Re: fio --direct=1 and Linux page cache effects

[Jens Axboe <axboe@kernel.dk>]

On 2012-12-11 21:10, Ken Raeburn wrote:
> On 12/11/12 08:29, Jens Axboe wrote:
>> You are right, blkid likely wont be done by then, so it's still down 
>> to timing whether it'll help or not. This is pretty annoying. This 
>> issue is due to the file being opened for write. But unfortunately we 
>> cannot open for read always, as fcntl() wont allow change of file 
>> access mode flags. So how about the below. Basically DON'T close the 
>> fd, defer that until we really close the file. This will keep one 
>> extra fd open until the original is closed, but I don't see that as 
>> being an issue.
> 
> This patch seems to be working just fine. I have reproduced the case 
> where it uses the shadow_fd field, but blkid doesn't run and the page 
> cache entries don't get loaded, and in my initial tests with dm-zero, it 
> looks like the bimodal performance distribution is gone.
> 
> Thanks!

Excellent, I think this is probably as good as it is going to get. I
have committed the fix. Thanks a lot for reporting it, especially in
such great detail. When a problem is fully understood, fixing it is then
the smallest part of the effort.

-- 
Jens Axboe