prefetch(8)                                                             prefetch(8)

NAME

       prefetch - NetWorker prefetch command

SYNOPSIS

       prefetch -v {ssh|acs} -j " jukebox1 centricstor1 " -j ...

DESCRIPTION

 	CentricStor (a Virtual tape library) can be used with NetWorker like
	real tape libraries. NetWorker sees a (virtual) ACS robotic interface
	and many (virtual) tape drives in a SAN. The timing behaviour of tape
	operations may be different from physical libraries. Since CentricStor
	uses a big internal disk cache for the virtual volumes, mount opera-
	tions are very fast in case of a cache hit. The mount operation will
	succeed immediately.
	
	On the other hand, if a (virtual) volume is requested, that is purged
	from the cache, the mount operation requires additional time. The real
	tape has to be loaded and the entire virtual volume has to be read into
	the cache. This would be a substantial disadvantage especially for
	huge recovers with a high probability of cache misses for the virtual
	volumes. Each volume needs the time to be read from the physical tape
	into the disk array plus the time to recover the data from the virtual
	tape drive back to its destination.
	
	Therefore, to achieve maximum recover speed, a mechanism for read ahead
	or prefetching of volumes is needed. Prefetching all needed volumes is
	undesirable, because the cache might fill up with volumes that are not
	needed (if the reocver process is aborted). Therefore, our prefetch
	mechanism uses a number of volumes as a window size for the read ahead.
	This window or bucket size is configurable with the variable
	
	PREFETCH_BUCKET_SIZE_<jukebox name>=<value>
	
	To avoid running out of prefetched volumes during the recover, the next
	bucket of volumes will be prefetched, once the number of prefetched but
	not yet read volumes has fallen below a low water mark. It is config-
	urable with the variable
	
	PREFETCH_LOW_WATER_MARK_<jukebox name>=<value>
	
	A prefetch notification, which is triggered by the nsrmmd daemon, exe-
	cutes the CentricStor prefetch command (usr/sbin/prefetch). A list of
	volume names, barcodes and locations is the input of the prefetch noti-
	fication.
	

CONFIGURATION

 	Set the following variables for nsrmmd:
	
	PREFETCH_BUCKET_SIZE_<jukebox name>=<number of volumes for the read
	ahead>
	
	PREFETCH_LOW_WATER_MARK_<jukebox name>=<value>
	
	<jukebox name> is the name attribute of the respective jukebox resource
	with lower case letters transformed to upper case letters and nonal-
	phanumeric characters transformed to "_".
	
	A notification has to be created with the following resource
	attributes:
	
	- action:
	
		/usr/sbin/prefetch -v {ssh|acs} -j "jb1 cs1" -j ...
	
	- event: Media
	
	- priority: Notice
	
	Parameter for prefetch:
	
	-v: Specifies the communication method to CentricStor
	
	ssh: Communication via ssh
	
	acs: Communication via ACSLS interface
	
	-j:
	
	Translates a jukebox name to the name of the corresponding CentricStor.
	More than one "-j" option can be specified. If the jukebox name con-
	tains blanks they have to be substituted by "_"
	
	Prefetch using SSH as remote command execution method
	-----------------------------------------------------
	
	With CentricStor V4 and later the command execution on the CS host is
	done with 'ssh'. For this to work one must create and install SSH keys
	on the NetWorker server and on the CentricStor VLP.
	
	On the NetWorker server, create a pair of SSH keys
	
	# mkdir /nsr/res/.ssh
	
	# ssh-keygen -t dsa -C 'NetWorker prefetch' -P '' -f
	/nsr/res/.ssh/prefetch_key
	
	This will create a key pair consisting of the files
	/nsr/res/.ssh/prefetch_key /nsr/res/.ssh/prefetch_key.pub
	
	Make sure the private key file /nsr/res/.ssh/prefetch_key is owned by
	root and only the owner can read the file.
	
	# ls -l /nsr/res/.ssh
	
	-rw------- 1 root root 668 Dec 20 12:56 prefetch_key
	
	-rw-r--r-- 1 root root 608 Dec 20 12:56 prefetch_key.pub
	
	Install the public key in the authorized_keys file of user 'root' on
	the CentricStor VLP
	
	Copy the public key /nsr/res/.ssh/prefetch_key.pub to the CentricStor
	VLP host.
	
	Log in to the CentricStor VLP host and add the public key to the
	allowed SSH keys:
	
	# cat prefetch_key.pub >>/root/.ssh/authorized_keys
	
	It should now be possible from the NetWorker server host to execute
	commands on the CentricStor VLP via SSH without a password.
	
	Example: On the NetWorker server execute
	
	# ssh -i /nsr/res/.ssh/prefetch_key root@VLP uname -a
	
	You should see the output of the 'uname -a' command without being asked
	for a password.

NetWorker 8.0.1 		Dec 02, 12 		prefetch(8)