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CHAPTER 12. dCache SPACE MANAGER

The spacemanager is a component in dCache that manages space reservations, a concept that was first introduced in SRM version 2.2. Space reservation guarantees that the requested amount of storage space of a specified type is made available by the storage system for a specified amount of time.



Utilization of space reservations for data storage

Users can create space reservations using an appropriate SRM client, although it is more common for the dCache administrator to make space reservations for VOs (see the section “SpaceManager configuration”. Each space reservation has an associated ID (or space token). VOs then can copy directly into space tokens assigned to them by the dCache administrator.

When a file is about to be transferred to a storage system, it is checked whether the space available in the space reservation can accomodate the entire file. If yes, this chunk of space is marked as allocated, so that it can not be taken by another, concurrently transferred file. If the file is transferred successfully, the allocated space becomes used space within the space reservation, else the allocated space is released back to the space reservation as free space.

SRM space reservation can be assigned a non-unique description which can be used to query the system for space reservations with a given description.

dCache only manages write space, i.e. space on disk can be reserved only for write operations. Once files are migrated to tape, and if no copy is required on disk, space used by these files is returned back into space reservation. When files are read back from tape and cached on disk, they are not counted as part of any space.

Properties of space reservation

A space reservation has a retention policy and an access latency.

Retention policy describes the quality of the storage service that will be provided for the data ( files) stored in the space reservation, and access latency describes the availability of this data. The SRM specification requires that if a space reservation is given on upload, then the specified retention policy and access latency must match those of the space reservation.

The default values for the retention policy and access latency can be changed in the file /etc/dcache/dcache.conf.

Retention policy The values of retention policy supported by dCache are REPLICA and CUSTODIAL.

  • REPLICA corresponds to the lowest quality of the service, usually associated with storing a single copy of each file on disk.

  • CUSTODIAL is the highest quality service, usually interpreted as storage of the data on tape.

Once a file is written into a given space reservation, it inherits the reservation’s retention policy.

If the space reservation request does not specify a retention policy, we will assign a value given by dcache.default-retention-policy. The default value is CUSTODIAL.

Edit the file /etc/dcache/dcache.conf to change the default value.

Example: Change the default value to REPLICA.

dcache.default-retention-policy=REPLICA

Access latency The two values allowed for access latency are NEARLINE and ONLINE.

  • NEARLINE means that data stored in this reservation is allowed to migrate to permanent media. Retrieving this data may result in delays associated with preparatory steps that the storage system has to perform to make this data available for user I/O (e.g., staging data from tape to a disk cache).

  • ONLINE means that data is readily available allowing for faster access.

In case of dCache, ONLINE means that there will always be a copy of the file on disk, while NEARLINE does not provide such guarantee. As with retention policy, once a file is written into a given space reservation, it inherits the reservation’s access latency.

If a space reservation request does not specify an access latency, we will assign a value given by dcache.default-access-latency. The default value is NEARLINE.

Edit the file /etc/dcache/dcache.conf to change the default value.

Example: Change the default value to ONLINE.

dcache.default-access-latency=ONLINE

Activating Spacemanager

In order to enable the SRM SpaceManager you need to add the spacemanager service to your layout file

[dCacheDomain]
[dCacheDomain/spacemanager]

Unless you have reason not to, we recommend placing the spacemanager service in the same domain as the poolmanager service.

Explicit and implicit space reservations for data storage in dCache

Explicit Space Reservations

Each SRM space reservation is made against the total available disk space of a particular link group. If dCache is configured correctly, each byte of disk space, that can be reserved, belongs to one and only one link group. See the section “SpaceManager configuration” for a detailed description.

IMPORTANT

Make sure that no pool belongs to more than one pool group, no pool group belongs to more than one link and no link belongs to more than one link group.

If a space reservation is specified during upload, the file will be stored in it.

Files written into a space made within a particular link group will end up on one of the pools belonging to this link group. The difference between the link group’s free space and the sum of all its space reservation’s unused space is the available space of the link group. The available space of a link group is the space that can be allocated for new space reservations.

The total space in dCache that can be reserved is the sum of the available spaces of all link groups. Note however that a space reservation can never span more than a single link group.

Implicit Space Reservations

dCache can perform implicit space reservations for non-SRM transfers, SRM Version 1 transfers and for SRM Version 2.2 data transfers that are not given the space token explicitly. The parameter that enables this behavior is srm.enable.space-reservation.implicit, which is described in the section “SRM configuration for experts”. If no implicit space reservation can be made, the transfer will fail.

Implicit space reservation means that the srm will create a space reservation for a single upload while negotiating the transfer parameters with the client. The space reservation will be created in a link group for which the user is authorized to create space reservations, which has enough available space, and which is able to hold the type of file being uploaded. The space reservation will be short lived. Once it expires, it will be released and the file it held will live on outside any space reservation, but still within the link group to which it was uploaded. Implicit space reservations are thus a technical means to upload files to link groups without using explicit space reservations.

The reason dCache cannot just allow the file to be uploaded to the link group without any space reservation at all is, that we have to guarantee, that space already allocated for other reservations isn’t used by the file being uploaded. The best way to guarantee that there is enough space for the file is to make a space reservation to which to upload it.

In case of SRM version 1.1 data transfers, where the access latency and retention policy cannot be specified, and in case of SRM V2.2 clients, when the access latency and retention policy are not specified, default values will be used. First SRM will attempt to use the values of access latency and retention policy tags from the directory to which a file is being written. If the tags are not present, then the access latency and retention policy will be set on basis of pnfsmanager defaults controlled by pnfsmanager.default-retention-policy and pnfsmanager.default-access-latency variables in /etc/dcache/dcache.conf.

You can check if the AccessLatency and RetentionPolicy tags are present by using the following command:

chimera lstag /path/to/directory
|Total: numberOfTags
|tag1
|tag2
|..
|AccessLatency
|RetentionPolicy

If the output contains the lines AccessLatency and RetentionPolicy then the tags are already present and you can get the actual values of these tags by executing the following commands, which are shown together with example outputs:

chimera readtag /data/experiment-a AccessLatency
|ONLINE
chimera readtag /data/experiment-a RetentionPolicy
|CUSTODIAL

The valid AccessLatency values are ONLINE and NEARLINE, valid RetentionPolicy values are REPLICA and CUSTODIAL.

To create/change the values of the tags, please execute :

chimera writetag /path/to/directory AccessLatency "<New AccessLatency>"
chimera writetag /path/to/directory RetentionPolicy "<New RetentionPolicy>"

NOTE

Some clients also have default values, which are used when not explicitly specified by the user. In this case server side defaults will have no effect.

NOTE

If the implicit space reservation is not enabled, pools in link groups will be excluded from consideration and only the remaining pools will be considered for storing the incoming data, and classical pool selection mechanism will be used.

Spacemanager configuration

SRM Spacemanager and Link Groups

SpaceManager is making reservations against free space available in link groups. The total free space in the given link group is the sum of available spaces in all links. The available space in each link is the sum of all sizes of available space in all pools assinged to a given link. Therefore for the space reservation to work correctly it is essential that each pool belongs to one and only one link, and each link belongs to only one link group. Link groups are assigned several parameters that determine what kind of space the link group corresponds to and who can make reservations against this space.

Making a Space Reservation

Now that the SRM SpaceManager is activated you can make a space reservation. As mentioned above you need link groups to make a space reservation.

Prerequisites for Space Reservations

Login to the admin interface and connect to the cell SrmSpaceManager.

ssh -p 22224 -l admin admin.example.org
|(local) admin > \c SrmSpaceManager

Type ls link groups to get information about link groups.

(SrmSpaceManager) admin > ls link groups

The lack of output tells you that there are no link groups. As there are no link groups, no space can be reserved.

The Link Groups

For a general introduction about link groups see the section called “Link Groups”.

Example:

In this example we will create a link group for the VO desy. In order to do so we need to have a pool, a pool group and a link. Moreover, we define unit groups named any-store, world-net and any-protocol. (See the section called “Types of Units”.)

Define a pool in your layout file, add it to your pool directory and restart the poolDomain.

[poolDomain]
[poolDomain/pool]
path=/srv/dcache/spacemanager-pool
name=spacemanager-pool
mkdir -p /srv/dcache/spacemanager-pool
dcache restart

In the admin interface, \c to the CELL-POOLMNGR and create a pool group, a link and a link group.

(local) admin > \c PoolManager
(PoolManager) admin > psu create pgroup spacemanager_poolGroup
(PoolManager) admin > psu addto pgroup spacemanager_poolGroup spacemanager-pool
(PoolManager) admin > psu removefrom pgroup default spacemanager-pool
(PoolManager) admin > psu create link spacemanager_WriteLink any-store world-net any-protocol
(PoolManager) admin > psu set link spacemanager_WriteLink -readpref=10 -writepref=10 -cachepref=0 -p2ppref=-1
(PoolManager) admin > psu addto link spacemanager_WriteLink  spacemanager_poolGroup
(PoolManager) admin > psu create linkGroup spacemanager_WriteLinkGroup
(PoolManager) admin > psu set linkGroup custodialAllowed spacemanager_WriteLinkGroup true
(PoolManager) admin > psu set linkGroup replicaAllowed spacemanager_WriteLinkGroup true
(PoolManager) admin > psu set linkGroup nearlineAllowed spacemanager_WriteLinkGroup true
(PoolManager) admin > psu set linkGroup onlineAllowed spacemanager_WriteLinkGroup true
(PoolManager) admin > psu addto linkGroup spacemanager_WriteLinkGroup spacemanager_WriteLink
(PoolManager) admin > save

Check whether the link group is available. Note that this can take several minutes to propagate to SPACEMNGR.

(local) admin > \c SrmSpaceManager
(SrmSpaceManager) admin > ls link groups
FLAGS CNT RESVD        AVAIL         FREE             UPDATED NAME
--rc:no 0     0 + 7278624768 = 7278624768 2024-04-30 12:12:51 spacemanager_WriteLinkGroup

The link group spacemanager_WriteLinkGroup was created. Here the flags indicate first the status (- indicates that neither the expired [e] nor the released flags [r] are set), followed by the type of reservations allowed in the link group (here replica [r], custodial [c], nearline [n] and online [o] files; output [o] files are not allowed - see help ls link groups for details on the format). No space reservations have been created, as indicated by the count field. Since no space reservation has been created, no space in the link group is reserved.

The SpaceManagerLinkGroupAuthorizationFile

Now you need to edit the LinkGroupAuthorization.conf file. This file contains a list of the link groups and all the VOs and the VO Roles that are permitted to make reservations in a given link group.

Specify the location of the LinkGroupAuthorization.conf file in the /etc/dcache/dcache.conf file.

spacemanager.authz.link-group-file-name=/path/to/LinkGroupAuthorization.conf

The file LinkGroupAuthorization.conf has following syntax:

LinkGroup followed by the list of the Fully Qualified Attribute Names (FQANs). Each FQAN is on a separate line, followed by an empty line, which is used as a record separator, or by the end of the file.

FQAN is usually a string of the form /Role=. Both and can be set to *, in this case all VOs or VO Roles will be allowed to make reservations in this link group. Any line that starts with # is a comment and may appear anywhere.

Rather than an FQAN, a mapped user name can be used. This allows clients or protocols that do not provide VOMS attributes to make use of space reservations.

#SpaceManagerLinkGroupAuthorizationFile

LinkGroup NameOfLinkGroup
/VO/Role=VORole

NOTE

You do not need to restart the DOMAIN-SRM or dCache after changing the LinkGroupAuthorization.conf file. The changes will be applied automatically after a few minutes.

Use update link groups to be sure that the LinkGroupAuthorization.conf file and the link groups have been updated.

(SrmSpaceManager) admin > update link groups
Update started.

Example:

In the example above you created the link group spacemanager_WriteLinkGroup. Now you want to allow members of the VO desy with the role production to make a space reservation in this link group.

#SpaceManagerLinkGroupAuthorizationFile
# this is comment and is ignored

LinkGroup spacemanager_WriteLinkGroup
#
/desy/Role=production

Example:

In this more general example for a SpaceManagerLinkGroupAuthorizationFile members of the VO desy with role test are authorized to make a space reservation in a link group called desy-test-LinkGroup. Moreover, all members of the VO desy are authorized to make a reservation in the link group called desy-anyone-LinkGroup and anyone is authorized to make a space reservation in the link group called default-LinkGroup.

#SpaceManagerLinkGroupAuthorizationFile
# this is a comment and is ignored

LinkGroup desy-test-LinkGroup
/desy/Role=test

LinkGroup desy-anyone-LinkGroup
/desy/Role=*

LinkGroup default-LinkGroup
# allow anyone :-)
*/Role=*

Making and Releasing a Space Reservation as dCache Administrator

Making a Space Reservation

Example:

Now you can make a space reservation for the VO desy.

(SrmSpaceManager) admin > reserve space -owner=/desy/Role=production -desc=DESY_TEST -lifetime=10000 -lg=spacemanager_WriteLinkGroup 5MB
110000 voGroup:/desy voRole:production retentionPolicy:CUSTODIAL accessLatency:NEARLINE
linkGroupId:0 size:5000000 created:Fri May 31 12:43:48 UTC 2024 lifetime:10000000ms expiration:Fri May 31 15:30:28 UTC 2024 description:DESY_TEST state:RESERVED used:0 allocated:0

The space token of the reservation is 110000. Check the status of the reservation by

(SrmSpaceManager) admin > ls spaces -e -h
 TOKEN RETENTION LATENCY FILES ALLO   USED   FREE   SIZE             EXPIRES DESCRIPTION
110000 CUSTODIAL NEARLINE    0   0B +   0B + 5.0M = 5.0M 2024-05-31 12:43:48 DESY_TEST

(SrmSpaceManager) admin > ls link groups -h
FLAGS CNT RESVD   AVAIL   FREE             UPDATED NAME
--rc:no 1  5.0M +  7.3G = 7.3G 2024-05-31 12:12:51 spacemanager_WriteLinkGroup

Here the -h option indicates that approximate, but human readable, byte sizes are to be used, and -e indicates that ephemeral (time limited) reservations should be displayed too (by default time limited reservations are not displayed as they are often implicit reservations). As can be seen, 5 MB are now reserved in the link group, although with approximate byte sizes, 5 MB do not make a visible difference in the 7.3 GB total size. You can now copy a file into that space token.

srmcp -space_token=110000 file://bin/sh \
|    srm://dcache.example.org/data/mydata

Now you can check via the Webadmin Interface or the Web Interface that the file has been copied to the pool spacemanager-pool.

There are several parameters to be specified for a space reservation.

(SrmSpaceManager) admin > reserve space [-al=online|nearline] [-desc=<string>] -lg=<name>
[-lifetime=<seconds>] [-owner=<user>|<fqan>] [-rp=output|replica|custodial] <size>

[-owner=|] The owner of the space is identified by either mapped user name or FQAN. The owner must be authorized to reserve space in the link group in which the space is to be created. Besides the dCache admin, only the owner can release the space. Anybody can however write into the space ( although the link group may only allow certain storage groups and thus restrict which file system paths can be written to space reservation, which in turn limits who can upload files to it).

[-al=] AccessLatency needs to match one of the access latencies allowed for the link group.

[-rp=] RetentionPolicy needs to match one of the retention policies allowed for the link group.

[-desc=] You can chose a value to describe your space reservation.

-lg= Which link group to create the reservation in.

The size of the space reservation should be specified in bytes, optionally using a byte unit suffix using either SI or IEEE prefixes.

[-lifetime=<lifetime]> The life time of the space reservation should be specified in seconds. If no life time is specified, the space reservation will not expire automatically.

Releasing a Space Reservation

If a space reservation is not needed anymore it can be released with

(SrmSpaceManager) admin > release space <spaceTokenId>

Example:

(SrmSpaceManager) admin > reserve space -owner=/desy -desc=DESY_TEST -lifetime=600 5000000
110042 voGroup:/desy voRole:production retentionPolicy:CUSTODIAL accessLatency:NEARLINE
linkGroupId:0 size:5000000 created:Fri May 31 12:00:35 UTC 2024 lifetime:600000ms
expiration:Fri May 31 12:10:35 UTC 2024 description:DESY_TEST state:RESERVED used:0
allocated:0

(SrmSpaceManager) admin > release space 110042
110042 voGroup:/desy voRole:production retentionPolicy:CUSTODIAL accessLatency:NEARLINE
linkGroupId:0 size:5000000 created:Fri May 31 12:00:35 UTC 2024 lifetime:600000ms
expiration:Fri May 31 12:10:35 UTC 2024 description:DESY_TEST state:RELEASED used:0
allocated:0

You can see that the value for state has changed from RESERVED to RELEASED.

Making and Releasing a Space Reservation as a User

If so authorized, a user can make a space reservation through the SRM protocol. A user is authorized to do so using the LinkGroupAuthorization.conf file.

VO based Authorization Prerequisites

In order to be able to take advantage of the virtual organization (VO) infrastructure and VO based authorization and VO based access control to the space in dCache, certain things need to be in place:

Only if these 3 conditions are satisfied the VO based authorization of the SpaceManager will work.

VO based Access Control Configuration

As mentioned above dCache space reservation functionality access control is currently performed at the level of the link groups. Access to making reservations in each link group is controlled by the SpaceManagerLinkGroupAuthorizationFile.

This file contains a list of the link groups and all the VOs and the VO Roles that are permitted to make reservations in a given link group.

When a SRM Space Reservation request is executed, its parameters, such as reservation size, lifetime, access latency and retention policy as well as user’s VO membership information is forwarded to the `SRM SpaceManager.

Once a space reservation is created, no access control is performed, any user can store the files in this space reservation, provided he or she knows the exact space token.

Making and Releasing a Space Reservation

A user who is given the rights in the SpaceManagerLinkGroupAuthorizationFile can make a space reservation by

srm-reserve-space -retention_policy=<RetentionPolicy> -lifetime=<lifetimeInSecs> -desired_size=<sizeInBytes> -guaranteed_size=<sizeInBytes>  srm://example.dcache.org/
|Space token =SpaceTokenId

and release it by

srm-release-space srm://dcache.example.org/ -space_token=SpaceTokenId

NOTE

Please note that it is obligatory to specify the retention policy while it is optional to specify the access latency.

Example:

srm-reserve-space -retention_policy=REPLICA -lifetime=300 -desired_size=5500000 -guaranteed_size=5500000  srm://dcache.example.org
|Space token =110044

The space reservation can be released by:

srm-release-space srm://dcache.example.org -space_token=110044

Space Reservation without VOMS certificate

If a client uses a regular grid proxy, created with grid-proxy-init, and not a VO proxy, which is created with the voms-proxy-init, when it is communicating with SRM server in dCache, then the VO attributes can not be extracted from its credential. In this case the name of the user is extracted from the Distinguished Name (DN) to use name mapping. For the purposes of the space reservation the name of the user as mapped by gplazma is used as its VO Group name, and the VO Role is left empty. The entry in the SpaceManagerLinkGroupAuthorizationFile should be:

#LinkGroupAuthorizationFile
#
<userName>

Space Reservation for non SRM Transfers

Edit the file /etc/dcache/dcache.conf to enable space reservation for non-SRM transfers.

spacemanager.enable.reserve-space-for-non-srm-transfers=true

If the spacemanager is enabled, spacemanager.enable.reserve-space-for-non-srm-transfers is set to true, and if the transfer request comes from a door, and there was no prior space reservation made for this file, the SpaceManager will try to reserve space before satisfying the request.

Possible values are true or false and the default value is false.

This is analogous to implicit space reservations performed by the srm, except that these reservations are created by the spacemanager itself. Since an SRM client uses a non-SRM protocol for the actual upload, setting the above option to true while disabling implicit space reservations in the srm, will still allow files to be uploaded to a link group even when no space token is provided. Such a configuration should however be avoided: If the srm does not create the reservation itself, it has no way of communicating access latency, retention policy, file size, nor lifetime to spacemanager.

dcache.enable.space-reservation

dcache.enable.space-reservation tells if the space management is activated.

Possible values are true and false. Default is true.

dcache.enable.overwrite

dcache.enable.overwrite tells SRM and GRIDFTP servers if overwriting is allowed. If enabled on the SRM node, it should be enabled on all GRIDFTP nodes.

Possible values are true and false. Default is false.

spaceManagerDatabaseHost

spacemanager.db.host tells SpaceManager which database host to connect to.

Default value is the property value of dcache.db.host, which is the default host of RDBMS used by various services.