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тАО01-16-2002 06:13 AM
тАО01-16-2002 06:13 AM
Solved! Go to Solution.
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тАО01-16-2002 06:18 AM
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тАО01-16-2002 06:20 AM
тАО01-16-2002 06:20 AM
Re: dbc_max_pct
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тАО01-16-2002 06:25 AM
тАО01-16-2002 06:25 AM
Re: dbc_max_pct
Your assumption is correct. You have another option and that is to fix the buffer cache at some value by setting bufpages to a non-zero value. e.g. bufpages=80000 will set the value to 320MB. I find that on even very large servers that typically the marginal improvements in buffer cache hit rates become very small above 300-400 MB. If your are using raw/io or the OnlineJFS mount options convosync=direct,mincache=direct which bypass the unix buffers, you can reduce the buffer cache even lower especially for a machine that is a pure database server. I really prefer fixed buffer cache because it allows one to tune other parameters while keeping the buffer cache constant.
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тАО01-16-2002 06:36 AM
тАО01-16-2002 06:36 AM
Re: dbc_max_pct
You are totally right. Check these threads for more information on dbc_max_pct:
http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0xbfa872234586d5118ff00090279cd0f9,00.html
http://www.docs.hp.com/cgi-bin/otsearch/getfile?id=/hpux/onlinedocs/os/KCparam.DBCmaxPct.html&searchterms=dbc_max_pct&queryid=20020116-064151
HTH,
Shiju
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тАО01-16-2002 06:47 AM
тАО01-16-2002 06:47 AM
Re: dbc_max_pct
It depends. On the Physical memory you have. It is not recommended to set it to any value more than 300MB. So, you can adjust your dbc_max_pct to get around 300MB. Another dependency is about your application. Having a very low buffer cache will degrade sequential reads. And if your application does it a lot, then you will be losing the performance.
-Sri
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тАО01-21-2002 12:51 PM
тАО01-21-2002 12:51 PM
Re: dbc_max_pct
My advice is to try on your own server. Good rule of thumb is on dbase server to do 5-10 for dbc_max_pct. Optimal performance will come when you bypass dynamic buffer cache sizing ( set nbufs )
mincache=direct,convosync=direct mount parameters will allow direct I/O which means the UNIX buffer cache is tottaly bypassed on read/writes. This is great for OSYNC writes, and non sequential reads.
However sequential read performance is hurt more than 2x on these filesystems. Also filesystem based I/O (such as backups, filecopies suffer horrendously). I haven't seen many databases which do not tablescan.
Final suggestion is to use mincache=direct,convosync=direct only on those filesystems which have mostly completely random I/O pattern ( no tablescan ), or heavy writes ( bypass double buffered write ). The gain is approximately 10% in these cases.
My suggestion is larger than the 300-800M for servers which are more than simply database servers. Too small of a buffer cache is a bad situation, because you encounter thrashing of the buffer cache. This occurs when you have many competing processes attempting to allocate space in the cache.
On NFS servers, Clearcase servers, servers where large files are moved, use as much buffer cache as the box can stand with improved performance.