On Sun, Feb 4, 2018 at 3:39 AM, Raghavendra Gowdappa <rgowdapp@xxxxxxxxxx> 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 feel that we have come a long way from the time the install guide was written and an update for removing the "staleness of content" might be in order there :-).
Regards,
Vijay
[1] http://docs.gluster.org/en/latest/Install-Guide/Overview/
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1512691
[3] https://bugzilla.redhat.com/show_bug.cgi?id=1390050
regards,
Raghavendra
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