Oracle database performance after server reboot

Question

hi masters,
this is not some kind of question, but a discussion. some statements come from our client that after weekly reboot of system, the oracle database performance is low for some time and increase after some time(say 2 days).
i think it is but obvious, because at reboot oracle flushes all cache, and temporary space, so it need to re parse the sql statements and perform some disc I/O's so it might need some time and hence performance will degrade.
but at the same time some people claim that after reboot their database performance is better than their normal performance for some days. it seems controversial that's why i am posting it here.
what might be the reason behind this?? prior can have a valid reason of hard parsing, but what with second case??
any clarification is highly appreciated...
thank you
regadrs
VD

Answer 1

Vikrant,
You should wait for some time buddy, its weekend ;-) .
this is not some kind of question, but a discussion. some statements come from our client that after weekly reboot of system, the oracle database performance is low for some time and increase after some time(say 2 days).i think it is but obvious, because at reboot oracle flushes all cache, and temporary space, so it need to re parse the sql statements and perform some disc I/O's so it might need some time and hence performance will degrade.
>
I would start from saying that checking the performance when the system just started, is a wrong approach. There would be lots of IOs , parsing, calculations(related to memory allocations) happening so there would be a delay/bad performance at that time. Very simple example can be parsing, another can be memory allocation. Oracle doesn't allocate the entire memory in the instance startup that is allocated to the memory areas but allocates just the bare minimum that is needed to start the instance and than after the startup, it keeps on allocating the memory. So surely enough, with the startup and after a while of it, there would be a different performance than that after the instance hsa already been started and the workload informations have started coming up.
Its correct that Oracle would deallocate all the caches with the reboot as the instance is on the memory(physical) and with the reboot , that would be flushed including the SGA which is allocated over it. Temporary tablespace is now not freed with the reboot. I guess its a rather illogical thing to do but that's what is there now. Oracle keeps the segment allocated even after the reboot is issued, hence the reason for larger temporary tablespaces.
>
but at the same time some people claim that after reboot their database performance is better than their normal performance for some days. it seems controversial that's why i am posting it here.
what might be the reason behind this?? prior can have a valid reason of hard parsing, but what with second case??
>
This should not come as a surprise once we understand what might be happening with this process. Assume a situation where you have undersized caches. For example, shared pool . which is very heavily used for database , if this is going to be undersized and you are not using automatic memory management, you won't be enjoying the dynamic management of this parameter. Now, if you do lots of parsing , thanks to your wrongly written queries, you would eventually end up filling up shared pool to its max thus leaving no space for incoming new hard parsed cursors. Here , if you can't manage to add more memory to add to it, the only solution left would be to flush the shared pool( as good as rebooting the db because this would do the same) and than make space for the new cursors. The performance is going to be better becausethe cursors would not be getting flushed out immediately and will be kept in the shared pool as long as its not filled up again.Once you have reached to limit of it, again there would be performance benefit. So there are always odds added to the statements like this that I rebuilt my index , I got better, I rebooted my server, my querie are much faster now. Most of the time when these kind of statements are given, they are based on what we have seen, without understading what actually might have happened. So I would siggest to hear the statement but not take them as a rule of thumb to follow.
Hope this all makes some sense for you and would help somewhat.
Aman....