linux kernel buffer memory is zero

Question

There are one qestion that i can`t find in google.

I have many linux boxes mostly with SLES or openSUSE, diffrent versions and kernels. On some of them i faced with slow oracle transactions problem. It time to time problem and when i log in the box on that time i see that oracle blocked in kernel function sync_page

# while :; do ps axo stat,pid,cmd,wchan | egrep '^D|^R'; echo --; sleep 5; done
D     3483 hald-addon-storage: polling ide_do_drive_cmd
Ds    4635 ora_dbw0_orcl               sync_page
Ds    4637 ora_lgwr_orcl               sync_page
Ds    4639 ora_ckpt_orcl               sync_page
D    11210 oracleorcl (LOCAL=NO)       sync_page
D    12457 [smtpd]                     sync_page
R+   12458 ps axo stat,pid,cmd,wchan   -
--
Ds    4635 ora_dbw0_orcl               sync_page
Ds    4637 ora_lgwr_orcl               sync_page
Ds    4639 ora_ckpt_orcl               sync_page
D    11210 oracleorcl (LOCAL=NO)       sync_page
R+   12501 ps axo stat,pid,cmd,wchan   -
--
Ds    4635 ora_dbw0_orcl               sync_page
Ds    4637 ora_lgwr_orcl               sync_page
Ds    4639 ora_ckpt_orcl               sync_page
D    11210 oracleorcl (LOCAL=NO)       sync_page
R+   12535 ps axo stat,pid,cmd,wchan   -
--
Ds    4635 ora_dbw0_orcl               sync_page
Ds    4637 ora_lgwr_orcl               sync_page
Ds    4639 ora_ckpt_orcl               sync_page
D    11210 oracleorcl (LOCAL=NO)       sync_page
R+   12570 ps axo stat,pid,cmd,wchan   -
--

so i think that box is run out of memory for disk buffers but memry is fine

             total       used       free     shared    buffers     cached
Mem:       4149084    3994552     154532          0          0    2424328
-/+ buffers/cache:    1570224    2578860
Swap:      3148700     750696    2398004

i think that this is the problem, buffer is zero and we must write directly to disk, but why buffer is zero ? - i try to google it and find nothing - is anyone can help ?

Answer 1

The Oracle DBMS bypasses the OS's buffers and writes directly to the block device. I don't think your slow transactions are caused by having no buffers in use.

Answer 2

free is giving you how much of the buffer the kernel is using, not how much is available. Kernel buffers are taken out of the total memory--so in this case, your kernel is using 0/4149084 available memory for buffers. I would not consider this to be your issue, and would create a new question trying to find out why you're experiencing Oracle-related issues.