[Lustre-discuss] How to you generated discarded readahead pages?

Mark Seger Mark.Seger at hp.com
Sat Mar 22 06:10:10 PDT 2008


I'm running a little script that does 3 consecutive small reads of 4K 
each and then sleeps for a second and as you can see from the following 
collectl output readahead is happening.  How do I know?  Because the 
network traffic is so high:

#         <----------Network----------><-------------Lustre 
Client-------------->
#Time     netKBi pkt-in netKBo pkt-out  Reads KBRead Writes KBWrite   
Hits Misses
09:31:54    1084    740     29     394      3     12      0       0      
0      3
09:31:55    1084    740     29     394      3     12      0       0      
0      3
09:31:56    1084    740     29     394      3     12      0       0      
0      3
09:31:57    1084    741     29     394      2      8      0       0      
0      2

I'm getting my data from 
/proc/fs/lustre/llite/spfs1-00000100768c2800/read_ahead_stats and when I 
look at the disards, they're not changing.  What I would like to know is 
why. as I'd think all that extra data would be discarded from the 
cache.  Is it because some other process needs to put pressure on it?  I 
have 4GB memory and just wrote a 5GB file which should certain force out 
anything already there but the counter never changed.

I do see "readpage not consecutive" changing,

-mark





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