[sheepdog] [PATCH] disk cache: drop O_DSYNC only when O_DIRECT is set

[MORITA Kazutaka]

At Tue, 30 Oct 2012 18:30:37 +0800,
Liu Yuan wrote:
> 
> On 10/30/2012 08:49 AM, MORITA Kazutaka wrote:
> > At Thu, 25 Oct 2012 13:52:01 +0900,
> > MORITA Kazutaka wrote:
> >>
> >> To make sure that data is not cached by the host os, we cannot drop
> >> O_DSYNC when O_DIRECT is not set.
> >>
> 
> Why data are cached in page cache a problem? We do sync(2) it afterwards.

For users who run VMs on storage nodes, more memory should be used for
guest OSes.  It's inefficient to cache data in both host and guest
page caches.

> 
> We don't set O_DIRECT for inode object, so sync(2) won't flush it to
> disk safely?

O_DIRECT imposes alignment restrictions on both the file offset and
the buffer length, so we cannot use it for any files other than data
objects.

Thanks,

Kazutaka