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