[Gluster-devel] [DHT] The myth of two hops for linkto file resolution

Xavier Hernandez xhernandez at datalab.es
Thu May 4 11:06:33 UTC 2017


On 30/04/17 06:03, Raghavendra Gowdappa wrote:
> All,
> Its a common perception that the resolution of a file having linkto file on the hashed-subvol requires two hops:
> 1. client to hashed-subvol.
> 2. client to the subvol where file actually resides.
> While it is true that a fresh lookup behaves this way, the other fact that get's ignored is that fresh lookups on files are almost always prevented by readdirplus. Since readdirplus picks the dentry from the subvolume where actual file (data-file) resides, the two hop cost is most likely never witnessed by the application.

This is true for workloads that list directory contents before accessing 
the files, but there are other use cases that directly access the file 
without navigating through the file system. In this case fresh lookups 
are needed.


> A word of caution is that I've not done any testing to prove this observation :).
> regards,
> Raghavendra
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