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XROOTD-L  June 2019

XROOTD-L June 2019

Subject:

Re: recommended settings for a distributed fs

From:

Andrew Hanushevsky <[log in to unmask]>

Reply-To:

Support use of xrootd by HEP experiments <[log in to unmask]>

Date:

Fri, 14 Jun 2019 12:12:07 -0700

Content-Type:

TEXT/PLAIN

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TEXT/PLAIN (81 lines)

Hi Afrian,

Answers withon the email...

On Fri, 14 Jun 2019, Adrian Sevcenco wrote:

> Hi! What would be the recommended settings for an xrootd installation
> that use a distributed file system like gluster?
I can't recommend specific settings but can indicate required directives 
to allow the whole thing to correctly function. You need to specify the
cms.dfs directive.

http://xrootd.org/doc/dev410/cms_config.htm#_Toc8247261

This tells the cmsd that you are using a distributed file system. It needs 
to know that to change file location to be consistent with DFS semantics.

> beside the dfs what other tweaks will make sure that the load is equally 
> distributed among the file servers?
You will need to enable load scheduling to get the best effect here. The 
default is round-robbin which works in many cases but is far from ideal 
when you want to avoid hot spots. Consult the cms.sched and cms.perf 
directives:

http://xrootd.org/doc/dev410/cms_config.htm#_Toc8247267
http://xrootd.org/doc/dev410/cms_config.htm#_Toc8247264

> it would seem that the redirector remembers the last data server that 
> answered a file query so the same server is used again ...how can this be 
> disabled?
No need to disable anything here if you use the cms.dfs directive.

> also, given the uniformity of the storage space, is there a need for a 
> redirector?(or cmsd)? could a simple xrootd (and with dns aliasing)
> distribute the load equally?
Using DNS will not distribute load equally and will cause severe latency 
issues should one or more of the servers become unavailable. While it may 
appear to work you will soon find that it works very poorly.

> also, for really large installations (and not only) i seen that there is a 
> mechanism for keeping a metadata cache of all namespace(s) - XrdCnsd
Yes, we have tried to deprecate that but people come up wanting it from 
time to time; so, it still has a life of its own.

> can this be used to bypass the metadata query of files and reduce the load of 
> the actual block device?
No, the XrdCnsd is meant to keep an inventory so that should you loose a 
disk array you will know what you lost. It is not a sufficiently 
real-time inventory suitable for file lookups.

> also, i seen the in the documentation the XrdCnsd is started by xrootd,
> which i do not think is a safe model (service started by a service)
> is there a way for ofs.notify to send the information to an already started 
> service?
Strange, without an xrootd running there is no reason to have a cnsd 
running so it makes sense to have the cnsd treated as a helper service to 
make sure you do not loose any events; which may happen if they are run as 
independent services. But to answer your question, yes, there is. Route 
the messages to a pipe and tell the cnsd to get the messages from the 
pipe. In order for this to be as rugged as you apparently want you would 
actually route the mesages to something like activeMQ which would in turn 
feed the cnsd, though that becomes relatively complicated and I would not 
recommend that. Also, the pipe solution comes with it's own set of issues 
as you can imagine.

> Also, this inventory is a collection of what? in the data servers the 
> namespace is a collection of symlinks
The inventory is a collection of files that exist in the xrootd exported 
namespace. It is nothing more than that. People wanted to track file 
creations and removals to have an inventory of what is on the server. It 
was specifically created for nothing-shared clusters. Nobody has used it 
in a DFS cluster; so we have no experience using it with a DFS.

Andy

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