LISTSERV mailing list manager LISTSERV 16.5

Help for XROOTD-L Archives


XROOTD-L Archives

XROOTD-L Archives


XROOTD-L@LISTSERV.SLAC.STANFORD.EDU


View:

Message:

[

First

|

Previous

|

Next

|

Last

]

By Topic:

[

First

|

Previous

|

Next

|

Last

]

By Author:

[

First

|

Previous

|

Next

|

Last

]

Font:

Proportional Font

LISTSERV Archives

LISTSERV Archives

XROOTD-L Home

XROOTD-L Home

XROOTD-L  August 2005

XROOTD-L August 2005

Subject:

0 Config et. al

From:

Andrew Hanushevsky <[log in to unmask]>

Date:

16 Aug 2005 23:30:02 -0700 (PDT)Tue, 16 Aug 2005 23:30:02 -0700 (PDT)

Content-Type:

TEXT/PLAIN

Parts/Attachments:

Parts/Attachments

TEXT/PLAIN (146 lines)

Hi,

I have been investigating on how to meet various needs in terms of
0-config clusters and simplifying the overall setup. The two main items
that were brought up: 1) 0-config, and 2) combining the xrootd and olbd so
that one does not have to deal with two daemons seem like workable
solutions that, in fact, are very related. So, here is a proposal and
please feel free to rip it apart :-)

In general, manager and supervisor nodes can combine the xrootd and olbd
as one daemon. This is because they perform one simple function -- lookup
and redirection. The thread demands are largely homogeneous and one would
expect a smooth flow through the daemon. Data servers, on the other hand,
pose a problem since combining the two functions is like mixing apples and
oranges. Data servers never really need olbd functionality and, indeed,
the thread demands for data serving would compete with the services a data
server olbd provides to the cluster. Depending on the thread  contention,
long delays can be introduced into the olbd path that would  cause
unpredictable behaviour in terms of locating files. Hence, the two
function really need to live in separate processes.

So, here is what I can do to work within these constraints.

1) We introduce a new directive (optional):

xrootd.olb <path>

which specifies the location of the olbd "plugin", libXrdOlb.so. The
default is to use whatever LD_LIBRARY_PATH happens to be set to.

2) Manager and supervisor xrootd’s simply load the plugin and use it via
an object interface. Data server xrootd’s load the plugin  then fork and
execute the plugin in a separate process. The process verifies that a
previous incarnation is not running and if it is, exits since the xrootd
will simply use the previous  incarnation. This allows all functionality
to be controlled by simply starting an xrootd with the appropriate
parameters. No more starting a separate daemon.

3) Introduce a new directive (mandatory for auto-config clusters):

olb.xrootd <command line>

This directive specifies how to start an xrootd that will function as a
supervisor. I suppose we can come up with defaults but the problem is that
the xrd layer strips out  parameters before passing the command line to
xrootd so we can never know things like where the log file should go.
However, I don’t think that this parameter is unwieldy since it’s pretty
much fixed for once you dream up the configuration.

4) Introduce a new directive (optional):

olb.ftlevel x%

This specifies the fault tolerance level (default is 20%). The manager
will start enough supervisors to handle x% more data servers than really
is needed (e.g., be default 1.2 times as many supervisors than would be
needed are started).

5) Modify the existing role directive:

olb.role {manager | server | supervisor | superserver ]} [if <conds>]

The difference is that you can specify that a data server olb can also
function as a supervisor olb if you specify

olb.role superserver

In general, auto-clusters would always have that directive (the default
being manual configuration). This also provides an convenient way to limit
which nodes can act as superservers.

The algorithm would work as follows:

1) As data server olb’s connect they tell the manager (as is now) how many
rounds they have done without finding a supervisor. Once that number
reaches 3 (arbitrary -- you can give me another one), the manager asks a
superserver that it has not asked before to start a supervisor and the
data server olbd is asked to restart it’s search.

2) When a superserver olbd is asked to start a supervisor (and it has not
done so already) it launches a supervisor using the olb.xrootd command.
The trick here (which I haven’t figured out yet) is how to know that a
supervisor has been launched across a restart of the data server
xrootd/olbd. Tricky, very tricky.

3) The manager tallies how many data servers it knows about and always
makes sure that olb.ftlevel supervisors have been started.

4) This leads to a good possibility that the superserver requests can be
cascaded so that auto-clustering can work past 4,096 servers.

There are still many details to work out (the devil is in the details):

a) How one controls this algorithm in the presence of load balanced
managers. That is, you can start x managers and, somehow, one has to
prevent these managers from starting supervisors willy-nilly. This is not
an easy problem to solve as managers work independently and are loath to
contact each other (in fact that’s one of the strengths of the current
scheme).

b) What are the administrative interface relationships? This is another
one that I haven’t solved. It’s easy when xrootd’s and olbd’s are separate
but difficult to address when some are and some are not.

c) What are the cache side-effects in the presence of combined
xrootd/olbd’s? Not clear. What I do know is that there will be more cache
activity as things come and go at the supervisor level. How that sorts out
is unknown. However, one good thing here is that this investigation did
bring to light a failing in the reconfiguration algorithm. Currently, the
system does not completely handle port reassignment across partial
reconfigurations (i.e., xrootd going then coming back with a different
port number). Something to fix.

e) How will this effect existing schemes to automatically restart failed
servers? Note that data servers start an ephemeral olbd. This puts the
xrootd in the situation where it has to make sure that the ephemeral olbd
is restarted should it fail. It also adds in the big nit that this
knowledge is lost across data server restarts and it’s not clear how to
handle that situation.

f) Should the architecture change in terms of the xroots/olbd
relationship? Currently, olbd interactions occur at the ofs layer. In a
combined xrootd/olbd these interactions could occur at the xroot protocol
layer. In some ways this is cleaner but it also is more restrictive in how
you can reuse components.

g) How long will it take to reach stability? The answer is obviously
longer than it takes now since, if for no other reason, supervisors cannot
be pre-started. Unknown what production effect this will have.

h) There are probably a lot of other end conditions that I don’t know
about. So, please speak up and ask questions on how things would be
handled in strange situations.

All in all, the above is a workable solution but not something that I can
implement in a day. So, please comment because once I star down this road
it will be hard to change things.

Andy






Top of Message | Previous Page | Permalink

Advanced Options


Options

Log In

Log In

Get Password

Get Password


Search Archives

Search Archives


Subscribe or Unsubscribe

Subscribe or Unsubscribe


Archives

March 2024
February 2024
January 2024
December 2023
November 2023
October 2023
September 2023
August 2023
July 2023
June 2023
May 2023
April 2023
March 2023
February 2023
January 2023
December 2022
October 2022
September 2022
August 2022
July 2022
June 2022
May 2022
April 2022
March 2022
February 2022
January 2022
December 2021
November 2021
October 2021
September 2021
August 2021
July 2021
June 2021
May 2021
April 2021
March 2021
February 2021
January 2021
December 2020
November 2020
October 2020
September 2020
August 2020
July 2020
June 2020
May 2020
April 2020
March 2020
February 2020
January 2020
December 2019
November 2019
October 2019
September 2019
August 2019
July 2019
June 2019
May 2019
April 2019
March 2019
February 2019
January 2019
December 2018
November 2018
October 2018
September 2018
August 2018
July 2018
June 2018
May 2018
April 2018
March 2018
February 2018
January 2018
December 2017
November 2017
October 2017
September 2017
August 2017
July 2017
June 2017
May 2017
April 2017
March 2017
February 2017
January 2017
December 2016
November 2016
October 2016
September 2016
August 2016
July 2016
May 2016
April 2016
March 2016
February 2016
January 2016
December 2015
November 2015
October 2015
September 2015
August 2015
July 2015
June 2015
May 2015
April 2015
March 2015
February 2015
January 2015
December 2014
November 2014
October 2014
September 2014
August 2014
July 2014
June 2014
May 2014
April 2014
March 2014
February 2014
January 2014
December 2013
November 2013
October 2013
September 2013
August 2013
July 2013
June 2013
May 2013
April 2013
March 2013
February 2013
January 2013
December 2012
November 2012
October 2012
September 2012
August 2012
July 2012
June 2012
May 2012
April 2012
March 2012
February 2012
January 2012
December 2011
November 2011
October 2011
September 2011
August 2011
July 2011
June 2011
May 2011
April 2011
March 2011
February 2011
January 2011
December 2010
October 2010
September 2010
August 2010
July 2010
June 2010
May 2010
April 2010
March 2010
February 2010
January 2010
December 2009
November 2009
October 2009
September 2009
July 2009
June 2009
May 2009
April 2009
March 2009
January 2009
December 2008
September 2008
August 2008
July 2008
June 2008
May 2008
April 2008
March 2008
February 2008
January 2008
December 2007
November 2007
October 2007
September 2007
August 2007
July 2007
June 2007
May 2007
April 2007
March 2007
February 2007
January 2007
December 2006
November 2006
October 2006
September 2006
August 2006
July 2006
June 2006
May 2006
April 2006
March 2006
February 2006
January 2006
December 2005
November 2005
October 2005
September 2005
August 2005
July 2005
June 2005
May 2005
April 2005
March 2005
February 2005
January 2005
December 2004
November 2004
October 2004
September 2004
August 2004

ATOM RSS1 RSS2



LISTSERV.SLAC.STANFORD.EDU

Secured by F-Secure Anti-Virus CataList Email List Search Powered by the LISTSERV Email List Manager

Privacy Notice, Security Notice and Terms of Use