WMS best practices

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Contents

Objective

This document objective is to to provide guidelines on how to improve the availability and load balancing of the WMS by addressing three main areas:


Hardware

Hardware (WMSMonitor)


Hardware (DNS if used only for this purpose)


Hardware (WMSs)

Each WMS should be installed in a dedicated HW (e.g.: no virtual machines). Number of cores, RAM, disk space and number of machine should be proportional to the VOs supported and the number of job submitted to the WMSs but should have at least:

This kind of machine is able to process about 10.000 jobs/day (repeated for several days in a single month) with peaks at 15.000 jobs/day. Better HW seems not to have show linear improvements in the job rate (Eg.: even if the performances are improved, doubling the number of cores and amount of RAM is not reflected in the number of job processed per day). We tested only systems with 2 SATA, ATA mirrored disks for the entire system. Better disks or SSD might improve the performances and allow a higher job rate. The directory that are more most used by the system are:

/var/lib/mysql
/var/glite/SandboxDir

it might improve the performances a system that stores those two directory in different physical disks.

Physical vs Virtual Machines

Given the minimum hardware requirements there should not be any difference in using a Physical or Virtual Machine for the WMSMonitor and the DNS. However in the case of WMSMonitor the use of a database and the frequent disk access could be a limiting factor in using a Virtual Machine. For a small number of clients this should not represent an issue. The use of virtio can improve performances.


DNS round robin load balacing

Load balancing is a technique to distribute workload evenly across two or more resources. A load balancing method, which does not necessarily require a dedicated software or hardware node, is called round robin DNS.

We can not assume that all the job submitted to the WMS will require the same amount of resources and thus generate the same resource load (this depends on the job request, if there are errors in the submission and the job needs to be resubmitted, how many times it must be resubmitted, etc.). The load depends also on the type of hardware in which the WMS is installed. For an effective load balancing, a pool of available WMS should be regularly updated and the WMSs that have a higher load should be removed from that pool. All the WMSs that are in the pool should be used in a round robin fashion based on the DNS name resolution. With the help of the sensors installed in each WMS, the loadbalancing takes care to add and remove the WMSs from the pool on the DNS by updating records mapping on the same hostname. This results in a hostname that maps to multiple IP addresses under the configured DNS zone. As an example: in dns.top.domain, add multiple A records mapping to the same hostname with multiple IP addresses

Zone wms.zone.domain
name.wms.zone.domain IN A x.x.x.x
name.wms.zone.domain IN A y.y.y.y
name.wms.zone.domain IN A z.z.z.z

The 3 records are always served as answer but the order of the records will rotate in each DNS query. If the metrics on the test performed on one of those WMS report problems, the WMS is removed from the pool by removing the corresponding entry on the DNS. This mechanism provides fault tolerance of the WMSs.

In a similar way, a configurable number of WMSs that is under highest utilization is kept out of the pool to allow the submission of new jobs only to the WMSs that have less load.


Implementation

The metric measured by the wmsmonitor rely on sensors installed on each WMS. The detailed procedures for the WMS sensors and WMSMonitor server installation are available at this address:

https://wiki.italiangrid.it/twiki/bin/view/WMSMonitor/InstallationProcedureV2_1

An updated version of sensors and servers for the upcoming release of the WMS released in EMI is in Pre-View testing now. Documentation is available at this address:

https://wiki.italiangrid.it/twiki/bin/view/WMSMonitor/WebDownload

and the packages are distributed on request.

WMS maintenance

During normal utilization, even if a WMS instance is working correctly it may require regular maintenance. The MySQL database usually grows for each new job that is submitted and does not shrink when they are removed which leads to a higher disk utilization. For this reason one of the two following operations (one which requires that the WMS is drained and one that does not require to put offline the WMS) can and should be performed if the free space is less than 20% on the storage used by MySQL:

innodb_file_per_table
default-storage-engine=InnoDB

in the [mysqld] section. When the available space is low it is now possible to use the following command:

mysqlcheck --optimize lbproxy -u root -p

that optimize and reduce the size of the MySQL tables and files on the disk.

service gLite stop
service mysqld stop

It is then possible to remove the MySQL files

rm -rf /var/lib/mysql

at this point a reconfiguration of the WMS is necessary. This will recreate the database and tables structure.

Another operation that might be required is to add a line in the configuration file of the WMS to limit the size of the OutputSandBox. WMS should not be used to transfer big files from the WN to the user. A suitable SE should be used instead. This can be enforced by adding a line similar to:

MaxOutputSandboxSize  =  55000000;

in WorkloadManager section of /opt/glite/etc/glite_wms.conf configuration file which limits, in this case, the OutputSandbox to about 55Mb. The service should be restarted after the change of configuration:

/opt/glite/etc/init.d/glite-wms-wm restart
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