Hub/Switch Installation Guide

Chapter 2 HPSS Planning
80 September 2002 HPSS Installation Guide
Release 4.5, Revision 2
requests to the DMAP Gateway. Migration processes (hpss_hdm_mig) migrate data to HPSS, and
purge processes (hdm_hdm_pur) purge migrated data from DFS and XFS. A set of processes
(hpss_hdm_tcp)accept requestsfromtheDMAPGateway,andperformtherequestedoperationin
DFS. A destroy process (hpss_hdm_dst) takes care of deleting files. Finally, XFS HDMs have a
processthatwatchesforstaleevents(hpss_hdm_stl)andkeepstheHDMfromgettingboggedown
by them.
There are three types of event handlers based on the type of activity that generates the events:
administrative, name space, and data. Administrative activities include mounting and
dismountingaggregates.Namespaceactivitiesincludecreating,deleting,orrenamingobjects,and
changingan object'sattributes.Data activitiesincludereadingandwriting filedata. Thenumberof
processes allocated to handle events generatedby these activities should be large enough to allow
a reasonable mix of these activities to run in parallel.
When the HDM fetches an event from DFS or XFS, it is put on a queue and assigned to an
appropriate event handler when one becomes free. The total number of entries allowed in the
queue is determined by a configuration parameter. If this value is not large enough to handle a
reasonable number of requests, some of the event handlers may be starved. For example, if the
queuefills upwithdataevents, thenamespace handlerswillbe starved.Section7.6.3.3.1: config.dat
File on page 449 discusses the criteria for selecting the size of an event queue.
HDMlogsoutstandingname spaceevents. Ifthe HDM isinterrupted,the logis replayedwhen the
HDM restarts to ensure that the events have been processed to completion and the DFS/XFS and
HPSS name spaces are synchronized. The size of the log is determined by a configuration
parameter, as discussed in Section 7.6.3.3.1: config.dat File on page 449.
HDMhastwo otherlogs, eachcontaininga listoffilesthat arecandidatesfor being destroyed.One
ofthelogs,calledthezap log, keepstrackoffilesonarchivedaggregatesandfile systems,whilethe
other, called the destroy log, keeps track of files on mirrored aggregates. Because of restrictions
imposed by the DFS SMR, the HDM cannot take the time to destroy files immediately, so the logs
serve asa record offiles that needto be destroyed bythe destroy process.The size ofthe zap log is
bounded only bythe file system wherethelog is kept, butthe size of the destroylog is determined
by a configuration parameter. If the destroy log is too small, the HDM will be forced to wait until
space becomes available.
Since the HDM may be running on a machine where it cannot write error messages to the HPSS
message log, it uses its own log. This HDM log consists of a configurable number of files (usually
2)that arewritten inround-robinfashion. Thesizes ofthesefiles aredeterminedbya configuration
parameter.
HDM logging policyallowsthesystemadministratorto determinethe typeof messageswritten to
the logfile: alarm, event, debug, and/ortrace messages. Typically, only alarmsshould be enabled,
although event messages can be useful, and do not add significant overhead. If a problem occurs,
activating debug and trace messages may provide additional information to help identify the
problem.However,thesemessagesaddoverhead,andthesystemwillperformbest ifmessagesare
kept toa minimum. Thetype of messageslogged is controlled by aparameter in theconfiguration
file and can be dynamically changed using the hdm_admin utility.
HDMmigratesandpurgesfilesbasedonpoliciesdefinedintheHDMpolicyconfigurationfile.The
administrator can establish different policies for each aggregate in the system. Migration policy
parametersincludethelengthoftimetowaitbetweenmigrationcyclesand theamountoftimethat
must elapse since a file was last accessed before it becomes eligible for migration. Purge policy
parameters include the length of time to wait between purge cycles, the amount of time that must
elapse since a file was last accessed, an upper bound specifying the percentage of DFS space that