Re: Flushing the (page?) cache for a block device

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Mar 30, 2009 at 03:40:52PM +0800, Wu Fengguang wrote:
> Hi Sascha,
> 
> On Sun, Mar 29, 2009 at 02:54:39PM +0200, Sascha Silbe wrote:
> > Hello!
> >
> > I'd like to do a verification step after writing to a USB stick (block  
> > device level, no filesystem involved). For this to work as intended, I  
> > need to ensure the reads are not satisfied from blocks still in cache,  
> > i.e.: flush "the cache" (page cache IIUC) for the block device in  
> > question (and _only_ this device).
> > Some research resulted in the following options:
> >
> > 1. Open the target file using O_DIRECT while writing.
> >      Doesn't feel like the right thing to do; only want to flush the  
> > cache, not circumvent it. Might severly limit performance as writes are  
> > synchronous.
> >      Interface isn't very nice (need to ensure alignment to page size in 
> > memory).
> >
> > 2. Use posix_fadvise() with POSIX_FADV_DONTNEED.
> >      Probably works with the current kernel (haven't tested yet), but  
> > there's absolutely no guarantee.
> >      Interface much better than O_DIRECT.
> 
> FYI, here is a handy fadvise tool:
>         http://www.zip.com.au/~akpm/linux/patches/stuff/ext3-tools.tar.gz
> 
> Usage:
>         fadvise /some/file 0 0 dontneed

And I'd recommend to make a fsync() call before posix_fadvise() to
makes it work more reliably, because dirty pages are not guaranteed
to be freed by posix_fadvise().

Thanks,
Fengguang