Checking AIX cache

The cache area is a memory area where the operating system places blocks of files used often, therefore reducing costly I/O to slower devices like hard disks. It is a good concept and very useful for boosting performance of file servers (NFS, samba), but is not so good for database software like Oracle because an instance manages its own cache area.

Therefore, if you are an Oracle administrator of databases in AIX servers it's a good practice to be aware of memory configuration and usage, and maybe help a bit your fellow sysadmin just in case. The important parameters to check are the minimum and maximum percentage of RAM for file caching purposes (minperm% and maxperm%):

root:myserver> vmstat -v
              2621440 memory pages
              2525761 lruable pages
               180000 free pages
                    4 memory pools
               647419 pinned pages
                 80.0 maxpin percentage
                 20.0 minperm percentage
                 80.0 maxperm percentage
                 24.8 numperm percentage
               627654 file pages
                  0.0 compressed percentage
                    0 compressed pages
                 24.8 numclient percentage
                 25.0 maxclient percentage
               627654 client pages
                    0 remote pageouts scheduled
                58246 pending disk I/Os blocked with no pbuf
             10544297 paging space I/Os blocked with no psbuf
                 2228 filesystem I/Os blocked with no fsbuf
                  805 client filesystem I/Os blocked with no fsbuf
              2215609 external pager filesystem I/Os blocked with no fsbuf
                    0 Virtualized Partition Memory Page Faults
                 0.00 Time resolving virtualized partition memory page faults

As you can see in this example, there is a lot of minimum memory allotted for file caching (20%) and wasted because Oracle caching is better for Oracle databases than operating system caching, but you can't assign that 20% RAM space to Oracle unless you change the virtual memory AIX parameters.

Moreover, the maximum memory reserved for file caching is limited by the maxclient percentage (25%) so in this system you can use up to 25% of RAM for file caching of non-JFS filesystem files and the other 55% for caching files of JFS filesystems. You can check JFS memory pages usage with this command:

root:myserver> svmon -G
               size       inuse        free         pin     virtual
memory      2621440     2442500      178940      647452     2186997
pg space    5373952      925157

               work        pers        clnt       other
pin          562525           0           0       84927
in use      1734676           0      707824

PageSize   PoolSize      inuse       pgsp        pin    virtual
s   4 KB          -    2326020     925157     571068    2070517
m  64 KB          -       7280          0       4774       7280

You might notice that there are 0 pages used for JFS file caching (pers), and that might be because there are no JFS file systems in this server:

root:myserver> mount
  node       mounted        mounted over    vfs       date        options      
-------- ---------------  ---------------  ------ ------------ --------------- 
         /dev/hd1         /                jfs2   Jun 05 19:09 rw,log=/dev/hd8 
         /dev/hd2         /usr             jfs2   Jun 05 19:09 rw,log=/dev/hd8 
         /dev/hd5         /var             jfs2   Jun 05 19:10 rw,log=/dev/hd8 
         /dev/hd3         /tmp             jfs2   Jun 05 19:10 rw,log=/dev/hd8 
         /dev/hd4         /home            jfs2   Jun 05 19:11 rw,log=/dev/hd8 
         /proc            /proc            procfs Jun 05 19:11 rw              
         /dev/hd6         /opt             jfs2   Jun 05 19:11 rw,log=/dev/hd8 
         /dev/fslv00      /myfs            jfs2   Jun 05 19:11 rw,log=/dev/loglv00

IBM recommends setting the parameters as follows:

maxperm = 90%
maxclient = 90%
minperm = 3%

More information:

Tuning the AIX file caches
Overview of AIX page replacement