The Dirty Region Logging feature of VERITAS Volume Manager (tm) for Windows 2000 and VERITAS Storage Foundation (tm) for Windows

Article:TECH17630  |  Created: 2001-01-28  |  Updated: 2004-01-03  |  Article URL http://www.symantec.com/docs/TECH17630
Article Type
Technical Solution


Environment

Issue



The Dirty Region Logging feature of VERITAS Volume Manager (tm) for Windows 2000 and VERITAS Storage Foundation (tm) for Windows

Solution



Dirty Region Logging (DRL) logically divides a volume into a set of consecutive regions. DRL then keeps track of volume regions that are being written to. A dirty region log is maintained that contains a status bit representing each region of the volume. For any write operation to the volume, the regions being written are marked dirty in the log before the data is written. If a write causes a log region to become dirty, when it was previously clean, the log is synchronously written to disk before the write operation can occur. On system restart, Volume Manager will recover only those regions of the volume that are marked as dirty in the dirty region log.

Log subdisks are used to store the dirty region log of a volume that has DRL enabled. A volume with DRL has at least one log subdisk; multiple log subdisks can be used to mirror the dirty region log. Each log subdisk is associated with one of the volume's plexes. Only one log subdisk can exist per plex.

If the plex contains only a log subdisk and no data subdisks, that plex can be referred to as a log plex. The log subdisk can also be associated with a regular plex containing data subdisks, in which case the log subdisk risks becoming unavailable in the event that the plex must be detached due to the failure of one of its data subdisks. If the vxassist command is used to create a dirty region log, it creates a log plex containing a single log subdisk, by default. A dirty region log can also be created manually by creating a log subdisk and associating it with a plex. In the latter case, the plex may contain both a log subdisk and data subdisks.

Only a limited number of bits can be marked dirty in the log at any time. The dirty bit for a region is not cleared immediately after writing the data to the region. Instead, it remains marked as dirty until the corresponding volume region becomes the least recently used. If a bit for a given region is already marked dirty and another write to the same region occurs, it is not necessary to write the log to the disk before the write operation can occur.

Caution: While DRL makes sure that the mirror copies of the mirrored volumes are in synchronisation following a system crash, it does not guarantee data integrity.

If DRL is not used and a system failure occurs, all mirrors of the volumes must be restored to a consistent state by copying the full contents of the volume between its mirrors. This process can be lengthy and I/O intensive; it may also be necessary to recover the areas of volumes that are already consistent.



Legacy ID



243080


Article URL http://www.symantec.com/docs/TECH17630


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