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

[Xavier Hernandez]

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
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