Directory Tags

Directory Tags
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In the Chimera namespace, each directory can have a number of tags. These directory tags may be used within dCache to control the file placement policy in the pools (see the section called “The Pool Selection Mechanism”). They might also be used by a tertiary storage system for similar purposes (e.g. controlling the set of tapes used for the files in the directory).

Note

Directory tags are not needed to control the behaviour of dCache. dCache works well without directory tags.

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Create, List and Read Directory Tags if the Namespace is not Mounted

You can create tags with

[user] $ /opt/d-cache/bin/chimera writetag <directory> <tagName> "<content>"

list tags with

[user] $ /opt/d-cache/bin/chimera lstag <directory>

and read tags with

[user] $ /opt/d-cache/bin/chimera readtag <directory> <tagName>

Example:

Create tags for the directory data with

[user] $ /opt/d-cache/bin/chimera writetag /data sGroup "myGroup"
[user] $ /opt/d-cache/bin/chimera writetag /data OSMTemplate "StoreName myStore"

list the existing tags with

[user] $ /opt/d-cache/bin/chimera lstag /data
Total: 2
OSMTemplate
sGroup

and their content with

[user] $ /opt/d-cache/bin/chimera readtag /data OSMTemplate
StoreName myStore
[user] $ /opt/d-cache/bin/chimera readtag /data sGroup
myGroup

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Create, List and Read Directory Tags if the Namespace is Mounted

If the namespace is mounted, change to the directory for which the tag should be set and create a tag with

[user] $ cd <directory>
[user] $ echo '<content1>' > '.(tag)(<tagName1>)'
[user] $ echo '<content2>' > '.(tag)(<tagName2>)'

Then the existing tags may be listed with

[user] $ cat '.(tags)()'
.(tag)(<tagname1>)
.(tag)(<tagname2>)

and the content of a tag can be read with

[user] $ cat '.(tag)(<tagname1>)'
<content1>
[user] $ cat '.(tag)(<tagName2>)'
<content2>

Example:

Create tags for the directory data with

[user] $ cd data
[user] $ echo 'StoreName myStore' > '.(tag)(OSMTemplate)'
[user] $ echo 'myGroup' > '.(tag)(sGroup)'

list the existing tags with

[user] $ cat '.(tags)()'
.(tag)(OSMTemplate)
.(tag)(sGroup)

and their content with

[user] $ cat '.(tag)(OSMTemplate)'
StoreName myStore
[user] $ cat '.(tag)(sGroup)'
 myGroup

A nice trick to list all tags with their contents is

[user] $ grep "" $(cat  ".(tags)()")
.(tag)(OSMTemplate):StoreName myStore
.(tag)(sGroup):myGroup

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Directory Tags and Command Files

When creating or changing directory tags by writing to the command file as in

[user] $ echo '<content>' > '.(tag)(<tagName>)'

one has to take care not to treat the command files in the same way as regular files, because tags are different from files in the following aspects:

The <tagName> is limited to 62 characters and the <content> to 512 bytes. Writing more to the command file, will be silently ignored.
If a tag which does not exist in a directory is created by writing to it, it is called a primary tag.
Tags are inherited from the parent directory by a newly created directory. Changing a primary tag in one directory will change the tags inherited from it in the same way. Creating a new primary tag in a directory will not create an inherited tag in its subdirectories.
Moving a directory within the Chimera namespace will not change the inheritance. Therefore, a directory does not necessarily inherit tags from its parent directory. Removing an inherited tag does not have any effect.
Empty tags are ignored.

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Directory Tags for dCache

The following directory tags appear in the dCache context:

OSMTemplate: Must contain a line of the form “StoreName <storeName>” and specifies the name of the store that is used by dCache to construct the storage class if the HSM Type is osm.
HSMType: The HSMType tag is normally determined from the other existing tags. E.g., if the tag OSMTemplate exists, HSMType=osm is assumed. With this tag it can be set explicitly. A class implementing that HSM type has to exist. Currently the only implementations are osm and enstore.
sGroup: The storage group is also used to construct the storage class if the HSMType is osm.
cacheClass: The cache class is only used to control on which pools the files in a directory may be stored, while the storage class (constructed from the two above tags) might also be used by the HSM. The cache class is only needed if the above two tags are already fixed by HSM usage and more flexibility is needed.
hsmInstance: If not set, the hsmInstance tag will be the same as the HSMType tag. Setting this tag will only change the name as used in the storage class and in the pool commands.
WriteToken: Assign a WriteToken tag to a directory in order to be able to write to a space token without using the SRM.

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Storage Class and Directory Tags

The storage class is a string of the form <StoreName>:<StorageGroup>@<hsm-type>, where <StoreName> is given by the OSMTemplate tag, <StorageGroup> by the sGroup tag and <hsm-type> by the HSMType tag. As mentioned above the HSMType tag is assumed to be osm if the tag OSMTemplate exists.

In the examples above two tags have been created.

Example:

[user] $ /opt/d-cache/bin/chimera lstag /data
Total: 2
OSMTemplate
sGroup

As the tag OSMTemplate was created the tag HSMType is assumed to be osm.

The storage class of the files which are copied into the directory /data after the tags have been set will be myStore:myGroup@osm.

If directory tags are used to control the behaviour of dCache and/or a tertiary storage system, it is a good idea to plan the directory structure in advance, thereby considering the necessary tags and how they should be set up. Moving directories should be done with great care or even not at all. Inherited tags can only be created by creating a new directory.

Example:

Assume that data of two experiments, experiment-a and experiment-b is written into a namespace tree with subdirectories /data/experiment-a and /data/experiment-b. As some pools of the dCache are financed by experiment-a and others by experiment-b they probably do not like it if they are also used by the other group. To avoid this the directories of experiment-a and experiment-b can be tagged.

[user] $ /opt/d-cache/bin/chimera writetag /data/experiment-a OSMTemplate "StoreName exp-a"
[user] $ /opt/d-cache/bin/chimera writetag /data/experiment-b OSMTemplate "StoreName exp-b"

Data from experiment-a taken in 2010 shall be written into the directory /data/experiment-a/2010 and data from experiment-a taken in 2011 shall be written into /data/experiment-a/2011. Data from experiment-b shall be written into /data/experiment-b. Tag the directories correspondingly.

[user] $ /opt/d-cache/bin/chimera writetag /data/experiment-a/2010 sGroup "run2010"
[user] $ /opt/d-cache/bin/chimera writetag /data/experiment-a/2011 sGroup "run2011"
[user] $ /opt/d-cache/bin/chimera writetag /data/experiment-b sGroup "alldata"

List the content of the tags by

[user] $ /opt/d-cache/bin/chimera readtag /data/experiment-a/2010 OSMTemplate
StoreName exp-a
[user] $ /opt/d-cache/bin/chimera readtag /data/experiment-a/2010 sGroup
run2010
[user] $ /opt/d-cache/bin/chimera readtag /data/experiment-a/2011 OSMTemplate
StoreName exp-a
[user] $ /opt/d-cache/bin/chimera readtag /data/experiment-a/2011 sGroup
run2011
[user] $ /opt/d-cache/bin/chimera readtag /data/experiment-b/2011 OSMTemplate
StoreName exp-b
[user] $ /opt/d-cache/bin/chimera readtag /data/experiment-b/2011 sGroup
alldata

As the tag OSMTemplate was created the HSMType is assumed to be osm.

The storage classes of the files which are copied into these directories after the tags have been set will be

exp-a:run2010@osm for the files in /data/experiment-a/2010
exp-a:run2011@osm for the files in /data/experiment-a/2011
exp-b:alldata@osm for the files in /data/experiment-b

To see how storage classes are used for pool selection have a look at the example ’Reserving Pools for Storage and Cache Classes’ in the PoolManager chapter.

There are more tags used by dCache if the HSMType is enstore.

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