Virtual Branches

from 

ACORN Software

"storage in a nutshell"


Virtual Branches

Product Description



VirtualBranches provides a "disk farm in a box". For a wide variety of 
Optical and CDROM data libraries each I/O surface in the data library 
appears to be an individual disk. No special action need be taken by the 
user of the disk, other than to mount it, in order to access data. The 
user simply reads and write data, VirtualBranches takes care of moving 
media to and from the physical drives in the data library while 
providing access to the entire contents of the data library. 
VirtualBranches provides two unique features available in no other 
product: 
  *	Correct MSCP serving of robot-served disk drives. 
  *	Fragmentation Avoidance. 



Product Availability

VirtualBranches V2.2-1 is available for a 45-day free trial and may be 
freely downloaded and installed via FTP or Gopher. Documentation, 
supplied in machine readable form with the installation kits, may be 
printed at will. If this is not convenient, then hard copies of the 
documentation are available from Acorn Software. If downloading is not 
possible or is not convenient, then VirtualBranches may be ordered from 
Acorn Software on most popular media (TK50, 4mm, 8mm, and MO). 

Price List




Virtual Branches and MSCP Serving

VirtualBranches disks are NOT MSCP served to a cluster. Rather, each 
cluster node has a server which communicates with the current master 
node (a master node is one which can control the robot inside a data 
library) to coordinate MSCP traffic. 
This is necessary because of a limitation in MSCP serving. If sufficient 
load is reached on MSCP served disks, and I/O to those disks cannot 
complete quickly enough, the underlying cluster communications services 
drops the connection to that served disk and attempts to rebuild it, 
putting all the mounted disks into mount verify, thus further increasing 
the I/O load at the MSCP level and causing a positive feedback that 
ultimately requires the rebooting of the entire cluster to clear the 
problem. If the VirtualBranches disks were MSCP served (as other 
products have them including Digital's OSMS), VirtualBranches would 
suffer from this problem as well. 
Thanks to the unique design of VirtualBranches, no MSCP traffic is 
queued for a VirtualBranches disk unless the associated media is known 
to be physically resident in a drive. Thus, cluster operation will 
remain stable with VirtualBranches regardless of the load level.


------------------------------------------------------------------------

Copyright (c) 1996 Acorn Software, Inc. All rights reserved 
