By Brian Edminster
First of two parts
Once store-to-disk backups are regularly being processed, it’s highly desirable to move them offsite — for the same reasons that it’s desirable to rotate tape media to offsite storage. You want to protect against site-wide catastrophic failures. It could be something as simple as fire, flood, or a disgruntled employee, or as unusual as earthquake or act of war.
Regardless of the most pressing reason, it really is important to keep at least some of your backups offsite, so as to facilitate rebuilding / recovering from scratch, either at your own facility, or at a backup/recovery site.
The problem comes in that the MPE/iX file system is far more structured than Unix, Windows, or any other non-MPE/iX file system-based storage mechanisms. While transferring a file off MPE/iX is easy via FTP, sftp/scp, or rsync, retrieving it is problematic, at least if you wish the retrieved files and the original store-to-disk files to be identical (i.e., with the same file characteristics: filecode, recsize, blockfactor, type, and so forth).
What would be optimal is automatic preservation of these attributes, so that a file could be moved to any offsite storage that could communicate with the MPE/iX system. Posix on MPE/iX comes to the rescue.
If you do not specify the ‘buildparms’ for a file being retrieved, it will default to the file-type implied by the FTP transfer mode: ASCII (the default), binary, or byte-stream (often called ‘tenex’ on Unix systems). The respective defaults used are shown below:
What follows is an example of automatic preservation of these attributes, so that a file could be moved to any offsite storage that could communicate with the MPE/iX system. And this is yet again where Posix comes to the rescue, via the venerable ‘tar’ (Tape ARchiver), or ‘pax’ archiving utilities.
‘pax’ is a newer backup tool, designed to be able to read/write with tar format archives, newer ‘ustar’ format (that includes Extended Attributes of files). At the same time it has a more ‘normal/consistent’ command syntax (as Unix/Posix stuff goes, anyway), plus a number of other improvements. Think of it as tar’s younger (and supposedly more handsome) brother.
A little known feature of most ‘late-model’ tar and all pax commands is the ability for it to recognize and utilize Extended Attributes. These will vary with the target implementation platform, but for the tar and pax commands included with releases after v5.5 of MPE/iX this capability is not only present — but contrary to the man command’s output and HP’s Posix Command Line manual, it’s the default! You use the -A switch to turn it off, returning tar to a bytestream-only tool.
While not externally documented, via a little experimentation I’ve determined that the following series of Extended Attributes value-pairs are in the MPE/iX Posix implementation of a tar or pax ‘file header’ for each non-Posix file archived:
MPE.RECORDSIZE= value in bytes
MPE.BLOCKFACTOR= integer value
MPE.RECORDFORMAT= integer value (0=unstructured?)
MPE.CCTL= integer value (0=nocctl)
MPE.ASCII= integer value (0=binary, 1=ascii)
MPE.FILECODE= integer value, absent for ‘0’
MPE.FILELIMIT= value in bytes
MPE.NUMEXTENTS= integer value, may be absent
MPE.NUMUSERLABELS= integer value (0=no user labels), and
MPE.USERLABELS=[binary content of user labels]