[suse-oracle] File system cache limits, vm.dirty_ratio

We want to limit amount of memory that SLES uses to buffer file systems. We have situation like this on our database server:

- SGA at around half of physical memory (SGA 16GB, total RAM 32GB)
- Remaining memory is for user sessions
- No swapping, everything is fine. Performance is great, no issues.

However, when we initiate backup (NetBackup client, simple file backups) memory usage peaks at 100% and swapping starts. When we terminate backup, everything slowly goes back to normal.

Our assumption is that file system caching kicks in when we start backup. We want to limit O/S cache for filesystems. Oracle doesn't use file system cache anyway, so it will not affect Oracle (OCFS2 with O_DIRECT).

vm.dirty_ratio is set to 10, as advised by Suse support. However, nothing has changed. This is free output:

free -m
             total       used       free     shared    buffers     cached
Mem:         32173      32107         65          0         66      25896
-/+ buffers/cache:       6145      26028
Swap:        32773        578      32194

Also, paging rate increases tremendously. Here is the part of sar output:

sar -B 5 20
Linux 2.6.16.46-0.12-smp (ai-sapgrt-pr1)        02/18/09

12:23:42     pgpgin/s pgpgout/s   fault/s  majflt/s
12:23:47     64715.80    916.60  13664.40      0.00
12:23:52     59247.31    710.98  12509.38      0.00
12:23:57     32867.47   1010.58  11054.29      0.00
12:24:02     49332.20    221.20  23691.20      0.00
12:24:07     43291.97   1972.89  12452.41      0.20
12:24:12     52531.94   1495.21  11647.31      0.00

12:24:12     pgpgin/s pgpgout/s   fault/s  majflt/s
12:24:17     30538.00   1095.80  13729.20      0.80
12:24:22     35545.00   2260.80  15230.00      0.20
12:24:27     59802.40   1898.80  15587.80      0.00
12:24:32     46216.57   2165.07  13352.69      1.00
12:24:37     44167.60   4573.20  13011.20      2.20
12:24:42     44947.40   2124.00  12552.80      0.20

Is there a way to really limit Linux buffers?

Regards,
Vladimir Barac
Database administrator | Alghanim Industries
P.O. Box 223, Safat 13003, Kuwait
Tel: +965 1881 111  / ext. 2960
[cid:[email protected]]


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