[Lustre-devel] open(), getattr(), and negative dentries

John Hammond jhammond at tacc.utexas.edu
Mon Aug 1 09:10:15 PDT 2011

On 07/27/2011 12:02 AM, Oleg Drokin wrote:
> Hello!
> On Jul 21, 2011, at 4:15 PM, John Hammond wrote:
>> It appears that stating a non-existent file on Lustre causes a negative
>> dentry to be created, while opening it does not.  If I understand
>> correctly, this also means that every open of a non-existent file that
>> misses the dentry cache causes an RPC to the MDS.  Is there a reason for
>> the difference?
> Open does not return a parent dir lock back to the client and this is what guards
> negative dentries.
> So if you do ls -l on the dir, and then do your open (without O_CREATE!)
> the negative dentry will be cached because of the pre-existing lock.
> In fact is the dir was last modified sufficiently long ago (like several seconds have passed),
> even lookup of the dir itself (even done as part of the path lookup to the filename)
> would return you the needed lock and negative dentry will be cached.

Thanks, that's helpful.  I assume that "sufficiently long ago" means
after ns_ctime_age_limit (10 by default) seconds have passed.  I
adjusted my test case to account for this but there still seems to be an
issue here, so I created LU-560.



John L. Hammond, Ph.D.
TACC, The University of Texas at Austin
jhammond at tacc.utexas.edu
(512) 471-9304

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