On Wed, Feb 22, 2012 at 7:09 PM, Bernd Oppolzer <bernd.oppol...@t-online.de> wrote:
> The database engine or process uses large memory buffers to > cache the (least recently used) database pages in REAL memory, > and database performance may degrade significantly, if there is > paging acitivity on the memory buffers of the database engine. > So the DB admins have to take care that the real storage on the > DB machine is large enough that there is no paging on the DB > buffers, and the DB buffers have to be defined only as large as the > available REAL memory (not the address space). I don't think anyone disagrees with your claim that a running DB system needs the code and data in memory to operate on it. But when the database utilization is only 5%, it only needs that 5% of the time. Not 100% of the time. This means that the paging subsystem has to page out the idle guest to make room for others, and page it back in when it becomes active. The challenge becomes a bit easier if the DBA reduces the peak memory requirements (smaller SGA and PGA target) even when that means a bit more overhead during peak times. > This is true for every DB system, regardless of name or vendor. > > So, of course, the performance of a DB system on a virtual system > will suffer - if you don't have features like V=R pages (on z/VM, for > example), where you can fix the real pages of the DB machine. It's natural for a DBA to care about his own databases and want those to be resident all the time. But sharing unused capacity is something else. To some extent, it's the VM systems programmer role to implement the business requirements to address that. And some push it way beyond that :-) Rob ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
