Handbook of Local Area Networks, 1998 Edition:LAN Management Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Data Migration Archiving data is an explicit action performed by a user or LAN administrator when files have been determined to be dormant. Sometimes the system will take this action according to previously published system rules. Files are removed from the primary system and the names are removed from the primary file system directory tree. Retrieving them is also an explicit act. This requires locating the desired file(s) in the file history data base and then restoring them. Archiving is performed as a routine practice for maintaining files over time and to keep them from cluttering expensive primary storage. Data migration provides an automatic method of moving data from primary to near-online storage in order to maintain more free primary storage. This is done automatically according to aging rules such as those discussed for archiving (e.g., files that have been dormant for three months would be moved to near-online storage). Exhibit 7-4-6 illustrates this concept. Exhibit 7-4-6.  Schematic of Data Migration With data migration, however, file names are not removed from the original name directory and will still appear to a user browsing those directories as if they are online. When a user attempts to access a file, the file is restored from near on-line to primary online storage. The user or LAN administrator does not have to do any explicit moving of data or tracking of files. These are performed automatically by the data migration system. Data migration must make use of near-online media or retrieve times become too long, and system time-outs become a potential problem. Typically near-online storage employs high-capacity, rotating, removable media (e.g., optical or Bernoulli disks). Before installing such a system, it is important to make sure it is matched to the environment. Automatic data migration is similar to providing virtual storage for the primary disk system. The rules for when files are moved and when and how they are restored are sensitive to various virtual memory afflictions such as thrashing. Using Hierarchical Storage Data migration, archiving, and backup can be planned as one logical, harmonious set of actions. Archiving and data migration are used for managing storage. When using both these methods, the communications manager would set up the data migration using, for example, a near-online optical disk jukebox. The communications manager might archive from there to an auto-loading tape device and eventually move the archive files off-site. If the archives must be kept for a long time, then specific optical media might be used for the permanent archives. Weekly and daily backups would be performed to the tape auto loader, with media being rotated from there off-site according to a schedule. The file history system would be used for tracking all copies of the files. Such a system is called hierarchical, because data moves down the hierarchy from primary online storage to fast-access near-online storage, to a slower access near-online storage, to offline and off-site storage. If all these levels of storage are to be managed, it has to be done automatically or the LAN administrator will be devoted solely to this task. DISK FRAGMENTATION Fragmentation has been a problem with storage systems since the introduction of direct access rotating media. When the file system needs to write data to a disk, it searches for the first available block and starts writing to this block. If the block is smaller than the amount of data that needs to be written, the storage algorithm will break up the data and store the remainder in another open block on the disk. If, for example, a 1M-byte file is being written to disk, the disk space found may be a 500K-byte open block. The first half of the file is written to this area and then the system continues seeking. The next free space may be 300K byte, and more of the file is written. Finally, 200K bytes of open space is located and the remainder of the file is laid down on disk. The single file has been fragmented into three pieces. If these three fragments are physically far apart on the disk, the read head will have to move to retrieve them. Moving the disk head is a relatively slow mechanical operation, thus fragmentation adversely affects performance. Normally, a file system will try to place all information on the disk contiguously. However, after files have been written and erased, holes are left. It is unfortunate that as disks fill up they are more prone to fragmentation, and the newest, most active files are usually the most severely fragmented. The effect of fragmentation can be minimized by systems employing efficient storage allocation techniques and disk caching. Administrators can help by ensuring that ample free space is available so the system has room to manage the storage. Even so, sometimes defragmentation is required. Defragmentation can be accomplished best using specialized utilities that have been designed for the task. These utilities rewrite data into continuous blocks to reduce disk arm movement. An alternate method for defragmenting a volume involves the deletion and restoration of all data. When data is laid down on a fresh, unused volume, it is written contiguously; therefore, the whole volume can be backed up, the data deleted, and then a complete restore done. This should not be undertaken lightly. But with a proper full volume backup system it can be a fairly painless operation. It would be prudent to undertake multiple full backups and some restores to test the media and system before undertaking this form of defragmentation. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.