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:

Monospaced Font

LISTSERV Archives

LISTSERV Archives

XROOTD-L Home

XROOTD-L Home

XROOTD-L  March 2005

XROOTD-L March 2005

Subject:

Re: cache validity

From:

Peter Elmer <[log in to unmask]>

Date:

3 Mar 2005 12:17:36 +0100Thu, 3 Mar 2005 12:17:36 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (88 lines)

  Hi Jean-Yves,

On Thu, Mar 03, 2005 at 11:02:04AM +0100, Jean-Yves Nief wrote:
> at the moment, it is a read only system.
> so the user recreated his file and migrate it to HPSS using the usual
> rfio command "rfcp".
> it can be considered as a backdoor and it is a special case (I did not
> think I would encounter that one) as usually the files are writen once /
> read many times, so the problem I mentionned is usually avoided in the
> HEP world but that can happen with user or privately produced files. I
> understand it will certainly rarely seen, but it could happen.

  Yes, this sounds like an artifact of using simultaneously two different
systems with different notions of how things should work...

> I think this issue can be encountered even if the writing is made within
> the xrootd service (as opposed to the case where a "backdoor" was used).
> There would be 2 cases:
>
> a) 1 pool allowing r/w: assuming one would like to update a file foo
> (initial version: v1), we can encounter the case where there are 2
> duplicates of foo_v1.root on 2 different servers s1 and s2. Then, let's
> assume that s1:foo_v1.root is being updated. We will then have on disk:
> s1:foo_v2.root
> s2:foo_v1.root
> and in HPSS, foo_v2.root will superceed foo_v1.root after migration.
> but then you have some inconsistency in the cache.
> So if someone who wants to read the latest version of foo can be
> directed to s2:foo_v1.root. That is not what we want and this is also
> why I don't really like the idea of mixing read and write in a single
> pool. But a way to avoid the situation would be to use some checking to
> the MSS core server as I proposed. I understand that when the MSS is
> down all the xrootd could be down a priori even if you want to access
> files already on disk: well in the case of a MSS failure, the checking
> operation would return an error and in that case, one could resume
> activity with the copy on disk.

  This was the idea behind the "oss.check" directive. As I understood it,
it would basically disallow write/update access once the file has been
migrated to the MSS. This is what we were intending to use for precisely
this purpose in the /store/users r/w space. Note that the check would have to
happen when files are opened for _write_, not read, which is again the
economy we want to achieve.

  Wilko and I intend set up the /store/users area up at SLAC after the
new production xrootd version is running everywhere next week. Note that we
don't expect to do _automatic_ migration from MSS from this area. The idea
is that a user will have to run some command to trigger the stage out. (In
this case in BaBar we will give users some command that puts the information
into the dataset bookkeeping and wraps the lower level command that actually
triggers the stageout.) When this command is issued, the file would have to
be removed from the "write" pool so it cannot be further updated. It can
be moved to a r/o pool as an optimization or just staged back from MSS to
the r/o machines when needed. The write machine is not allowed to stage files
in from the MSS, hence the file can no longer be updated. Thus when a user
runs this command they are basically saying "this is the final version of
this file, from now on it is read-only". And the r/o system stays simple and
scalable.

  Does this seem reasonable? Wilko/Andy, did I miss anything?

  (Unless Andy or you have a surprise for me, Artem and the pud daemon will
perhaps get their revenge here as there is currently no mechanism I can
see to trigger the stage out via xrootd itself... ;-) Presumably the mps
scripts have to be used directly until such a general mechanism is provided.

> b) 1 pool for read access + 1 for writing:
> clearly, if you got updates of files in the writing pool, then the read
> pool would have its cache not up-to-date and you will have to find a
> mechanism to update it. It could be done using xrdcp.

  I think that a combined r/w + ro pool can be done as described above, with
the caveat that a command would have to be run explicitly to trigger a
stage-out. This is probably sufficient for the user areas. (True production
areas could of course have automated migration to MSS as they like as long
they are willing to say "once I drop the file in it can be considered r/o".)

  Comments? (From anyone.)

                                   Pete

-------------------------------------------------------------------------
Peter Elmer E-mail: [log in to unmask] Phone: +41 (22) 767-4644
Address: CERN Division PPE, Bat. 32 2C-14, CH-1211 Geneva 23, Switzerland
-------------------------------------------------------------------------


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