Gluster users - Apr 2017 - [DHT] The myth of two hops for linkto file resolution

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.

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

regards,
Raghavendra

On Sun, Apr 30, 2017 at 9:33 AM, Raghavendra Gowdappa <rgowdapp at
redhat.com>
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.
>
> A word of caution is that I've not done any testing to prove this
> observation :).
>
May be you should do it and send an update. That way we can use the
knowledge to do something.

>
> regards,
> Raghavendra
> _______________________________________________
> Gluster-users mailing list
> Gluster-users at gluster.org
> http://lists.gluster.org/mailman/listinfo/gluster-users
>


-- 
Pranith
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Hi,

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.

Xavi
>
> A word of caution is that I've not done any testing to prove this
observation :).
>
> regards,
> Raghavendra
> _______________________________________________
> Gluster-devel mailing list
> Gluster-devel at gluster.org
> http://lists.gluster.org/mailman/listinfo/gluster-devel
>