Re: Is it really necessary to mirror the physical log?

Without mirroring your physical log you're wasting disk space mirroring your logical log. If you loose your physical log there wouldn't be any "before pages" to restore so that you can roll forward the logical logs from. The recovering of the before images from the physical log is the first step in the fast recovery. Without them, your instance will not go "online" and you will have to restore from archive. The reason Informix will not go online is that the data will not be consistent (all transactions committed or rolled back). Say, you crash 3 minutes after your last checkpoint. During those 3 minutes page cleaners may have written modified data to disk and the transactions that modified the data was logged. Some modified data may be on disk and some still in memory. For Informix to guarantee consistency, fast recovery first restores the physical log to memory from the last checkpoint. In this case it's 3 minutes of physical log. Then it checkpoints again, to write the data to disk and reposition the start pointer in the physical log. Then it finds the last checkpoint in the logical log and replays the transactions until the end (this includes writing "before pages" back to the physical log). When it gets to the end of the log it rolls back any uncommitted transactions. Hope this helps, Scott Kolaya Jacob Salomon wrote in message <3563594E.9634D3C1@garpac.com>... >Hi family. > >I am about ot start configuring a new site. I am setting up for many >dbspaces but mirroring is basically a luxury here. I have made it my >policy to *always* mirror the root dbspace and that dbspace containing >the logical logs, because if either of those goes south, there goes my >system and I MUST recover from archive tapes. > >Q: If I choose not too mirror the dbspace containing the physical log >and that DBspace gets corrupted, is recovery still possible without >going back to archive tapes? Would fast recovery still work, albeit >with the possible loss of some recent updates? > >My suspicion is that mirroring the physical log is a waste of disk space >as well as I/O bandwidth when it gets flushed. > >Qualified opinions anyone? > >Thanks. >-- > -- Jake (Retrospectively realizes there is no future in hindsight) >+------------------------------------------------------------+ >| The expedient performance of a task with excessive concern | >| regarding its duration-to-completion engenders a virtual | >| certainty of diminished benefit therefrom. | >| -- Benjamin Franklin (but he said it in 3 words) | >+------------------------------------------------------------+