The PPD dCache Storage Element

DCache is the main disk storage system in us on the PPD Unix/Linux systems. It allows us to aggregate the storage space on many servers and present this as a unified namespace to users.

Files in dCache can be read using the SRM "grid" protocol or the "local" protocols dcap and xrootd. For writing files only the SRM grid protocolcan be used and requires you to have a valid voms proxy for a supported VO.

The dCache namespace can also be browsed like a normal filesystem on the PPD linux nodes but the contents of the files cannot be accessed through this (i.e. ls or find will work cp, open or mv will fail).

VOs and/or groups have area to store files under their name dCache namespace in /pnfs/pp.rl.ac.uk/data/

Inspecting a file using the pnfs mount

Simply use ls as you would with a local file.

ls -l /pnfs/pp.rl.ac.uk/data/vo/path/to/file

Converting a local file path in PNFS to a URL to access the file

For srm add srm://heplnx204.pp.rl.ac.uk to the path beginning /pnfs

i.e.

srm://heplnx204.pp.rl.ac.uk/pnfs/pp.rl.ac.uk/data/vo/path/to/file

For dcap add dcap://dcap.pp.rl.ac.uk to the path beginning /pnfs

i.e.

dcap://dcap.pp.rl.ac.uk/pnfs/pp.rl.ac.uk/data/vo/path/to/file

Note that the hosts are different heplnx204.pp.rl.ac.uk for srm and dcap.pp.rl.ac.uk for dcap

Copying/Reading a file using the Grid tools

To copy a file into dCache without registering it in a Logical File Catalogue

lcg-cp -v file:/path/to/local/file srm://heplnx204.pp.rl.ac.uk/pnfs/pp.rl.ac.uk/data/vo/path/to/put/file

To copy a file into dCache and register it with a Logical File Catalogue use the lcg-cr command instea, VOs that use the LFC may have some rules about how the LFC namespace and the SRM namespace should be related to one another.

Reading a file using local protocols

copying a file out of dCache using dcap:

dccp dcap://dcap.pp.rl.ac.uk/pnfs/pp.rl.ac.uk/data/vo/path/to/your/file ./file

Or, if pnfs is locally mounted:

dccp /pnfs/pp.rl.ac.uk/data/vo/path/to/your/file ./file

Access a file directly from dCache using root:

xrdcp root://dcap.pp.rl.ac.uk///pnfs/pp.rl.ac.uk/data/vo/path/to/your/file ./file

Deleting a file from dCache

If the file is not registered in the LFC:

lcg-del -l srm://heplnx204.pp.rl.ac.uk/pnfs/pp.rl.ac.uk/data/vo/path/to/file

If it is registered in the LFC remove the "-l" option:

lcg-del srm://heplnx204.pp.rl.ac.uk/pnfs/pp.rl.ac.uk/data/vo/path/to/file

-- ChrisBrew / RobHarper - 2010-03-12