hi list:
im working in the setup of a warm standby server. im using
postgres-8.2.5 in the slave and 8.2.4 in master right now, in
production it will be only 8.2.5, everything in debian etch.
we decided this solution because we want an easy to
migrate/implement/adminstrate backup sever, and we have a few minutes
of data loss (if we can be precise with the "starting point in time"
of that loss).
by now, i have the warm standby server working fine (at least, it logs
that processes the archives...).
if i am undestanding, the "complete" (in broad strokes) process for
WAL archiving and warm standby, is something like this:
1) the primary server processes some transactions and
generates some "WAL segment files" in the $DATA/pg_xlog/ dir of 16MB
each (by default).
2) if archive_command is activated and working, the primary
server sends (preferably with rsync or some other "atomic tool") the
NEW WAL files to the standby server. Later, at some point, the primary
server will delete this files when considers that are not necessary
anymore.
3) the standby server receives the archive files and processes
them somehow (almost everybody does this with pg_standby by now?) and
keep waiting for a new WAL file or the trigger file.
now, as you can see, im not a expert on database tech, maybe only a
"brave admin", so, reading the docs and seeing the warm standby
"working", i have a few questions (in any case, links to docs a
welcome!!):
-a "WAL segment file" is the same that a "log file segment"?
-what are the "log file segments" referenced by checkpoint_segments?
are the "binary logs" where postgres stores lasts transactions, the
ones in $DATA/pg_xlog dir? if this is true, then:
what means "Maximum distance between automatic WAL checkpoints"???
this is how often, in "log file segments", postgres will perform a
checkpoint, generating a special checkpoint record from which to start
the redo operation, dont?
-what is the "restartpoint" named in the "Warm Standby Servers for
High Availability" page?
(http://www.postgresql.org/docs/8.2/static/warm-standby.html) i cant
find a definition in any other doc...
-how often a new WAL file is generated? this depends on the server load?
-in one WAL file i could have one, more than one and even an
incomplete "transaction"?? (where is the doc i need to read?!)
-if i have incomplete transactions on a WAL, how the standby server
processes that? what if the rest of the transaction never reaches the
standby server?
-how do i know exactly at which point in time (in transactions) my
standby server is, if i have to switch to primary role?
-how many archive files is safe to keep in the standby server? right
now, i have "-k 100" in the pg_standby opts but in the pg_standby
README says:
"You should be wary against setting this number too low,
since this may mean you cannot restart the standby. This
is because the last restartpoint marked in the WAL files
may be many files in the past and can vary considerably.
This should be set to a value exceeding the number of WAL
files that can be recovered in 2*checkpoint_timeout seconds,
according to the value in the warm standby postgresql.conf.
It is wholly unrelated to the setting of checkpoint_segments
on either primary or standby."
i cant use the %r because im in 8.2 and not thinking in upgrade by now...
this is related to the restartpoint in my previous question... but i
dont know exactly what it is so i cant make a more detailed analysis.
finally: does anybody have a config like this one working with
heartbeat? is that recommendable?
wow! sorry for the huge post... i think i tied some loose ends just by
writing this email!! :D
thanks in advance!!
--
Roberto Scattini
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