NFS Performance Tuning for HP-UX 11.0 and 11i Systems

nfs performance tuning for hp-ux 11.0 and 11i systems page 107

Notes:

Page 107July 22, 2002

buffer

cache

What is buffer cache memory?

• Portion of physical memory dedicated to storing file data

• NFS read performance is increased when requested data

is present in the cache, avoiding physical disk read

• NFS write performance is increased by allowing writing

process to post data to cache instead of to server’s disk

• HP-UX uses a split memory cache system, employing both

a buffer cache (used for storing data) and a page cache

(used for storing executables, libraries, mmap files)

Buffer cache is a portion of physical memory that is allocated for storing blocks of

file data. HP-UX uses this memory to speed up file operations such as read() and

write(). Since memory access times are so much faster than disk access times,

generally the more file system requests that can be satisfied by buffer cache the

better overall I/O performance will be. NFS read operations can be satisfied

without waiting for a physical disk I/O if the requested data is already residing in

the cache. NFS write performance can be dramatically increased by allowing a

writing process to post the data to local buffer cache and continue, letting the

system migrate the pages to the server’s physical disk in the background.

Unlike most vendor’s UNIX implementations, HP-UX currently uses a split memory

cache system where some memory pages are cached in the buffer cache and

others in the page cache. While the buffer cache is used to store data pages, the

page cache is used when pages are brought into or pushed out from memory using

the paging interface, either explicitly by mapping a file into memory through a call

to mmap(2), or implicitly for objects such as executable programs or shared

libraries which are mapped into memory on behalf of a running program.