Experiences Deploying Xrootd at RAL

Chris Brew (RAL)

Contents

• Introduction to xrootd– What is xrootd?– How does xrootd work?– What are xrootd’s advantages?

• Deploying xrootd at RAL

What is xrootd?

• xrootd (eXtended Root Daemon) was written at SLAC and INFN Padova as part of the work to migrate the BaBar event store from Objectivity to Root I/O

• It’s a fully functional suite of tools for serving data, including server daemons and clients which take to each other using the xroot protocol

• Since Sept 2004 (Root V4.01-02) it has been distributed as part of the Root distribution as well as being separately available from http://xrootd.slac.stanford.edu

Xrootd Goals

• High Performance File-Based Access

• Fault tolerance• Flexible Security• Simplicity• Generality

xrootd Architecture

Protocol LayerProtocol Layer

Filesystem Logical LayerFilesystem Logical Layer

Filesystem Physical LayerFilesystem Physical Layer

Filesystem ImplementationFilesystem Implementation

Protocol ManagerProtocol Manager

(included in distribution)

Performance Features

• Scalable request/response protocol– Connection multiplexing– Heavily Multithreaded– Request redirection supported– Request deferral– Unsolicited Reverse Request Mode

• Adaptive Reconfigurationhttp://xrootd.slac.stanford.edu/presentations/Xrootd_Performance.htm

Simple Multi-Server Example

Data Serverxroot daemonolb daemon

Load Balancerolb daemonxrootd daemon

Client

Simple Load Balanced Example

Load Balanced Example with MSS

The Cache File System

• Xrootd allows us to span a single namespace across multiple servers

• CacheFS allows us to span a single namespace across multiple disks on the same server

• Distributed with xrootd but not a part of it• Provides scripts to manage namespace

– Namespace served by xrootd is just a tree of links /some/file/path/…

– Links point to /cacheNN/some%file%path%...

• Links to the…

MPS (Migration, Purging and Staging) Tools

• The MPS commands manage the interface between the disk cache and the MSS– Migration step copies new data from the disk to MSS– Purge step deletes unused data from disk– Stage step copies data back from MSS to disk when

requested by xrootd

• Supports pre-staging and stage request queuing • Files can be pinned on disk for a set period or

indefinitely• Calls user defined commands to interface to any

MSS

Deploying Xrootd at RAL

• Where we started– BaBar at RAL had 40TB increasing to

75TB of disk on 57 filesystems spread across 26 servers

– Single namespace was implemented using a large directory tree containing links to files on the disks

– Very difficult to maintain

• Where we wanted to get to:– All BaBar data files server via xrootd– Files backed up in ADS– No single point of failure– Simple deployment

Server Layers

• Xrootd/Olbd system serves files to clients

• CacheFS aggregates multiple FS’s on each server

• MPS manages the disk space, calls …

─────────────────────────• ADS link to put/retrieve files from … • Atlas Datastore, tape backup

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BaBar RAL Data Model

• Client’s locate data via two olb manager machines with one DNS alias

• Disks are held at 95-97% full so if a server fails requests for the data files it held automatically cause files to be staged from the ADS to the spare space

• Stage requests can also be triggered by excessive loads on a server

• A purge process on each server will eventually delete these extra files returning us to the initial position

• Daily catalogues of each filesystem should mean that we can rebuild lost filesystems from the ADS

Installation

• Rpm from SLAC provides basic software plus some additional packages but does not include:– Configuration– Start up scripts– Interface to our MSS

• These are provided by a local config rpm

• Both rpms are installed/monitored by RAL’s Yum/YumIT infrastrucure

RAL Config rpm

• Installs the ADS link software• Creates init.d entries for xrootd and olbd

and registers them with chkconfig• Builds server config file from common and

machine specific files• Sets up the CacheFS system across the

disk• Starts, condrestarts or stops the services

for install, upgrade or removal of the rpm

ADS Link

• Locally written PERL module with wrapper utilities

• Chains together sysreq path, datastore and tape commands to allow easy access to ADS entities by filename

• Supported operations:– put– get– stat– rm

Where we are now?

• Most of the above has been deployed very painlessly– xrootd/olbd and the CacheFS has been

deployed on 26 servers with > 70TB of disk and > 65TB of data on it

– The MPS ADS Link ADS system has been written and tested, it is in successful production on one server and will be deployed on more in the very near future

Benefits

• For Users:– Jobs don’t crash if a disk/server goes down,

they back off, contact the olb manager and get the data from somewhere else

– Queues aren’t stopped just because 2% of the data is offline

• For Admins:– No need for heroic efforts to recover

damaged filesystems– Much easier to schedule maintenance

Conclusion

• Xrootd has proved to be easy to configure and link to our MSS. Initial indications are that the production service is both reliable and performant

• This should improve both the lives of the users and sysadmins with huge advances in both the robustness of the system and it’s maintainability without sacrificing performance

• Talks, software (binaries and source), documentation and example configurations are available at http://xrootd.slac.stanford.edu