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dCache/OSM

The Mass Storage Service (dCache/OSM)

Computer Center

The Mass Storage Service (dCache/OSM)


1. Before you use the mass storage service
2. About the software
3. About the hardware
4. When is it reasonable to use the mass storage ?
5. Access management
6. The migration directory
6.1 Command restrictions
6.2 This is forbidden
6.3. dccp is deprecated
7 Examples
7.1 Check the return code

1. Before you use the mass storage service

Please read this page completely. In case of questions about the usage please contact uco-zn@desy.de.

In case of changes or problems concerning the dCache service, messages will be sent to the mailing list dcache-user-zeuthen. Subscribe by clicking on the link and send the mail.

Here you may query the status of dCache: dCache-status
Some dCache documentation

2. About the software

A disc cache system (dCache) is optimizing the access to the mass storage consisting of a tape library and many disk servers. The disk status can be queried. The dCache system provides two kinds of storage backends: disk-only (online) and tape-backed disks (nearline).
Data can be accessed using Posix NFS-4.1 mount (on WGS /pnfs/ifh.de/acs/…), by using https (WebDAV), xrootd or GridFTP protocol.

If directories are configured to use tape back-end, an Open Storage Manager (OSM) is performing the access to the files on tape transparently.
Reading from tape is triggered by accessing the file, if it is not available on disk cache.
Writing is triggered asynchronously by writing in a directory, which has been configured to write to tape. This configuration means also, that files - which are already written to tape - might be purged (according to last recently used) from disk if not enough disk space is left. Selection of tapes is done automatically by defined group (again defined per directory).

Unique and difficult to recreate data can have a second copy on a different tape, again this is defined per directory and should be requested via uco-zn@desy.de. Please also contact us in case you plan to massively stage data from tape.

3. About the hardware

The mass storage device is a robot with around 4900 slots for LTO-cartridges, from which 4000 are used for dCache tape backend.
The mass storage uses 5 tape drives of type LTO 6 and 2 tape drives of type LTO 4.
Native LTO 4 tape capacity for uncompressed data: 780 GB
Native LTO 6 tape capacity for uncompressed data: 2.4 TB

4. When is it reasonable to use the mass storage ?

You should use the mass storage only for files not smaller than 220 MB
The transfer of small files to tape results in
* start/stop operation for the tape drives
* dramatic loss of transfer speed
* high stress on tape material and on tape drives
* lower reliability

The maximum file size of 1TB is due to the some technical reasons. Currently it is not recommended to write that big files: 1000000MB/150MB/s~2hours, the client would always receive an error as its time-out is around 1h although the file might be written correctly.
Keep in mind that reading transfer rate is 160 MB/s, locating the file on tape takes about 1-80s, mounting the tape needs round about 30s, so transfer time for a file less than 1.6GB will mostly be from tape overhead.

If possible you should avoid to use it on a machine which does not have the file you want to copy to the mass storage on it's local file system.
Otherwise too much network traffic will be produced.

5. Access management

The access to the mass storage tape back-end is organized groupwise:

The user has no influence on the particular tape his file is transferred to. Tapes are organized by groups assigned to subdirectories in the migration directory. It may be subdivided like a usual UNIX directory, but in general only main directories have different groups. You see these directories under the name /acs/... on any machine on which this service is available.

It may be useful to assign separate tape pools to certain subdirectories of a group. For example for data that might be deleted altogether or for data that represent simply backup versions of your current data. Keep in mind that with 2.4 TB Tapes the amount of data shouldn't be much smaller than a tape, otherwise we waste slots in the robot. Tapes can be easier reused when you delete all data belonging to one group at a certain moment, or we can temporarily place tapes of a certain pool outside the robot in case we run out of empty tape slots.

In case you need to store data that would be unreproducible in the rare case of tape corruption we can arrange for a second copy to a different tape-group. Please contact uco when this is needed.
 
On which machine is the service available?

On any machine that has setup a link named /pnfs/ifh.de/acs. E.g. all farm machines and WGS. For data transfers to/from other (external) sites you should use the machine transfer.zeuthen.desy.de

Please send requests to setup the service on a certain machine, to setup /acs for your group or to setup tape pools to uco-zn@desy.de .

6. The migration directory

The migration directory is implemented as a posix filesystem (NFS-4.1). It shows the names and sizes of the files transferred to the mass storage. It looks like a usual UNIX directory but is simulated by a database. Most of the UNIX commands work in the migration directory as usual (e.g. cp, mkdir, ls, rm). Files cannot be modified, though. In case of a modification the file gets completely re-written.

The migration directory is provided by automounter on all machines which are configured for dCache as

/pnfs/ifh.de/acs

In addition pnfs maintains invisible for the user a mirror of the migration directory. It contains for each entry in the migration directory a short file (stubfile) with information about the transferred files necessary to find them on the mass storage.

For each group there is a subdirectory under /pnfs/ifh.de/acs. It may be subdivided like a usual UNIX directory.

6.1 Command restrictions

These commands are not available:

mv [src-file] [dest-file]

If [src-file] OR [dest-file] is part of the migration directory tree
use instead: dccp and rm (see example below).
note: If [src-file] AND [dest-file] are part of the migration directory tree "mv" is available,but it works safely only if the path of src and dest are the same.

6.2 This is forbidden in the migration directory:

mv [dir-name] ../[any-dir]

To move a subdirectory into another branch of the migration directory tree may cause problems, but no error messages are generated.
To move a subdirectory within the same directory (simple rename) is safe.

This has no affect for the migration directory:
sticky bit

The files you copy into the migration directory will have your current unix group. In case you want these files to have a unix group other than your unix login group you can use the command newgrp.

6.3. dccp is deprecated

The dccp client is deprecated! It has known limitations (can only access data which is readable by 'other' - e.g. mask 0644) and should no longer be used.

7 Usage Examples:

To simplify the access to the migration directory symbolic links can be used, e.g.:

cp /path/to/big/file /pnfs/ifh.de/acs/group/...

Archive a directory from Lustre:

cd /lustre/fs23/group/...
tar -cjvf /pnfs/ifh.de/acs/group/.../myarchive.tar.bz2 ./my-dataset

7.1 You must check the return code

After a successful data transfer cp returns 0. It's highly recommended to check the return code (e.g. $? in /bin/sh).

You can not overwrite an existing file in the migration directory. If a file is obsolete you need to remove it first.
In case of problems like for example you start cp and it is hanging for hours please send an email to uco-zn@desy.de

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