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

[Ken Raeburn <raeburn@permabit.com>]

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!

Ken