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When the XRootD component was originally written for Ceph by Sebastien Ponce it was designed in two parts. He create a libradosstriper which is part of the Ceph code and broke large files into small objects which were written as separate Objects in Ceph. The first Object has some metadata attached which contains the checksum, the file size and other information to access the complete file. More details of how the striping works can be found in the Ceph documentation. https://docstracker.ceph.com/enprojects/latestceph/architecture/wiki/Object_striping_in_librados .
The XrdCeph plugin is a Storage level plugin relies on libradosstriper to split up its
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libradosstriper is not officially supported by Cephmaintained by Redhat and there is no external development of the code. | If there is a problem we will have to fix it ourselves. We do not have experience in commiting code to the Ceph base. |
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XCaches do not work as reliably as originally anticipated. | Jobs that use Vector reads have performance problems / higher failure rates. There is an extra level of complication both from our point of view maintain many XCaches and from a user point of view when it comes to reading/writing to different endpoints.Deletes need to be sped up |
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Deletes of large files are slow because the chunks are deleted in series. | Currently large files can take several seconds to delete if not longer. This is not what VOs / software developers expect and causes problems. |
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Checksuming is currently performed by a script that downloads the file after a transfer is complete. | For large files this can take a long time and possibly cause a time out. It slows down the perceived performance on the cluster. |
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XrdCeph does not support Vector reads, so any series of small reads is simply executed in series. | This appears to cause significant issues for jobs that use Vector reads as the Caching layer infront often doesn’t work as expected. |
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Small read/writes against Echo are not efficient. | This results in very slow external transfers unless a buffer is added to allow the data to be buffered on the gateway before a larger transfers is done to Echo. |
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Unstable under high load | Disruptive for VOs and can impact them even if load is caused by another user. |
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\uD83D\uDC65 Stakeholders
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\uD83C\uDFAF Goals
The goal is to have a single code base combining libradosStriper and XrdCeph, which could be easily maintained by RAL. We would like to parellise trans
\uD83D\uDC63 Next steps
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