[Gluster-users] Native client reads from a replication volume

[Kal Black]

Hi All,
I am new to Gluster and trying to figure out how the things actually work.
Have been reading the doc, lists and blogs but still have some doubts. Did
a little test trying to understand how a native client would read from a
replicated volume and need some help.

My setup:
0. Using Gluster 3.4.0 of CentOS 6.4 64bit servers.
1. Replicated gluster volume, using two bricks on two different gluster
servers.
2. Dedicated apache web server, mounting the replicated gluster volume,
using gluster native client, having it's document root directory, with 10
000 100K unique files, on the gluster mount.
3. Dedicated workstation that queries each unique 100K file from the web
server using  httperf:
httperf --hog --server=192.168.29.45 --uri= --port=80 --wlog
y,/var/tmp/100K_urls --num-calls 1 --timeout 5 --num-conns 10000  --rate 100


Findings:

1. Gluster servers will cache requests and then will serve subsequent
request from the cache (no disk i/o will be issued). Clearing the mem cache
(sync; echo "3" >/proc/sys/vm/drop_caches) will trigger reading the files
from disk again.
  1.1 Not sure if the caching is due to gluster caching (the
performance/io-cache translator is enabled by default) or it is due to the
system caching.
  1.2 During the initial caching on the gluster servers, the load (top) on
the web server would go above 200. It will go to normal level, once the i/o
on the gluster servers goes down (server starts serving requests from the
cache). There are any to very little blocked process on the web server
(vmstat) and performance looks good. Apparently this should be attributed
somehow to the gluster client  .. but why?

2. In order to figure out how the gluster client reads from the gluster
servers, I started iptraf on the gluster client (the apache server) and on
both gluster servers. After that, I stared the httperf from the
workstation, requesting  each of the 10 000, 100K files located on the
gluster volume (a total of 1G). In my understanding, the client would make
sure that the requested file is the right one by checking file's metadata
on all gluster servers, part of the replication volume, but will fetch it
from only one of them.

To my surprise, according to iptraf, each gluster server sent 1038M (close
to the total size of all files) to the gluster client. And the gluster
client received ~98001K from each gluster server:

Proto/Port   Pkts      Bytes    PktsTo    BytesTo  PktsFrom BytesFrom

Gluster server 1:
TCP/1020  543629   1072M   101758   1038M    441871  34578324

Gluster server 2:
TCP/1019  546148   1072M   103366   1038M    442782  34625676

Gluster Client:
TCP/1020  1087627  981701K  697846  949912K  389781
31789104

TCP/1019  1087516  981973K  697993  950220K  389523  31753440

In the other hand, each gluster server cached ~500MB while the gluster
client cached ~1GB

Gluster server 1:
# free -m
             total       used       free     shared    buffers     cached
Mem:          3821       1072       2748          0         66        520
-/+ buffers/cache:        485       3336
Swap:         3951          0       3951

Gluster server 1:
# free -m
             total       used       free     shared    buffers     cached
Mem:          3821       1085       2735          0         65        522
-/+ buffers/cache:        497       3324
Swap:         3951          3       3948

Gluser Client:
# free -m
             total       used       free     shared    buffers     cached
Mem:          3821       1330       2490          0          1        993
-/+ buffers/cache:        336       3485
Swap:         3951          0       3951


Can someone explain how exactly a single native guster client would read
from a replication volume.
Is there any mechanism (any details) that would spread requests (from a
single client) among all servers, taking part in a replica volume, making
it faster and spread the load among all servers?
Would a client take a decision where to fetch a file from each time based
on some criteria (details) or it will be bound to a give server all the
time?

Is there any low level documentation since I find the one provided on the
gluster site to be very basic?

Thanks!
Kal
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