[Gluster-users] Gluster 3.5.2 gluster, how does cache work?

[Ben Turner]

----- Original Message -----
> From: "Juan José Pavlik Salles" <jjpavlik at gmail.com>
> To: gluster-users at gluster.org
> Sent: Thursday, August 21, 2014 4:07:28 PM
> Subject: [Gluster-users] Gluster 3.5.2 gluster, how does cache work?
> 
> Hi guys, I've been reading a bit about caching in gluster volumes, but I
> still don't get a few things. I set up a gluster replica 2 volume like this:
> 
> [root at gluster-test-1 ~]# gluster vol info vol_rep
> Volume Name: vol_rep
> Type: Replicate
> Volume ID: b77db06d-2686-46c7-951f-e43bde21d8ec
> Status: Started
> Number of Bricks: 1 x 2 = 2
> Transport-type: tcp
> Bricks:
> Brick1: gluster-test-1:/ladrillos/l1/l
> Brick2: gluster-test-2:/ladrillos/l1/l
> Options Reconfigured:
> performance.cache-min-file-size: 90MB
> performance.cache-max-file-size: 256MB
> performance.cache-refresh-timeout: 60
> performance.cache-size: 256MB
> [root at gluster-test-1 ~]#
> 
> Then I mounted the volume with gluster client on another machine. I created
> an 80Mbytes file called 80, and here you have the reading test:
> 
> [root at gluster-client-1 gluster_vol]# dd if=/mnt/gluster_vol/80 of=/dev/null
> bs=1M
> 80+0 records in
> 80+0 records out
> 83886080 bytes (84 MB) copied, 1,34145 s, 62,5 MB/s
> [root at gluster-client-1 gluster_vol]# dd if=/mnt/gluster_vol/80 of=/dev/null
> bs=1M
> 80+0 records in
> 80+0 records out
> 83886080 bytes (84 MB) copied, 0,0246918 s, 3,4 GB/s
> [root at gluster-client-1 gluster_vol]# dd if=/mnt/gluster_vol/80 of=/dev/null
> bs=1M
> 80+0 records in
> 80+0 records out
> 83886080 bytes (84 MB) copied, 0,0195678 s, 4,3 GB/s
> [root at gluster-client-1 gluster_vol]#

You are seeing the effect of client side kernel caching.  If you want to see the actual throughput for reads run:

sync; echo 3 > /proc/sys/vm/drop_caches; dd blah

Kernel caching happens on both the client and server side, when I want to see uncached performance I drop caches on both clients and servers:

run_drop_cache()
{
    for host in $MASTERNODE $NODE $CLIENT
    do
        ssh -i /root/.ssh/my_id root@${host} echo "Dropping cache on $host"
        ssh -i /root/.ssh/my_id root@${host} sync
        ssh -i /root/.ssh/my_id root@${host} "echo 3 > /proc/sys/vm/drop_caches"
    done
}

HTH!

-b
 
> Cache is working flawlessly, (even though that 80 Mbytes is smaller than the
> min-file-size value, but I don't care about it right now) what I don't get
> is where cache is being stored. Is it stored on the client side or on the
> server side? According to documentation, the io-cache translator could be
> loaded on both sides (client and server), how can I know where it is being
> loeaded? It looks like as it was being stored locally because of the speed,
> but I'd like to be sure.
> 
> Thanks!
> 
> --
> Pavlik Salles Juan José
> Blog - http://viviendolared.blogspot.com
> 
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