CHAPTER 8. FILE HOPPING

File hopping on arrival from outside dCache

File hopping is a collective term in dCache, summarizing the possibility of having files being transferred between dCache pools triggered by a variety of conditions. The most prominent examples are:

If a file is requested by a client but the file resides on a pool from which this client, by configuration, is not allowed to read data, the dataset is transferred to an “allowed” pool first.
If a pool encounters a steady high load, the system may, if configured, decide to replicate files to other pools to achieve an equal load distribution.
HSM restore operations may be split into two steps. The first one reads data from tertiary storage to an “HSM connected” pool and the second step takes care that the file is replicated to a general read pool. Under some conditions this separation of HSM and non-HSM pools might become necessary for performance reasons.
If a dataset has been written into dCache it might become necessary to have this file replicated instantly. The reasons can be, to either have a second, safe copy, or to make sure that clients don’t access the file for reading on the write pools.

File hopping on arrival from outside dCache

File Hopping on arrival is a term, denoting the possibility of initiating a pool to pool transfer as the result of a file successfully arriving on a pool from some external client. Files restored from HSM or arriving on a pool as the result of a pool to pool transfer will not yet be forwarded.

Forwarding of incoming files can be enabled by setting the pool.destination.replicate property in the /etc/dcache/dcache.conf file or per pool in the layout file. It can be set to on, PoolManager or HoppingManager, where on does the same as PoolManager.

The pool is requested to send a replicateFile message to either the PoolManager or to the HoppingManager, if available. The different approaches are briefly described below and in more detail in the subsequent sections.

The replicateFile message is sent to the PoolManager. This happens for all files arriving at that pool from outside (no restore or p2p). No intermediate HoppingManager is needed. The restrictions are
- All files are replicated. No pre-selection, e.g. on the storage class can be done.
- The mode of the replicated file is determined by the destination pool and cannot be overwritten. See the section called “File mode of replicated files”
The replicateFile message is sent to the HoppingManager. The HoppingManager can be configured to replicate certain storage classes only and to set the mode of the replicated file according to rules. The file mode of the source file cannot be modified.

File mode of replicated files

The mode of a replicated file can either be determined by settings in the destination pool or by the HoppingManager. It can be cached or precious.

If the PoolManager is used for replication, the mode of the replicated file is determined by the destination pool. The default setting is cached.
If a HoppingManager is used for file replication, the mode of the replicated file is determined by the HoppingManager rule responsible for this particular replication. If the destination mode is set to keep in the rule, the mode of the destination pool determines the final mode of the replicated file.

File hopping managed by the poolmanager

To enable replication on arrival by the PoolManager set the property pool.destination.replicate to PoolManager for the particular pool

[exampleDomain]
[exampleDomain/pool]
pool.destination.replicate=PoolManager

or for several pools in the /etc/dcache/dcache.conf file.

pool.destination.replicate=PoolManager

File hopping configuration instructs a pool to send a replicateFile request to the PoolManager as the result of a file arriving on that pool from some external client. All arriving files will be treated the same. The PoolManager will process this transfer request by trying to find a matching link (Please find detailed information at Chapter 7, The poolmanager Service.

It is possible to configure the CELL-POOLMNGR such that files are replicated from this pool to a special set of destination pools.

Example: Assume that we want to have all files, arriving on pool ocean to be immediately replicated to a subset of read pools. This subset of pools is described by the poolgroup ocean-copies. No other pool is member of the poolgroup ocean-copies. Other than that, files arriving at the pool mountain should be replicated to all read pools from which farm nodes on the 131.169.10.0/24 subnet are allowed to read. The layout file defining the pools ocean and mountain should read like this:

[exampleDomain]
[exampleDomain/pool]
name=ocean
path=/path/to/pool-ocean
pool.wait-for-files=${path}/data
pool.destination.replicate=PoolManager

[exampleDomain/pool]
name=mountain
path=/path/to/pool-mountain
pool.wait-for-files=${path}/data
pool.destination.replicate=PoolManager

In the layout file it is defined that all files arriving on the pools ocean or mountain should be replicated immediately. The following PoolManager.conf file contains instructions for the CELL-POOLMNGR how to replicate these files. Files arriving at the ocean pool will be replicated to the ocean-copies subset of the read pools and files arriving at the pool mountain will be replicated to all read pools from which farm nodes on the 131.169.10.0/24 subnet are allowed to read.

#
# define the units
#
psu create unit -protocol   */*
psu create unit -net        0.0.0.0/0.0.0.0
psu create unit -net        131.169.10.0/255.255.255.0
# create the faked net unit
psu create unit -net        192.1.1.1/255.255.255.255
psu create unit -store      *@*
psu create unit -store      ocean:raw@osm
#
#
#  define unit groups
#
psu create ugroup  any-protocol
psu create ugroup  any-store
psu create ugroup  ocean-copy-store
psu create ugroup farm-network
psu create ugroup ocean-copy-network
#
psu addto ugroup any-protocol */*
psu addto ugroup any-store    *@*
psu addto ugroup ocean-copy-store ocean:raw@osm
psu addto ugroup farm-network  131.169.10.0/255.255.255.0
psu addto ugroup ocean-copy-network  192.1.1.1/255.255.255.255
psu addto ugroup allnet-cond 0.0.0.0/0.0.0.0
psu addto ugroup allnet-cond 131.169.10.0/255.255.255.0
psu addto ugroup allnet-cond 192.1.1.1/255.255.255.255
#
#
#  define the write-pools
#
psu create pool ocean
psu create pool mountain
#
#
#  define the write-pools poolgroup
#
psu create pgroup write-pools
psu addto pgroup write-pools ocean
psu addto pgroup write-pools mountain
#
#
#  define the write-pools-link, add write pools and set transfer preferences
#
psu create link write-pools-link any-store any-protocol allnet-cond
psu addto link write-pools-link write-pools
psu set link farm-read-link -readpref=0 -writepref=10 -cachepref=0 -p2ppref=-1
#
#
#  define the read-pools
#
psu create pool read-pool-1
psu create pool read-pool-2
psu create pool read-pool-3
psu create pool read-pool-4
#
#
#  define the farm-read-pools poolgroup and add pool members
#
psu create pgroup farm-read-pools
psu addto pgroup farm-read-pools read-pool-1
psu addto pgroup farm-read-pools read-pool-2
psu addto pgroup farm-read-pools read-pool-3
psu addto pgroup farm-read-pools read-pool-4
#
#
#  define the ocean-copy-pools poolgroup and add a pool
#
psu create pgroup ocean-copy-pools
psu addto pgroup ocean-copy-pools  read-pool-1
#
#
# define the farm-read-link, add farm-read-pools and set transfer preferences
#
psu create link farm-read-link any-store any-protocol farm-network
psu addto link farm-read-link farm-read-pools
psu set link farm-read-link -readpref=10 -writepref=0 -cachepref=10 -p2ppref=-1
#
#
# define the ocean-copy-link, add ocean-copy-pools and set transfer preferences
#
psu create link ocean-copy-link ocean-copy-store any-protocol ocean-copy-network
psu addto link ocean-copy-link ocean-copy-pools
psu set link ocean-copy-link -readpref=10 -writepref=0 -cachepref=10 -p2ppref=-1
#
#

While 131.169.10.1 is a legal IP address e.g. of one of your farm nodes, the 192.1.1.1 IP address must not exist anywhere at your site.

File hopping managed by the Hoppingmanager

With the HoppingManager you have several configuration options for file hopping on arrival, e.g.:

With the HoppingManager you can define a rule such that only the files with a specific storage class should be replicated.
You can specify the protocol the replicated files can be accessed with.
You can specify from which ip-adresses it should be possible to access the files.

Starting the FileHopping Manager service

Add the hoppingManager service to a domain in your layout file and restart the domain.

[<DomainName>]
[<DomainName>/hoppingmanager]

Initially no rules are configured for the HoppingManager. You may add rules by either edit the file /var/lib/dcache/config/HoppingManager.conf and restart the hoppingmanager service, or use the admin interface and save the modifications by the save command into the HoppingManager.conf

Configuring pools to use the HoppingManager

To enable replication on arrival by the CELL-HOPMNGR set the property pool.destination.replicate to HoppingManager for the particular pool

[exampleDomain]
[exampleDomain/pool]
pool.destination.replicate=HoppingManager

or for several pools in the /etc/dcache/dcache.conf file.

pool.destination.replicate=HoppingManager

HoppingManager Configuration Introduction

The HoppingManager essentially receives replicateFile messages from pools, configured to support file hopping, and either discards or modifies and forwards them to the PoolManager, depending on rules described below.
The HoppingManager decides on the action to perform, based on a set of configurable rules. Each rule has a name. Rules are checked in alphabetic order concerning their names.
A rule it triggered if the storage class matches the storage class pattern assigned to that rule. If a rule is triggered, it is processed and no further rule checking is performed. If no rule is found for this request the file is not replicated.
If for whatever reason, a file cannot be replicated, NO RETRY is being performed.
Processing a triggered rule can be :
- The message is discarded. No replication is done for this particular storage class.
- The rule modifies the replicateFile message, before it is forwarded to the PoolManager.
  
  An ip-number of a farm-node of the farm that should be allowed to read the file can be added to the replicateFile message.
  
  The mode of the replicated file can be specified. This can either be precious, cached or keep. keep means that the pool mode of the source pool determines the replicated file mode.
  
  The requested protocol can be specified.

HoppingManager Configuration Reference

define hop OPTIONS <name> <pattern> precious|cached|keep
OPTIONS
    -destination=<cellDestination> # default : PoolManager
    -overwrite
    -continue
    -source=write|restore|*   #  !!!! for experts only      StorageInfoOptions
    -host=<destinationHostIp>
    -protType=dCap|ftp...
    -protMinor=<minorProtocolVersion>
    -protMajor=<majorProtocolVersion>

name This is the name of the hopping rule. Rules are checked in alphabetic order concerning their names.

pattern pattern is a storage class pattern. If the incoming storage class matches this pattern, this rule is processed.

precious|cached|keep precious|cached|keep determines the mode of the replicated file. With keep the mode of the file will be determined by the mode of the destination pool.

-destination This defines which cell to use for the pool to pool transfer. By default this is the CELL-POOLMNGR and this should not be changed.

-overwrite In case, a rule with the same name already exists, it is overwritten.

-continue If a rule has been triggered and the corresponding action has been performed, no other rules are checked. If the continue option is specified, rule checking continues. This is for debugging purposes only.

-source -source defines the event on the pool which has triggered the hopping. Possible values are restore and write. restore means that the rule should be triggered if the file was restored from a tape and write means that it should be triggered if the file was written by a client.

-host Choose the id of a node of the farm of worker-nodes that should be allowed to access the file. Configure the POOLMNGR respectively.

-protType, -protMajor, -protMinor Specify the protocol which should be used to access the replicated files.

HoppingManager configuration examples

In order to instruct a particular pool to send a replicateFile message to the HOPPINGMANAGER service, you need to add the line pool.destination.replicate=HoppingManager to the layout file.

[exampleDomain]
[exampleDomain/pool]
name=write-pool
path=/path/to/write-pool-exp-a
pool.wait-for-files=${path}/data
pool.destination.replicate=HoppingManager

Assume that all files of experiment-a will be written to an expensive write pool and subsequently flushed to tape. Now some of these files need to be accessed without delay. The files that need fast acceess possibility will be given the storage class exp-a:need-fast-access@osm.

In this example we will configure the file hopping such that a user who wants to access a file that has the above storage info with the NFSv4.1 protocol will be able to do so.

Define a rule for hopping in the /var/lib/dcache/config/HoppingManager.conf file.

define hop nfs-hop exp-a:need-fast-access@osm cached -protType=nfs -protMajor=4 -protMinor=1

This assumes that the storage class of the file is exp-a:nfs@osm. The mode of the file, which was precious on the write pool will have to be changed to cached on the read pool.

The corresponding /var/lib/dcache/config/poolmanager.conf file could read like this:

#
# define the units
#
psu create unit -protocol   */*
psu create unit -net        0.0.0.0/0.0.0.0
psu create unit -store      exp-a:need-fast-access@osm
#
#
#  define unit groups
#
psu create ugroup  any-protocol
psu create ugroup  exp-a-copy-store
psu create ugroup allnet-cond
#
psu addto ugroup any-protocol */*
psu addto ugroup exp-a-copy-store    exp-a:need-fast-access@osm
psu addto ugroup allnet-cond 0.0.0.0/0.0.0.0
#
#
#  define the write-pool
#
psu create pool write-pool
#
#
#  define the read-pool
#
psu create pool read-pool
#
#
#  define the exp-a-read-pools poolgroup and add a pool
#
psu create pgroup exp-a-read-pools
psu addto pgroup exp-a-read-pools read-pool
#
#
#  define the exp-a-write-pools poolgroup and add a pool
#
psu create pgroup exp-a-write-pools
psu addto pgroup exp-a-write-pools write-pool
#
#
# define the exp-a-read-link, add exp-a-read-pools and set transfer preferences
#
psu create link exp-a-read-link exp-a-copy-store any-protocol allnet-cond
psu addto link exp-a-read-link exp-a-read-pools
psu set link exp-a-read-link -readpref=10 -writepref=0 -cachepref=10 -p2ppref=-1
#
#
# define the exp-a-write-link, add exp-a-write-pools and set transfer preferences
#
psu create link exp-a-write-link exp-a-copy-store any-protocol allnet-cond
psu addto link exp-a-write-link exp-a-write-pools
psu set link exp-a-write-link -readpref=0 -writepref=10 -cachepref=0 -p2ppref=-1
#
#
#