Mincache and Convosync access

[Wilder Mellotto]

Hi, we need some information about direct i/o.

We have one 9000/800/SD16A running HP-UX B.11.11 connected to one storage EMC DMX4 with 2 HBAs. The LUN paths was tested and all 4 path for a single LUN are working with load balance, that we can view with "powermt watch"

We have some issues with Oracle performance and we changed some attributes for some filesystems with oracle datafiles and here some tests:

Options - rw,delaylog,largefiles 0 2

# time prealloc tempfile 524288000;rm tempfile
real 8.9
user 0.0
sys 3.0

For Oracle cookbook performance is recommended to do direct i/o and then:

Options - rw,delaylog,largefiles,mincache=direct,convosync=direct 0 2

# time prealloc tempfile 524288000;rm tempfile

real 1:14.3
user 0.0
sys 11.6

What is causing this huge diference timing? Can anyone help?

Thanks.

[Matti_Kurkela]

If you are tuning a filesystem for maximum Oracle performance, you should measure it with an Oracle-level benchmark, not with an OS-level benchmark.

The options "mincache=direct,convosync=direct" are effectively disabling OS-level caching for the filesystem for most operations. It makes sense, because Oracle already has its own set of caches - and because Oracle "knows what it is doing", its cache can support Oracle-level operations more efficiently than the standard OS-level cache. But when you disable OS-level caching, any operations done by generic programs to that particular filesystem can of course be significantly slower.