[Gluster-devel] What's the correct way to enable direct-IO?

[Raghavendra Gowdappa]

----- Original Message -----
> From: "Paul Cuzner" <pcuzner at redhat.com>
> To: "Raghavendra Gowdappa" <rgowdapp at redhat.com>
> Cc: "Krutika Dhananjay" <kdhananj at redhat.com>, "Gluster Devel" <gluster-devel at gluster.org>
> Sent: Thursday, February 25, 2016 10:51:17 AM
> Subject: Re: What's the correct way to enable direct-IO?
> 
> This is great info - with a lot of options to take in :)
> 
> To summarise, to enable direct-io and bypass the kernel filesystem cache
> for a volume
> 1. Mount the brick with direct-io-mode=enable option
> 2. run vol set <vol> performance.strict-o-direct on

Would individual files be opened with O_DIRECT? If yes, this is sufficient. But if we want to do that for all files, the only way to do this is to disable write-behind.

> 3. update the vol files with 'o-direct' option in storage/posix (at least
> for now)

4. Quick read should be made aware of O_DIRECT (it doesn't as of now). Again this is on the read-path. If you don't want O_DIRECT semantics for read path (is there one?), this is fine.

> 
> Is that right?
> 
> 
> 
> On Thu, Feb 25, 2016 at 5:56 PM, Raghavendra Gowdappa <rgowdapp at redhat.com>
> wrote:
> 
> >
> >
> > ----- Original Message -----
> > > From: "Krutika Dhananjay" <kdhananj at redhat.com>
> > > To: "Gluster Devel" <gluster-devel at gluster.org>, "Raghavendra Gowdappa"
> > <rgowdapp at redhat.com>
> > > Cc: "Paul Cuzner" <pcuzner at redhat.com>
> > > Sent: Thursday, February 25, 2016 7:28:30 AM
> > > Subject: What's the correct way to enable direct-IO?
> > >
> > > Hi,
> > >
> > > git-grep tells me there are multiple options in our code base for
> > enabling
> > > direct-IO on a gluster volume, at several layers in the translator stack:
> > > i) use the mount option 'direct-io-mode=enable'
> >
> > This option is between kernel and glusterfs. Specifically it asks fuse
> > kernel module to bypass page-cache. Note that when this option is set,
> > direct-io is enabled for _all_ fds irrespective of whether applications
> > have used O_DIRECT in their open/create calls or not.
> >
> > > ii) enable 'network.remote-dio' which is a protocol/client option using
> > > volume set command
> >
> > This is an option introduced by [1] to _filter_ O_DIRECT flags in
> > open/create calls before sending those requests to server. The option name
> > is misleading here. However please note that this is the key (alias?) used
> > by glusterd. The exact option name used by protocol/client is
> > "filter_O_DIRECT" and its fine. Probably we should file a bug on glusterd
> > to change the name?
> >
> > Coming to your use case, we don't want to filter O_DIRECT from reaching
> > brick. Hence, we need to set this option to _off_ (by default its
> > disabled).
> >
> > I am still not sure what is the relevance of this option against the bug
> > it was introduced. If we need direct-io, we've to pass it to brick too, so
> > that backend fs on brick is configured appropriately.
> >
> > [1] http://review.gluster.org/4206
> > [2] https://bugzilla.redhat.com/show_bug.cgi?id=845213
> >
> > > iii) enable performance.strict-o-direct which is a
> > performance/write-behind
> > > option using volume-set command
> >
> > Yes, write-behind honours O_DIRECT only if this option is set. So, we need
> > to enable this for your use-case. Also, note that applications still need
> > to use O_DIRECT in open/create calls.
> >
> > To summarize, following are the ways to bypass write-behind cache:
> > 1. disable write-behind :).
> > 2. applications use O_SYNC/O_DSYNC in open calls
> > 3. enable performance.strict-o-direct _and_ applications should use
> > O_DIRECT in open/create calls.
> >
> > > iv) use 'o-direct' option in storage/posix, volume-set on which reports
> > that
> > > the option doesn't exist.
> >
> > The option exists in storage/posix. But, there is no way to set it through
> > cli (probably you can send a patch to do that if necessary). With this
> > option, O_DIRECT is passed with _every_ open/create call on the brick.
> >
> > >
> > > So then the question is - what is a surefire way to get direct-io-like
> > > behavior on gluster volume(s)?
> >
> > There is no one global option. You need to configure various translators
> > in the stack. Probably [2] was asking for such a feature. Also, as you
> > might've noticed above the behavior/interpretation of these options is not
> > same across all translators (like some are global and some are local only
> > to an fd etc).
> >
> > Also note that apart from the options you listed above,
> > 1. Quick-read is not aware of O_DIRECT. We need to make it to disable
> > caching if open happens with O_DIRECT.
> > 2. Handling of Quota Marker xattrs is not synchronous (though not exactly
> > an O_DIRECT requirement) as marking is done after sending reply to calls
> > like writev.
> >
> > On a related note, found article [3] to be informative.
> >
> > [1] http://review.gluster.org/4206
> > [2] https://bugzilla.redhat.com/show_bug.cgi?id=845213
> > [3] https://lwn.net/Articles/457667/
> >
> > regards,
> > Raghavendra.
> >
>