Re: ORA-1555 Snapshots Too Old Error[Q]
Date: 1995/06/07
Message-ID: <3r3jpn$lob_at_newsbf02.news.aol.com>#1/1
Regards ==> ORA 1555 and Rollback Segment setup
Chapter 9 in the Administrators Guide gives lots of advice on how to deal with rollback segments. I have found that it really depends on your mix of short transaction type quries/updates and long running batch type work.
I have found that rollback segments should have several preallocated extents at all times. Oracle will assign more than one transation to a rollback segment, but one transaction can not span multiple rollback segments. Allocation of additional extents whether for a table, index, or rollback segment is expensive so preallocate what you think your normal needs will be. You need to try to have one preallocated extent available for every concurrent user, but as it is wasteful to assign 5 meg extents to everyone when one one or two transactions a day will generate this much much change data so make sure you allow each rollback segment to take a fair number of extents.
Something like storage ( initial 512K next 512K
minextents 10 maxextents 50 )
may well suits your needs better that what you said you had. I like the
optimal parameter and set it to equal the size of minextents. If you have
a couple of large transactions or batch jobs that need lots of rollback
you can allocate a rollback segment with a higher number of minextents,
maxextents, and size then try to assign transactions these transactions to
it each time they run. Remember that Oracle assigns transactions to
rollback segments in a round robin fashion and you can not reserve this
large rollback segment so it needs to have multiple preallocated extents
to guarentee that it is available.
Be advised that Oracle calculates how many rollback segments it wants. This normally does not affect you unless you try to go parallel.
This is only my opinion. Read the chapter again and best of luck.
Mark D. Powell -- No Witty Disclaimer -- Received on Wed Jun 07 1995 - 00:00:00 CEST