Hi,
On 1/17/19 7:27 PM, Void Star Nill wrote:
Hi,
We am trying to use Ceph in our products to address some of the use
cases. We think Ceph block device for us. One of the use cases is that
we have a number of jobs running in containers that need to have
Read-Only access to shared data. The data is written once and is
consumed multiple times. I have read through some of the similar
discussions and the recommendations on using CephFS for these
situations, but in our case Block device makes more sense as it fits
well with other use cases and restrictions we have around this use case.
The following scenario seems to work as expected when we tried on a
test cluster, but we wanted to get an expert opinion to see if there
would be any issues in production. The usage scenario is as follows:
- A block device is created with "--image-shared" options:
rbd create mypool/foo --size 4G --image-shared
- The image is mapped to a host, formatted in ext4 format (or other
file formats), mounted to a directory in read/write mode and data is
written to it. Please note that the image will be mapped in exclusive
write mode -- no other read/write mounts are allowed a this time.
- The volume is unmapped from the host and then mapped on to N number
of other hosts where it will be mounted in read-only mode and the data
is read simultaneously from N readers
There is no read-only ext4. Using the 'ro' mount option is by no means a
read-only access to the underlying storage. ext4 maintains a journal for
example, and needs to access and flush the journal on mount. You _WILL_
run into unexpected issues.
There are filesystems that are intended for this use case like ocfs2.
But they require extra overhead, since any parallel access to any kind
of data has its cost.
Regards,
Burkhard
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