[nfsv4] Performance drop in iozone for reading large files

[Dean Hildebrand]

To prove/disprove the sever caching theory, here are a couple *echm* fun 
things you can try:
1) While writing and reading the file, watch the memory on the server 
using 'top'.  You should see it rise up an amount roughly equal to the 
size of the file.  Once the file is deleted by iozone, the memory will 
drop back down again.
2) After writing the file do not delete it (use -w I believe). Umount 
the server, reboot the server, then mount the server and read the file.  
This will read the file directly from disk.  Compare this performance to 
your original read performance (which used the server cache).

Dean

Trond Myklebust wrote:
> On Tue, 2006-09-05 at 15:55 -0700, Bryce Harrington wrote:
>   
>>> Is the readahead algorithm working correctly? Could you please try
>>> applying the following patch, and then check your syslog for warnings?
>>>       
>> I just want to follow up on this one and make sure I understand it
>> properly.
>>
>> Dean says this is due to hitting the memory limit of server-side
>> caching, which seems to explain it.  Our server has 1G memory, and when
>> shrinking it to lower ram, the issue does occur earlier.
>>     
>
> Do you see the same dropoff when running iozone locally on the server?
> Personally, I would expect that the server itself should be doing
> readahead too in order to mitigate these effects.
>
> Cheers,
>   Trond
>
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-- 
Dean Hildebrand
Ph.D. Candidate
University of Michigan