16 December 2015

DPM Workshop 2015

This year, the annual DPM Collaboration Workshop was hosted by the core DPM development team themselves, at CERN.

As usual, it was a two-day affair, headed by regional site reports from around the world - Italy, France, Australia and the UK reported.

One common topic between the site reports was discussion of the different approaches to configuring/maintaining DPM instances - France being a Quattor-dominant zone, with separately maintained configuration, Italy using a Foreman-driven puppet (requiring some modification of the "standard" DPM puppet scripts), and Australia and the UK using the standard puppet local config or hand-configured systems.
Another common topic was discussion of the recent ATLAS-driven storage tasks - the decommissioning of the large amount of data in PRODDISK, and the push for the new data dumps for consistency checking. People were generally rather unhappy about the PRODDISK task, especially in how slow it was to actually delete all the data from the token using the rfio-based admin tools. The newer davix-tools, which should be much faster, are unfortunately much less well known at present.

(Several sites also called out Andrey Kirianov's dpm-dbck tool for deep database linting and consistency checking as being especially useful - and he had a talk later in the workshop describing his future release plans, incorporating feedback.)

The Core DPM updates followed, beginning with the news that the DPM (and thus also LFC) team is being brought back into the fold of CERN IT-DSS, where they'll join the EOS, Castor and FTS teams. The DPM devs were each at pains to indicate that this would have no visible effects in the "short term".
As for the future plans of the group, they focussed much on the well-known existing topics of the last year: removing the dependance on rfio, and srm, and leveraging this freedom to improve other aspects of the system (for example, better support for multiple checksums on files).
Already, DPM 1.8.10 provides the "beta" versions of the new DPM Space Reporting functionality (which is intended to supplant the use of SRM for space reporting), although it is currently off by default (and is unable to track storage changes made via SRM). This functionality also broke the beta EGI Storage Accounting tools, until I submitted a patch to stop directory sizes being counted as well as the size of the individual files contained (essentially n-ple counting every file in the final tally, with n the directory depth of the file!).
GridFTP Redirection is another feature which the DPM team were keen to point to as a functioning enhancement in DPM 1.8.10. As with the Space Reporting, however, it comes with caveats: the enhanced performance gained from handing off from the Head nodes' GridFTP service to the correct pool nodes' GridFTP as part of the handshake is only possible for clients which support the Delayed PASV operation mode. Neither gfal1 (ie lcg-cp) nor uberftp support this, causing the new GridFTP to fall back to a much less efficient mechanism, which actually has worse network characteristics than the old GridFTP. Additionally, as this mechanism depends on patching GridFTP itself, new releases are needed every time a new Globus release happens...

The focus of the DPM core in the next year is to be removing the rfio underpinnings of DPM, which are used for all of the low-level communication between the head and pool nodes, in favour of a new, RESTful interface based on FastCGI. This is based on some exploratory work by Eric Cheung, and is also planned to bring in additional enhancements for non-SRM DPMs (for example, fully supported space management with directory quotas).

While I did the UK report, Bristol's Luke Kreszko headed up the afternoon's talks with a great talk on the experience of running a lightweight "dmlite" style DPM on HDFS, without an SRM. While Luke had some bugs to report, he was keen to make clear that Andrea and the rest of the DPM core team had responded extremely quickly to each bug report and snag, and that the system was improving over time. (Outstanding issues include a lack of a way to throttle load on any particular service (which is also a wider concern of the UK), and a desire to have more intelligent selection of servers for each request.)

There was also an interesting series of talks on the HTTP provision for Experiments - firstly, from the perspective of the HTTP Deployment Task Force, which exists to manage and enable the deployment of HTTP/WebDAV interfaces at sites with the functionality needed to support the Experiments, and secondly from the announcement of an initiative to attempt to run ATLAS work against a "pure" HTTP DPM. (This was a little short of the promise of the title, as data transfers were envisaged to occur over GridFTP, using the new GridFTP redirection, rather than HTTP.)

We also learned, from the Belle 2 Experiment, that they were currently rolling out their storage and data management infrastructure based entirely around DPM and the LFC as a file catalog!

Heading up the day, there was also a talk on the CMS "performance testing" for DPM sites using the CMS AAA Xrootd Federation. I have to say, I wasn't entirely convinced by some of the "improvements" demonstrated by the graphs, as the "new" tests seemed to stop before approaching the kinds of load which were problematic for the "old" tests. However, it was an interesting insight into CMS's thinking about dealing with the fact that not all sites will meet their performance standards for AAA, with the idea of two Federations (a "proper" AAA, and a "not as good" AAA) existing, with sites being shunted transparently from the "proper" one to the backup if their performance characteristics dropped sufficiently to compromise the federation.

The rest of the meeting was mostly feedback and live demos of the Puppet configuration and existing tools, which was useful and involving.

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