[Gluster-devel] Glusterfs and Structured data

[Raghavendra Gowdappa]

+gluster-users

Another guideline we can provide is to disable all performance xlators for workloads requiring strict metadata consistency (even for non gluster-block usecases like native fuse mount etc). Note that we might still can have few perf xlators turned on. But, that will require some experimentation. The safest and easiest would be to turn off following xlators:

* performance.read-ahead
* performance.write-behind
* performance.readdir-ahead and performance.parallel-readdir
* performance.quick-read
* performance.stat-prefetch
* performance.io-cache

performance.open-behind can be turned on if the application doesn't require the functionality of file accessible through an fd opened on a mountpoint while file is deleted from a different mount point. As far as metadata inconsistencies go, I am not aware of any issues with performance.open-behind.

Please note that as has been pointed out by different mails in this thread, perf-xlators is one part (albeit larger one) of the bigger problem of metadata inconsistency. 

regards,
Raghavendra

----- Original Message -----
> From: "Vijay Bellur" <vbellur at redhat.com>
> To: "Raghavendra G" <raghavendra at gluster.com>
> Cc: "Raghavendra Gowdappa" <rgowdapp at redhat.com>, "Gluster Devel" <gluster-devel at gluster.org>
> Sent: Friday, February 9, 2018 1:34:25 PM
> Subject: Re: [Gluster-devel] Glusterfs and Structured data
> 
> On Wed, Feb 7, 2018 at 10:35 PM, Raghavendra G <raghavendra at gluster.com>
> wrote:
> 
> >
> >
> > On Tue, Feb 6, 2018 at 8:15 PM, Vijay Bellur <vbellur at redhat.com> wrote:
> >
> >>
> >>
> >> On Sun, Feb 4, 2018 at 3:39 AM, Raghavendra Gowdappa <rgowdapp at redhat.com
> >> > wrote:
> >>
> >>> All,
> >>>
> >>> One of our users pointed out to the documentation that glusterfs is not
> >>> good for storing "Structured data" [1], while discussing an issue [2].
> >>
> >>
> >>
> >> As far as I remember, the content around structured data in the Install
> >> Guide is from a FAQ that was being circulated in Gluster, Inc. indicating
> >> the startup's market positioning. Most of that was based on not wanting to
> >> get into performance based comparisons of storage systems that are
> >> frequently seen in the structured data space.
> >>
> >>
> >>> Does any of you have more context on the feasibility of storing
> >>> "structured data" on Glusterfs? Is one of the reasons for such a
> >>> suggestion
> >>> "staleness of metadata" as encountered in bugs like [3]?
> >>>
> >>
> >>
> >> There are challenges that distributed storage systems face when exposed
> >> to applications that were written for a local filesystem interface. We
> >> have
> >> encountered problems with applications like tar [4] that are not in the
> >> realm of "Structured data". If we look at the common theme across all
> >> these
> >> problems, it is related to metadata & read after write consistency issues
> >> with the default translator stack that gets exposed on the client side.
> >> While the default stack is optimal for other scenarios, it does seem that
> >> a
> >> category of applications needing strict metadata consistency is not well
> >> served by that. We have observed that disabling a few performance
> >> translators and tuning cache timeouts for VFS/FUSE have helped to overcome
> >> some of them. The WIP effort on timestamp consistency across the
> >> translator
> >> stack, patches that have been merged as a result of the bugs that you
> >> mention & other fixes for outstanding issues should certainly help in
> >> catering to these workloads better with the file interface.
> >>
> >> There are deployments that I have come across where glusterfs is used for
> >> storing structured data. gluster-block  & qemu-libgfapi overcome the
> >> metadata consistency problem by exposing a file as a block device & by
> >> disabling most of the performance translators in the default stack.
> >> Workloads that have been deemed problematic with the file interface for
> >> the
> >> reasons alluded above, function well with the block interface.
> >>
> >
> > I agree that gluster-block due to its usage of a subset of glusterfs fops
> > (mostly reads/writes I guess), runs into less number of consistency issues.
> > However, as you've mentioned we seem to disable perf xlator stack in our
> > tests/use-cases till now. Note that perf xlator stack is one of worst
> > offenders as far as the metadata consistency is concerned (relatively less
> > scenarios of data inconsistency). So, I wonder,
> > * what would be the scenario if we enable perf xlator stack for
> > gluster-block?
> >
> 
> 
> tcmu-runner opens block devices with O_DIRECT. So enabling perf xlators for
> gluster-block would not make a difference as translators like io-cache &
> read-ahead do not enable caching for open() with O_DIRECT. In addition,
> since bulk of the operations happen to be reads & writes on large files
> with gluster-block, md-cache & quick-read are not appropriate for the stack
> that tcmu-runner operates on.
> 
> 
> * Is performance on gluster-block satisfactory so that we don't need these
> > xlators?
> >   - Or is it that these xlators are not useful for the workload usually
> > run on gluster-block (For random read/write workload, read/write caching
> > xlators offer less or no advantage)?
> >   - Or theoretically the workload is ought to benefit from perf xlators,
> > but we don't see them in our results (there are open bugs to this effect)?
> >
> 
> 
> Owing to the reasons mentioned above, most performance xlators do not seem
> very useful for gluster-block workloads.
> 
> 
>  Regards,
> Vijay
>