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Hi Paul,

On Wed, 6 Apr 2011, Paul T. Keener wrote:

> The centralized storage, both SRM and general use, are physically located on 
> same machine and either or both may be made up of several distinct 
> filesystems.
>
> The main motivation for this configuration is to preserve flexibility. To 
> some extend, I would rather have a single namespace, but I want to be able to 
> absolutely reserve space on the SRM side, and that doesn't seem possible in 
> the current software.
Actually, it is. You would have to setup a separate space group that would 
be tied to one or more filesystem partitions. Then the SRM would need to 
specify (normally via cgi information) the space name to be used for 
allocating space. So, when the SRM creates a file, the space would come 
out of special partitions and everyone else would use whatever you allow 
otherwise (i.e. the default space). Example:

in your config:
oss.space srm    /the_srm_partition_mount_point
oss.space public /the_default_partition_mount_point

Using a target filename of the form:

/path?cgroup=srm

would allocate space from the srm partition while not specifying the 
cgroup would allocate space from the "public" partition. You can have as 
many partitions as you want in each category. This way you can segragate 
space allocation within a common name space.

In practice, this may be difficult because you might not be able to add 
cgi information to the SRM file name. So, separate name space would be 
needed.

> Is it correct that I should run two data server xrootd processes, one to 
> handle the SRM namespace and one to handle the central general use namespace?
That would be one way of doing it. However, if you are using separate name 
spaces *and* you only need one special partition to reserve space you can 
do it with one xrootd and use symlinks. Example

/xrootd/srm -> the reserved partion (i.e., you precreate this in the 
server).

Then anyone allocating files in /xrootd would use the common partition 
while files in /xrootd/srm would vector into the special partition. Things 
get a bit more complicated if you want more partitions in the public area. 
Using an LVM (Logical Volume Manager) normally solves this problem.

If you want to leave it open-ended and can't use an LVM, then yes, you 
would need two xrootd's.

> Is there any problem running the three namespaces through a single
> redirector?
Not at all, you can have as many as you want and they can be disjoint 
between data servers.

> Is there anything special I should be aware of in trying to set this up?
Other than keeping tin mind what I outlined, I don't think so.

Andy