Update faq's.
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Bruce Momjian
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This outlines how to increase the number of shared memory buffers
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supported by BSD/OS. By default, only 4MB of shared memory is supported
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by BSDI.
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Bruce Momjian (pgman@candle.pha.pa.us)
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---------------------------------------------------------------------------
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First, increase SHMMAXPGS by 1024 for every additional 4MB of shared
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memory:
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/sys/sys/shm.h:69:#define SHMMAXPGS 1024 /* max hardware pages...
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The default setting of 1024 is for a maximum of 4MB of shared memory.
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Second, use bpatch to find the sysptsize value for the current kernel.
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This is computed dynamically at bootup.
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$ bpatch -r sysptsize
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0x9 = 9
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Next, change SYSPTSIZE to a hard-coded value. Use the bpatch value,
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plus add 1 for every additional 4MB of shared memory you desire.
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/sys/i386/i386/i386_param.c:28:#define SYSPTSIZE 0 /* dynamically...
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sysptsize can not be changed by sysctl on the fly.
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This should clearly be easier to do on BSDI. I will add a BSDI FAQ for
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PostgreSQL to cover this. One downside is that shared memory is not
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pageable. It is locked in RAM.
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@ -43,7 +43,7 @@ From owner-pgsql-hackers@hub.org Fri Dec 24 10:01:18 1999
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@ -129,7 +129,7 @@ From owner-pgsql-hackers@hub.org Fri Dec 24 18:31:03 1999
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@ -486,7 +486,7 @@ From owner-pgsql-hackers@hub.org Sun Dec 26 08:31:09 1999
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@ -905,3 +905,100 @@ Sys Admin
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************
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From owner-pgsql-hackers@hub.org Thu Dec 30 08:01:09 1999
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Thu, 30 Dec 1999 06:30:58 -0400
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From: Duane Currie <dcurrie@sandman.acadiau.ca>
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Message-Id: <199912301030.GAA18698@sandman.acadiau.ca>
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Subject: Re: [HACKERS] database replication
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In-Reply-To: <OFD38C9424.B391F434-ON85256851.0054F41A@black-oak.COM> from "DWalker@black-oak.com" at "Dec 24, 99 10:27:59 am"
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To: DWalker@black-oak.com
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Date: Thu, 30 Dec 1999 10:30:58 +0000 (AST)
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Status: OR
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Hi Guys,
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Now for one of my REALLY rare posts.
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Having done a little bit of distributed data systems, I figured I'd
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pitch in a couple cents worth.
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> 2) The replication system will need to add at least one field to each
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> table in each database that needs to be re plicated. This
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> field will be a date/time stamp which identifies the " last
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> update" of the record. This field will be called PGR_TIME
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> for la ck of a better name. Because this field will be used
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> from within programs and triggers it can be longer so as to not
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> mistake it for a user field.
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I just started reading this thread, but I figured I'd throw in a couple
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suggestions for distributed data control (a few idioms I've had to
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deal with b4):
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- Never use time (not reliable from system to system). Use
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a version number of some sort that can stay consistent across
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all replicas
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This way, if a system's time is or goes out of wack, it doesn't
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cause your database to disintegrate, and it's easier to track
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conflicts (see below. If using time, the algorithm gets
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nightmarish)
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- On an insert, set to version 1
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- On an update, version++
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- On a delete, mark deleted, and add a delete stub somewhere for the
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replicator process to deal with in sync'ing the databases.
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- If two records have the same version but different data, there's
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a conflict. A few choices:
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1. Pick one as the correct one (yuck!! invisible data loss)
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2. Store both copies, pick one as current, and alert
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database owner of the conflict, so they can deal with
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it "manually."
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3. If possible, some conflicts can be merged. If a disjoint
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set of fields were changed in each instance, these changes
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may both be applied and the record merged. (Problem:
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takes a lot more space. Requires a version number for
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every field, or persistent storage of some old records.
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However, this might help the "which fields changed" issue
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you were talking about in #6)
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- A unique id across all systems should exist (or something that
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effectively simulates a unique id. Maybe a composition of the
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originating oid (from the insert) and the originating database
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(oid of the database's record?) might do it. Store this as
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an extra field in every record.
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(Two extra fieldss so far: 'unique id' and 'version')
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I do like your approach: triggers and a separate process. (Maintainable!! :)
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Anyway, just figured I'd throw in a few suggestions,
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Duane
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************
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