That’s what CERN produces from their 6 experiments each year. How this data is subsequently accessed and replicated around the world is an interesting tale in multi-tier data grids.
When an experiment is run at CERN, the data is captured locally in what’s called Tier 0. After some initial processing, this data is stored on tape at Tier 0 (CERN) and then replicated to over 10 Tier 1 locations around the world which then become a second permanent repository for all CERN experiment data. Tier 2 centers can request data from Tier 1 locations and process the data and return results to Tier 1 for permanent storage. Tier 3 data centers can request data and processing time from Tier 2 centers to analyze the CERN data.
Each experiment has it’s own set of Tier 1 data centers that store its results. According to the latest technical description I could find, the Tier 0 (at CERN) and most Tier 1 data centers provide a tape storage permanent repository for experimental data frontended by a disk cache. Tier 2 can have similar resources but are not expected to be a permanent repository for data.
Each Tier 1 data center has it’s own hierarchical management system (HMS) or mass storage system (MSS) based on any number of software packages such as HPSS, CASTOR, Enstore, dCache, DPM, etc., most of which are open source products. But regardless of the HMS/MSS implementation they all a set of generic storage management services based on the Storage Resource Manager (SRM) as defined by a consortium of research centers and provide a set of file transfer protocols defined by yet another set of standards by Globus or gLite.
Each Tier 1 data center manages their own storage element (SE). Each experiment storage element has disk storage with optionally tape storage (using one or more of the above disk caching packages) and provides authentication/security, provides file transport, and maintains catalogs and local databases. These catalogs and local databases index the data sets or files available on the grid for each experiment.
Data stored in the grid are considered read-only and can never be modified. It is intended for users that need to process this data read it from Tier 1 data centers, process the data and create new data which is then stored in the grid. New data added Data to be placed in the grid must be registered to the LCG file catalogue and transferred to a storage element to be replicated throughout the grid.
CERN data grid file access
“Files in the Grid can be referred to by different names: Grid Unique IDentifier (GUID), Logical File Name (LFN), Storage URL (SURL) and Transport URL (TURL). While the GUIDs and LFNs identify a file irrespective of its location, the SURLs and TURLs contain information about where a physical replica is located, and how it can be accessed.” (taken from the gLite user guide).
- GUIDs look like guid:<unique_string> files are given an unique GUID when created and can never be changed. The unique string portion of the GUID is typically a combination of MAC address and time-stamp and is unique across the grid.
- LFNs look like lfn:<unique_string> files can have many different LFNs all pointing or linking to the same data. LFN unique strings typically follow unix-like conventions for file links.
- SURLs look like srm:<se_hostname>/path and provide a way to access data located at a storage element. SURLs are transformed to TURLs. SURLs are immutable and are unique to a storage element.
- TURLs look like <protocol>://<se_hostname>:<port>/path and are obtained dynamically from a storage element. TURLs can have any format after the // that uniquely identifies the file to the storage element but typically they have a se_hostname, port and file path.
GUIDs and LFNs are used to lookup a data set in the global LCG file catalogue . After file lookup a set of site specific replicas are returned (via SURLs) which are used to request file transfer/access from a nearby storage element. The storage element accepts the file’s SURL and assigns a TURL which can then be used to transfer the data to wherever it’s needed. TURLs can specify any file transfer protocol supported across the grid
CERN data grid file transfer protocols supported for transfer and access currently include:
- GSIFTP – a grid security interface enabled subset of the GRIDFTP interface as defined by Globus
- gsidcap – a grid security interface enabled feature of dCache for ftp access.
- rfio – remote file I/O supported by DPM. There is both a secure and an insecure version of rfio.
- file access – local file access protocols used to access the file data locally at the storage element.
While all storage elements provide the GSIFTP protocol, the other protocols supported depend on the underlying HMS/MSS system implemented by the storage element for each experiment. Most experiments use one type of MSS throughout their world wide storage elements and as such, offer the same file transfer protocols throughout the world.
If all this sounds confusing, it is. Imagine 15PB a year of data replicated to over 10 Tier 1 data centers which can then be securely processed by over 160 Tier 2 data centers around the world. All this supports literally thousands of scientist who have access to every byte of data created by CERN experiments and scientists that post process this data.
Just exactly how this data is replicated to the Tier 1 data centers and how a scientists processes such data must be the subject for other posts.
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