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General SRM Concepts (for developers)

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The SRM service

dCache SRM is implemented as a Web Service running in a Jetty servlet container and an Axis Web Services engine. The Jetty server is executed as a cell, embedded in dCache and started automatically by the SRM service. Other cells started automatically by SRM are SpaceManager, PinManager and RemoteGSIFTPTransferManager. Of these services only SRM and SpaceManager require special configuration.

The SRM consists of the five categories of functions:

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Space Management Functions

SRM version 2.2 introduces a concept of space reservation. 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.

We use three functions for space management:

  • srmReserveSpace
  • SrmGetSpaceMetadata
  • srmReleaseSpace

Space reservation is made using the srmReserveSpace function. In case of successful reservation, a unique name, called space token is assigned to the reservation. A space token can be used during the transfer operations to tell the system to put the files being manipulated or transferred into an associated space reservation. A storage system ensures that the reserved amount of the disk space is indeed available, thus providing a guarantee that a client does not run out of space until all space promised by the reservation has been used. When files are deleted, the space is returned to the space reservation.

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

Properties of the SRM space reservations can be discovered using the SrmGetSpaceMetadata function.

Space Reservations might be released with the function srmReleaseSpace.

For a complete description of the available space management functions please see the SRM Version 2.2 Specification.

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Data Transfer Functions

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SURLs and TURLs

SRM defines a protocol named SRM, and introduces a way to address the files stored in the SRM managed storage by site URL (SURL of the format srm://<host>:<port>/[<web service path>?SFN=]<path>.

Example:

Examples of the SURLs a.k.a. SRM URLs are:

srm://fapl110.fnal.gov:8443/srm/managerv2?SFN=//pnfs/fnal.gov/data/test/file1
srm://fapl110.fnal.gov:8443/srm/managerv1?SFN=/pnfs/fnal.gov/data/test/file2
srm://srm.cern.ch:8443/castor/cern.ch/cms/store/cmsfile23

A transfer URL (TURL) encodes the file transport protocol in the URL.

Example:

gsiftp://gridftpdoor.fnal.gov:2811/data/test/file1

SRM version 2.2 provides three functions for performing data transfers:

  • srmPrepareToGet
  • srmPrepareToPut
  • srmCopy

(in SRM version 1.1 these functions were called get, put and copy).

All three functions accept lists of SURLs as parameters. All data transfer functions perform file/directory access verification and srmPrepareToPut and srmCopy check if the receiving storage element has sufficient space to store the files.

srmPrepareToGet prepares files for read. These files are specified as a list of source SURLs, which are stored in an SRM managed storage element. srmPrepareToGet is used to bring source files online and assigns transfer URLs (TURLs) that are used for actual data transfer.

srmPrepareToPut prepares an SRM managed storage element to receive data into the list of destination SURLs. It prepares a list of TURLs where the client can write data into.

Both functions support transfer protocol negotiation. A client supplies a list of transfer protocols and the SRM server computes the TURL using the first protocol from the list that it supports. Function invocation on the Storage Element depends on implementation and may range from simple SURL to TURL translation to stage from tape to disk cache and dynamic selection of transfer host and transfer protocol depending on the protocol availability and current load on each of the transfer server load.

The function srmCopy is used to copy files between SRM managed storage elements. If both source and target are local to the SRM, it performes a local copy. There are two modes of remote copies:

  • PULL mode : The target SRM initiates an srmCopy request. Upon the client\u0411\u2500\u2265s srmCopy request, the target SRM makes a space at the target storage, executes srmPrepareToGet on the source SRM. When the TURL is ready at the source SRM, the target SRM transfers the file from the source TURL into the prepared target storage. After the file transfer completes, srmReleaseFiles is issued to the source SRM.

  • PUSH mode : The source SRM initiates an srmCopy request. Upon the client\u0411\u2500\u2265s srmCopy request, the source SRM prepares a file to be transferred out to the target SRM, executes srmPrepareToPut on the target SRM. When the TURL is ready at the target SRM, the source SRM transfers the file from the prepared source into the prepared target TURL. After the file transfer completes, srmPutDone is issued to the target SRM.

When a specified target space token is provided, the files will be located in the space associated with the space token.

SRM Version 2.2 srmPrepareToPut and srmCopy PULL mode transfers allow the user to specify a space reservation token or a retention policy and access latency. Any of these parameters are optional, and it is up to the implementation to decide what to do, if these properties are not specified. The specification requires that if a space reservation is given, then the specified access latency or retention policy must match those of the space reservation.

The Data Transfer Functions are asynchronous, an initial SRM call starts a request execution on the server side and returns a request status that contains a unique request token. The status of request is polled periodically by SRM get request status functions. Once a request is completed and the client receives the TURLs the data transfers are initiated. When the transfers are completed the client notifies the SRM server by executing srmReleaseFiles in case of srmPrepareToGet or srmPutDone in case of srmPrepareToPut. In case of srmCopy, the system knows when the transfers are completed and resources can be released, so it requires no special function at the end.

Clients are free to cancel the requests at any time by execution of srmAbortFiles or srmAbortRequest.

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Request Status Functions

The functions for checking the request status are:

  • srmStatusOfReserveSpaceRequest
  • srmStatusOfUpdateSpaceRequest
  • srmStatusOfChangeSpaceForFilesRequest
  • srmStatusOfChangeSpaceForFilesRequest
  • srmStatusOfBringOnlineRequest
  • srmStatusOfPutRequest
  • srmStatusOfCopyRequest

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Directory Functions

SRM Version 2.2, interface provides a complete set of directory management functions. These are

  • srmLs, srmRm
  • srmMkDir, srmRmDir
  • srmMv

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Permission functions

SRM Version 2.2 supports the following three file permission functions:

  • srmGetPermission
  • srmCheckPermission and
  • srmSetPermission

dCache contains an implementation of these functions that allows setting and checking of Unix file permissions.