Update mmap details:
Update mmap details:

<   portability issues. Anonymous mmap is required to prevent I/O
<   overhead.
>   portability issues. Anonymous mmap (or mmap to /dev/zero) is required
>   to prevent I/O overhead.
>
> * Consider mmap()'ing files into a backend?
>
>   Doing I/O to large tables would consume a lot of address space or
>   require frequent mapping/unmapping.  Extending the file also causes
>   mapping problems that might require mapping only individual pages,
>   leading to thousands of mappings.  Another problem is that there is no
>   way to _prevent_ I/O to disk from the dirty shared buffers so changes
>   could hit disk before WAL is written.

chrishajas/gpdb,cjcjameson/gpdb,ovr/postgres-xl,ahachete/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,jmcatamney/gpdb,adam8157/gpdb,foyzur/gpdb,ahachete/gpdb,xinzweb/gpdb,atris/gpdb,Quikling/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,edespino/gpdb,tangp3/gpdb,tpostgres-projects/tPostgres,rvs/gpdb,ashwinstar/gpdb,lpetrov-pivotal/gpdb,zaksoup/gpdb,janebeckman/gpdb,cjcjameson/gpdb,zeroae/postgres-xl,yuanzhao/gpdb,CraigHarris/gpdb,jmcatamney/gpdb,snaga/postgres-xl,rubikloud/gpdb,snaga/postgres-xl,greenplum-db/gpdb,janebeckman/gpdb,yuanzhao/gpdb,snaga/postgres-xl,cjcjameson/gpdb,lintzc/gpdb,chrishajas/gpdb,tangp3/gpdb,royc1/gpdb,snaga/postgres-xl,tangp3/gpdb,0x0FFF/gpdb,kmjungersen/PostgresXL,greenplum-db/gpdb,janebeckman/gpdb,rvs/gpdb,lisakowen/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,rvs/gpdb,kaknikhil/gpdb,edespino/gpdb,snaga/postgres-xl,pavanvd/postgres-xl,atris/gpdb,xuegang/gpdb,arcivanov/postgres-xl,xuegang/gpdb,chrishajas/gpdb,zeroae/postgres-xl,zaksoup/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,adam8157/gpdb,lpetrov-pivotal/gpdb,Postgres-XL/Postgres-XL,CraigHarris/gpdb,zaksoup/gpdb,50wu/gpdb,adam8157/gpdb,randomtask1155/gpdb,lintzc/gpdb,kaknikhil/gpdb,0x0FFF/gpdb,Chibin/gpdb,lintzc/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,janebeckman/gpdb,tangp3/gpdb,yuanzhao/gpdb,yazun/postgres-xl,ovr/postgres-xl,arcivanov/postgres-xl,xuegang/gpdb,foyzur/gpdb,Quikling/gpdb,ashwinstar/gpdb,royc1/gpdb,postmind-net/postgres-xl,tangp3/gpdb,Chibin/gpdb,Chibin/gpdb,zeroae/postgres-xl,foyzur/gpdb,rvs/gpdb,postmind-net/postgres-xl,Quikling/gpdb,tpostgres-projects/tPostgres,oberstet/postgres-xl,greenplum-db/gpdb,zaksoup/gpdb,randomtask1155/gpdb,cjcjameson/gpdb,lisakowen/gpdb,zeroae/postgres-xl,randomtask1155/gpdb,xuegang/gpdb,lpetrov-pivotal/gpdb,yuanzhao/gpdb,lintzc/gpdb,tangp3/gpdb,ashwinstar/gpdb,ahachete/gpdb,0x0FFF/gpdb,ahachete/gpdb,50wu/gpdb,tangp3/gpdb,rubikloud/gpdb,kaknikhil/gpdb,royc1/gpdb,lisakowen/gpdb,yuanzhao/gpdb,Postgres-XL/Postgres-XL,cjcjameson/gpdb,kaknikhil/gpdb,pavanvd/postgres-xl,zaksoup/gpdb,foyzur/gpdb,CraigHarris/gpdb,edespino/gpdb,zeroae/postgres-xl,Chibin/gpdb,yuanzhao/gpdb,cjcjameson/gpdb,randomtask1155/gpdb,edespino/gpdb,ahachete/gpdb,xuegang/gpdb,xuegang/gpdb,ashwinstar/gpdb,kaknikhil/gpdb,lisakowen/gpdb,ovr/postgres-xl,kmjungersen/PostgresXL,rvs/gpdb,tpostgres-projects/tPostgres,yazun/postgres-xl,yuanzhao/gpdb,Quikling/gpdb,lisakowen/gpdb,Chibin/gpdb,CraigHarris/gpdb,yuanzhao/gpdb,arcivanov/postgres-xl,Postgres-XL/Postgres-XL,50wu/gpdb,lisakowen/gpdb,CraigHarris/gpdb,edespino/gpdb,techdragon/Postgres-XL,royc1/gpdb,xinzweb/gpdb,lintzc/gpdb,foyzur/gpdb,atris/gpdb,randomtask1155/gpdb,pavanvd/postgres-xl,Chibin/gpdb,xinzweb/gpdb,Quikling/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,ahachete/gpdb,0x0FFF/gpdb,cjcjameson/gpdb,arcivanov/postgres-xl,janebeckman/gpdb,xinzweb/gpdb,50wu/gpdb,cjcjameson/gpdb,atris/gpdb,lpetrov-pivotal/gpdb,tpostgres-projects/tPostgres,Postgres-XL/Postgres-XL,oberstet/postgres-xl,greenplum-db/gpdb,postmind-net/postgres-xl,arcivanov/postgres-xl,Chibin/gpdb,rvs/gpdb,xuegang/gpdb,lisakowen/gpdb,foyzur/gpdb,Quikling/gpdb,xinzweb/gpdb,atris/gpdb,postmind-net/postgres-xl,foyzur/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,janebeckman/gpdb,rvs/gpdb,foyzur/gpdb,royc1/gpdb,kaknikhil/gpdb,xuegang/gpdb,kmjungersen/PostgresXL,greenplum-db/gpdb,chrishajas/gpdb,adam8157/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,CraigHarris/gpdb,kmjungersen/PostgresXL,postmind-net/postgres-xl,edespino/gpdb,janebeckman/gpdb,randomtask1155/gpdb,jmcatamney/gpdb,Quikling/gpdb,lpetrov-pivotal/gpdb,Quikling/gpdb,ahachete/gpdb,lintzc/gpdb,kaknikhil/gpdb,lisakowen/gpdb,chrishajas/gpdb,50wu/gpdb,atris/gpdb,cjcjameson/gpdb,0x0FFF/gpdb,chrishajas/gpdb,royc1/gpdb,lintzc/gpdb,zaksoup/gpdb,ashwinstar/gpdb,royc1/gpdb,rubikloud/gpdb,ashwinstar/gpdb,rvs/gpdb,techdragon/Postgres-XL,lintzc/gpdb,CraigHarris/gpdb,zaksoup/gpdb,kmjungersen/PostgresXL,CraigHarris/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,edespino/gpdb,edespino/gpdb,janebeckman/gpdb,kaknikhil/gpdb,ovr/postgres-xl,pavanvd/postgres-xl,oberstet/postgres-xl,atris/gpdb,tpostgres-projects/tPostgres,randomtask1155/gpdb,0x0FFF/gpdb,yazun/postgres-xl,yazun/postgres-xl,rubikloud/gpdb,rvs/gpdb,lpetrov-pivotal/gpdb,yazun/postgres-xl,rubikloud/gpdb,ahachete/gpdb,yuanzhao/gpdb,rubikloud/gpdb,lpetrov-pivotal/gpdb,adam8157/gpdb,rvs/gpdb,zaksoup/gpdb,chrishajas/gpdb,50wu/gpdb,Chibin/gpdb,cjcjameson/gpdb,lpetrov-pivotal/gpdb,edespino/gpdb,lintzc/gpdb,oberstet/postgres-xl,Chibin/gpdb,randomtask1155/gpdb,rubikloud/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,kaknikhil/gpdb,chrishajas/gpdb,rubikloud/gpdb,royc1/gpdb,atris/gpdb,arcivanov/postgres-xl,ovr/postgres-xl,edespino/gpdb,janebeckman/gpdb,jmcatamney/gpdb,tangp3/gpdb,techdragon/Postgres-XL,0x0FFF/gpdb,Chibin/gpdb,Quikling/gpdb,kaknikhil/gpdb,0x0FFF/gpdb,xuegang/gpdb,techdragon/Postgres-XL,janebeckman/gpdb,CraigHarris/gpdb,Quikling/gpdb