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

[Raghavendra G]

On Thu, May 4, 2017 at 4:36 PM, Xavier Hernandez <xhernandez at datalab.es>
wrote:

> 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.
>

I agree, if the contents of parent directory are not listed at least once,
 penalty is still there.


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