is a control utility for accessing and manipulating file layout
and location data in the Ceph distributed file system.
Choose one of the following three commands:
show_layout View the layout information on a file or directory
set_layout Set the layout information on a file or directory.
show_location View the location information on a file
Your applicable options differ depending on whether you are setting
or viewing layout/location.
Specify an offset for which to retrieve location data
Set the size of each stripe
Set the number of stripes per object
Set the size of the objects to stripe across
Set the pool (by numeric value, not name!) to use
Set the preferred OSD to use as the primary
When setting layout data, the specified stripe unit and stripe count
must multiply to the size of an object. Any parameters you don't set explicitly are left at the system defaults.
Obviously setting the layout of a file and a directory means different things.
Setting the layout of a file specifies exactly how to place the individual file. This must be done before writing *any* data to it. Truncating a file does not allow you to change the layout either.
Setting the layout of a directory sets the "default layout", which is used to set the file layouts on any files subsequently created in the directory (or any subdirectory). Pre-existing files do not have their layouts changed.
You'll notice that the layout information allows you to specify a preferred OSD for placement. This is allowed but is not recommended since it can dramatically unbalance your storage cluster's space utilization.