What is there for a user or admin or programmer to know about the new WAL
stuff? What all does it do? Does it allow for an audit file to be created,
which can be used to playback and/or rewind the transactions on the database
by user/admin commands? How do checkpoints limit or affect how far back
recovery is possible (if at all)? When is a checkpoint made? Does it allow
for online recovery or only offline? What are the settings/parameters that
control it (if any, like size of log at which to cut off a new one)? Can the
WAL files that are made be read by humans and where are they stored? How
transparent is this feature? Maybe I'm confused!
I have some familiarity with a mainframe database that made audit tapes of
all transactions. A nightly full dump was also made. Kinda old stuff. The
tapes were kept for something like 2 weeks before being recycled. If the
database crashed, it was possible to restore the database back to any time by
using a full dump and some audit tapes. The dumps and audit tapes were
specific to a database, not the whole DBMS. Other databases could be up and
running normally while one was being rolled back and then forward again after
fixing some problem. Even the one being restored could be online, queuing or
processing some queries until the recovery was done. Something like that, it
was a hospital environment. How does WAL compare to any of this, if at all?
Can WAL be described as a deferred fsync of a batch of transactions?
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