[Gluster-users] Read from fastest node only

[Yaniv Kaul]

On Wed, Jul 28, 2021 at 5:50 AM David Cunningham <dcunningham at voisonics.com>
wrote:

> Hi Yaniv,
>
> It may be my lack of knowledge, but I can't see how the fastest response
> time could differ from file to file. If that's true then it would be enough
> to only periodically test which node is fastest for this client, not having
> to do it for every single read.
>

In real life, the 'best' node is the one with the highest overall free
resources, across CPU, network and disk IO. So it could change and it might
change all the time.
Network, disk saturation might be common, disk performing garbage
collection, CPU being hogged by something, noisy neighbor, etc...

Our latency check is indeed not per file, AFAIK.
Y.


> Thanks for the tip about read-hash-mode. I see the help is as below. Value
> 4 may help, but not if the latency is tested for every file read. Value 0
> may help, but it depends how the children are ordered. Does anyone know
> more about how these work?
>
> Option: cluster.read-hash-mode
> Default Value: 1
> Description: inode-read fops happen only on one of the bricks in
> replicate. AFR will prefer the one computed using the method specified
> using this option.
> 0 = first readable child of AFR, starting from 1st child.
> 1 = hash by GFID of file (all clients use same subvolume).
> 2 = hash by GFID of file and client PID.
> 3 = brick having the least outstanding read requests.
> 4 = brick having the least network ping latency.
>
> Thanks again.
>
>
> On Tue, 27 Jul 2021 at 19:16, Yaniv Kaul <ykaul at redhat.com> wrote:
>
>>
>>
>> On Tue, Jul 27, 2021 at 9:50 AM David Cunningham <
>> dcunningham at voisonics.com> wrote:
>>
>>> Hello,
>>>
>>> We have a replicated GlusterFS cluster, and my understanding is that the
>>> GlusterFS FUSE client will check the file with all nodes before doing a
>>> read.
>>>
>>> For our application it is not actually critical to be certain of having
>>> the latest version of a file, and it would be preferable to speed up the
>>> read by simply reading the file from the fastest node. This would be
>>> especially beneficial if some of the other nodes have higher latency from
>>> the client.
>>>
>>
>> How do you define, in real time, per file, which is the fastest node?
>> Maybe you are looking for read-hash-mode volume option?
>> Y.
>>
>>>
>>> Is it possible to do this? Thanks in advance for any assistance.
>>>
>>> --
>>> David Cunningham, Voisonics Limited
>>> http://voisonics.com/
>>> USA: +1 213 221 1092
>>> New Zealand: +64 (0)28 2558 3782
>>> ________
>>>
>>>
>>>
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>>
>
> --
> David Cunningham, Voisonics Limited
> http://voisonics.com/
> USA: +1 213 221 1092
> New Zealand: +64 (0)28 2558 3782
>
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