On Fri, Jan 18, 2019 at 9:25 AM Burkhard Linke
<burkhard.li...@computational.bio.uni-giessen.de> wrote:
>
> 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.
Only if the journal needs replaying. If you ensure a clean unmount
after writing the data, it shouldn't need to write to the underlying
block device on subsequent read-only mounts.
As an additional safeguard, map the image with -o ro. This way the
block device will be read-only from the get-go.
Thanks,
Ilya
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