1009465
http://www.cyberciti.biz/tips/vmware-esx-server-scsi-timeout-for-linux-guest.html
However, this will only "postpone" your problems, because the kernel only gets more time before the filesystem will be set read-only. (I.e., you do not solve the cause of the problem.)
My experience (several years with VMware) is that this problem only exists with Linux kernels (we're using RHEL and SLES) and not with Windows servers. Also, this problem occurs on all sorts of storage – FC, iSCSI, local storage. For us, the most critical (and expensive) component in our virtual infrastructure is storage. (We're now using HP LeftHand with 1 Gbps iSCSI connections, and have not had any storage issues ever since. We chose LeftHand (over traditional FC-solutions) for its scalability.
You can skip the first step, and simply:
sudo losetup /dev/loop0 /path/to/disk.img
mkdir /mnt/image
sudo mount /path/to/disk.img /mnt/image
Best Answer
Use a bind mount:
You can also put these in
/etc/fstab:The rbind option is needed to make submounts visible, for example because
/projects/apollois on a separate partition. ro can be used to mount it read-only.More details are available in the mount(8) man page, in the "Bind mount operation" section.