Chapter 2 HPSS PlanningHPSS Installation Guide September 2002 131Release 4.5, Revision 2• Encina SFS DataActual HPSS metadata is stored in SFS data volumes. SFS data volumes store the actual SFS recorddata as well as associated index and B-tree overhead information. SFS must have sufficient diskspace allocated for its data volumes in order to store the projected amount of HPSS metadata.The metadata sizing spreadsheet, discussed in the previous section, needs to be used to calculatethe size of each SFS data volume and the distribution of the SFS data files across the data volumes.Any SFS data file that is estimated to be greater than 1 GB needs to reside on its ownSFS data volume.• Encina Transaction logThe Encina transaction log is a wrap-around log used to store transaction information. It records allchanges in SFS data on a per-transaction basis. In the event of a system or server crash, upon restartthe SFS server reads the transaction log to determine what, if any, transactions were in progress butnot committed and properly aborts all data changes associated with that transaction. Likewise, itproperly updates all SFS data for transactions that had been committed but not written to disk inthe SFS data volumes.The optimal type of disk used for the transaction log is a low-latency disk since the I/O isperformed in many small chunks. To ensure full recoverability in the event of a media failure onthe transaction log disk, it is important to mirror the transaction log on a separate physical disk.The size of the transaction log is dependent on the volume of HPSS transactions. Start with no lessthan 256 MB (512 MB for larger installations). It is best to start small and increase the size ifnecessary.A large log volume will increase SFS start time.Since I/O delays associated with the transaction log can affect HPSS performance, it isrecommended that few, if any, other logical volumes be placed on the same disks used for thetransaction log. For example, it would be unwise to use the rootvg volume group for the transactionlog or put the transaction log on the same disk where paging space has been created.• Encina Media Recovery Archive Files (MRA files)Because media failures may occur between SFS backups, it is important to be able to replay all SFStransactions from the last SFS backup up to the point of a media failure. Encina “media archiving”is the feature that enables this level of recovery. When media archiving is enabled, a runningbackup of the transaction log is written to disk. The transaction log wraps around, but mediaarchive files do not. The administrator can then move these media archive files to offline storage forsafe keeping. If media archiving has not been enabled prior to a media failure, the SFS data cannotbe restored.Because media failures can occur where these backup files are written, mirroring can be consideredfor improved reliability. It is recommended that at least 2 GB of disk space be available for themedia recovery archive files.
