[Gluster-devel] Regarding the issues gluster DHT and Layouts of bricks

[Subrata Ghosh]

Sorry for the wrong incomplete message sent by mistake earlier.

Hi Sussant,

Extremely sorry for the belated reply. Thanks  for your input. We will try having rebalance then create a set of small files with a random pattern generation and check where it falls in by DM_TYPE  DHT.

I have one short query:
We were also thinking to use (in case we need ) Translators/cluster/unify along with NUFA or ALU scheduler. However we noticed Translators/cluster/unify became  obsolete/legacy in gluster site , does it mean automatically ALU, NUFA,rr scheduler also became obsolete or not supported ? Can you please clarify ?

Is the Translators/cluster/unify is supported in 3.3.2 ( we are currently using) and following configuration would work ?

If not can you please suggest you how to use ALU/NUFA scheduler with 3.3.2 if there is any specific mechanism?

volume unify
   type cluster/unify
   subvolumes brick1 brick2 brick3 brick4
   option namespace brick-ns # should be a node which is not present in 'subvolumes'
   option scheduler NUFA/ALU/rr    # NUFA/ALU or Simple round-robin scheduler
end-volume

Thanks again for your time and advice.

Regards,

-----Original Message-----
From: Susant Palai [mailto:spalai at redhat.com]
Sent: Thursday, May 21, 2015 6:17 PM
To: Subrata Ghosh
Cc: gluster-devel at gluster.org; gluster-users at gluster.org; Nobin Mathew; Vijay Bellur
Subject: Re: Regarding the issues gluster DHT and Layouts of bricks

Commets inline.

----- Original Message -----
> From: "Subrata Ghosh" <subrata.ghosh at ericsson.com<mailto:subrata.ghosh at ericsson.com>>
> To: gluster-devel at gluster.org<mailto:gluster-devel at gluster.org>, gluster-users at gluster.org<mailto:gluster-users at gluster.org>
> Cc: "Nobin Mathew" <nobin.mathew at ericsson.com<mailto:nobin.mathew at ericsson.com>>, "Susant Palai" <spalai at redhat.com<mailto:spalai at redhat.com>>, "Vijay Bellur"
> <vbellur at redhat.com<mailto:vbellur at redhat.com>>
> Sent: Thursday, 21 May, 2015 4:26:05 PM
> Subject: Regarding the issues gluster DHT and  Layouts of bricks
>
>
> Hi  All,
>
> Could you please guide us  to solve the following DHT and brick layout
> problem we are  dealing with ? Questions are marked bold.
>
> Problem statement :
>
>
> 1.      We have a requirement to achieve maximum write and read performance
> and we have to meet some committed performance metrics.
>
>                Our goal is to place each file into different bricks to get
>                optimal performance and also observer the nature of the
>                throughput , hence need to have a mechanism  to generate
>                different hash using gluster glusterfs.gf_dm_hashfn,
> (assuming number of files are : N , Number of Bricks :N)  to place
> spate bricks.
>
>
> -        How to make sure each file has different hash and   falls to
> different bricks ?
>
>
>
> -        Other way to put the question if I  know the range of the brick
> layout or more precisely if I know the  hex value of the desired hash
> ( so that it will be placed desired brick)  that we need to generate
> from Davis-Meyer algorithm used in gluster,  Can we create a file name
> such that, that also solve our problem to some extent?
>
>
> 2.      We tried to experiment to see  how a file in gluster is decided to be
> placed in a particular brick following gluster glusterfs.gf_dm_hashfn
> and took some idea from
>        some articles  like
>        http://gluster.readthedocs.org/en/latest/Features/dht/ ,
>        https://joejulian.name/blog/dht-misses-are-expensive/ page which
>        describes layout for that brick  and calculate a hash for the file.
>
>
>         To minimize collisions or generating different hash in such way to
>         place each file in different bricks ( file 1 => brick A, file 2 =>
>         Brick B, file 3=>  Brick C, file 4 => brick D)
>
>                We use kind of similar script to get the hash value for
> a file
>
> def gf_dm_hashfn(filename):
>     return ctypes.c_uint32(glusterfs.gf_dm_hashfn(
>         filename,
>         len(filendame)))
>
> if __name__ == "__main__":
>     print hex(gf_dm_hashfn(sys.argv[1]).value)
>
> We can then calculate the hash for a filename:
> # python gf_dm_hash.py file1
> 0x99d1b6fL
>
>
> Extended attribute is fetch to check the range and try to match the
> above generated hash value.
>
> getfattr -n trusted.glusterfs.dht -e hex file1
>
>
>       However we are not able to exactly follow till this point ,  how the
>       hash value matched to one of the layout assignments, to yield what we
>       call a hashed location.
>
>
> -        My question is if I  know the range of brick lay out ( say
> 0xc0000000 to  0xffffffff, is range  select a hash 0xc0070000 ) where
> to be placed the next file can we generate the name ( kind of reverse
> of  gluster
> glusterfs.gf_dm_hashfn) ?

I am not aware of any such mechanism.  You will have to generate file names manually and run them through your script to check whether it falls in the brick range.

>
> PS :  Susant : Can you throw some light or suggest  a method we are
> trying to solve.
>
> Thanks for your time.
>
>
> Best Regards,
> Subrata Ghosh
>
>
>
>
>
>
>
Regards,
Susant

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