Re: How many Mandatory Process are there in Postgres

Hi All,

Thank you for the prompt reply on this.

Please find the output of the top command and the process availabe. Could
explain what logger process is here for

top - 12:41:57 up 17:51, 3 users, load average: 0.00, 0.04, 0.01
Tasks: 141 total, 1 running, 139 sleeping, 0 stopped, 1 zombie
Cpu(s): 0.3%us, 1.4%sy, 0.0%ni, 98.3%id, 0.0%wa, 0.0%hi, 0.0%si,
0.0%st
Mem: 1417256k total, 1300888k used, 116368k free, 41468k buffers
Swap: 2097144k total, 22156k used, 2074988k free, 1017980k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23466 postgres 15 0 4664 1508 1228 S 0.0 0.1 0:00.10 -bash
25983 postgres 15 0 44620 3032 2680 S 0.0 0.2 0:00.43
/usr/local/pgsql/bin/postgres
25984 postgres 18 0 8608 632 296 S 0.0 0.0 0:00.04 postgres:
logger process
25986 postgres 15 0 44604 1656 1292 S 0.0 0.1 0:00.10 postgres:
writer process
25987 postgres 15 0 44604 932 588 S 0.0 0.1 0:00.11 postgres: wal
writer process
25988 postgres 15 0 44756 1232 680 S 0.0 0.1 0:00.17 postgres:
autovacuum launcher process
25989 postgres 18 0 8604 648 304 S 0.0 0.0 0:00.04 postgres:
archiver process
25990 postgres 15 0 8740 900 408 S 0.0 0.1 0:00.31 postgres: stats
collector process
25993 postgres 15 0 4964 1464 1196 S 0.0 0.1 0:00.17 psql
25994 postgres 15 0 46264 6212 4672 S 0.0 0.4 0:00.23 postgres:
postgres postgres [local] idle

Regards
Raghavendra

On Fri, Mar 26, 2010 at 9:22 AM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>wrote:

> On Thu, Mar 25, 2010 at 9:03 PM, Tadipathri Raghu <traghu(dot)dba(at)gmail(dot)com>
> wrote:
> > Hi Scott, Thomas,
> >
> > Thank you for the update.
> >
> >>
> >> >> Oracle uses a completely different implementation of MVCC
> architecture.
> >> >> It
> >> >> overwrites the data and then uses rollback segments to provide
> >> >> 'previous
> >> >> versions' to running transactions etc.
> >> >>
> >> >> PostgreSQL does not overwrite the data - it just creates a copy of
> the
> >> >> row
> >> >> and then decides which version should each session see (depending on
> >> >> the
> >> >> transaction IDs etc.). So it does not need to do rollbacks the way
> >> >> Oracle
> >> >> does, but it has to remove stale copies of the rows (such that no
> >> >> running
> >> >> transaction can see) - that's why there is VACUUM.
> >> >
> >> >
> >> > Here, if you have issued a command pg_start_backup() at that time the
> >> > cluster is freezed, and if any transaction takes place before the
> >> > pg_stop_backup() issued at that time where the transaction data will
> be
> >> > kept
> >> > if the undo's are not there.
> >>
> >>
> >> What do you mean by 'freezed'? The cluster operates normally, the
> >> pg_start_backup() just creates a backup label (and performs a
> checkpoint),
> >> but that's not a problem. OK, there could be a performance decrease
> >> because of full page writes, but the data will be processed as if there
> is
> >> no backup running. PostgreSQL does not need the checkpoints to perform
> >> backup.
> >
> >> The 'UNDO' data is just kept in the main data files. Then, based on
> what
> >> your xid is, and the xmin / xmax on each row, it's either visible or
> not.
> >> That's what vacuum does, clean up the rows that are for 'undo' and can
> >> never been seen anymore. The REDO itself is kept in the transaction
> logs.
> >
> >
> >>
> >> Like Thomas says, the pg_start_backup() just creates a label that tells
> >> the restoring database what pg_xlog record to start with when you
> 'restore'
> >> your data. The 'UNDO' type of data (just the old rows actually) is just
> >> kept in the main table until vacuum nukes 'em.
> >
> > --Scott
> > Q1. Does vacuum process come into existence even you turn off the
> > autovacuum.? What is the main work for vacuum process, to the clean the
> > buffers or work only when autovacuum in on and to clean up the rows that
> are
> > 'undo'
>
> 1a: Yes, if you are approaching transaction id wraparound (txid wrap)
> then autovacuum will crank up a vacuum to fix that situation whether
> you like it or not. The alternative is db shutdown.
> 1b: The main job of vacuum is to free dead tuples, which postgresql
> accumulates as it updates or deletes tuples. When you have several old
> versions of a tuple to collect, vacuum recycles the space and makes it
> available to other updates to now use instead of allocating on the end
> of the relation.
>
> > Q2. Do you mean to say there is a vacuum process which will clean up the
> > buffers that are for 'undo'?
>
> No buffers for undo. It's all stored on disc, ready for instant
> access. Try it on a test db. begin; load a table with 1M rows;
> rollback; sub second response. A million changes just disappeared
> instantly. And now you've got 1M dead tuples in that table. Vacuum's
> job is to make them available for re-use. But they're all in one big
> consecutive file so the performance is actually pretty good as the
> table repopulates, especially if it'll get back up to 1M records again
> in the near future. As long as it got vacuumed.
>
> > Q3. Fine, if the vacuum is off then your data files will hold the undo
> data
> > also in it?
>
> Yep. Until a forced vacuum due to txid wrap occurs.
>