Hi,
we are currently evaluating GlusterFS for a production environment. Our
focus is on the high-availability features of GlusterFS. However, our tests
have not worked out well. Hence I am seeking feedback from you.
In our planned production environment, Gluster should provide shared
storage for VM disk images. So, our very basic initial test setup is as
follows:
We are using two servers, each providing a single brick of a replicated
gluster volume (Gluster 3.4.1). A third server runs a test-VM (Ubuntu 13.04
on QEMU 1.3.0 and libvirt 1.0.3) which uses a disk image file stored on the
gluster volume as block device (/dev/vdb). For testing purposes, the root
file system of this VM (/dev/vda) is a disk image NOT stored on the gluster
volume.
To test the high-availability features of gluster under load, we run FIO
inside the VM directly on the vdb block device (see configuration below).
Up to now, we tested reading only. The test procedure is as follows:
1. We start FIO inside the VM and observe by means of "top" which of the
two servers receives the read requests (i.e., increased CPU load of the
glusterd process). Let?s say that Server1 has the CPU load by glusterfsd.
2. While FIO is running, we take down the network of this Server1 and
observe if the Server2 takes over.
3. This ?fail over? works (almost 100% of the times), we see the CPU load
from glusterfsd on Server2. As expected, Server1 does not have any load
because is ?offline?.
4. After a while we bring up the NIC on Server1 again. In this step we
realized that the expected behavior is that when bringing up this NIC, this
server should take over again (something like active-passive behavior) but
this happens only 5-10% of the times. The CPU load is still on Server2.
5. After some time, we bring down the NIC on Server2 expecting that Server1
takes over. This second "fail over" crashes. The VM complains about I/O
errors which can only be resolved by restarting the VM and sometimes even
removing and creating the volume again.
After some test, we realized that if restarting the glusterd daemon
(/etc/init.d/glusterd restart) on Server1 after step 3 or before step 4,
the Server1 takes over automatically without bringing down Server2 or
anything like that.
We tested this using the normal FUSE mount and libgfapi. If using FUSE, the
local mount sometimes becomes unavailable (ls shows not more files) if the
failover fails.
We have a few fundamental questions in this regard:
i) Is Gluster supposed to handle such a scenario or are we making wrong
assumptions? Because the only solution we found is to restart the daemon
when a network outage occurs, but this is not acceptable in a real scenario
with VMs running real applications.
ii) What is the recommended configuration in terms of caching (QEMU:
cache=none/writethrough/writeback) and direct I/O (FIO and Gluster) to
maximize the reliability of the failover process? We varied the parameters
but could find a working configuration. Do these parameters have an impact
at all?
FIO test specification:
[global]
direct=1
ioengine=libaio
iodepth=4
filename=/dev/vdb
runtime=300
numjobs=1
[maxthroughput]
rw=read
bs=16k
VM configuration:
<domain type='kvm' id='6'>
<name>testvm</name>
<uuid>93877c03-605b-ed67-1ab2-2ba16b5fb6b5</uuid>
<memory unit='KiB'>2097152</memory>
<currentMemory unit='KiB'>2097152</currentMemory>
<vcpu placement='static'>1</vcpu>
<os>
<type arch='x86_64' machine='pc-1.1'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='utc'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/kvm</emulator>
<disk type='block' device='disk'>
<driver name='qemu' type='raw' cache='writethrough'/>
<source dev='/mnt/local/io-perf.img'/>
<target dev='vda' bus='virtio'/>
<alias name='virtio-disk0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04'
function='0x0'/>
</disk>
<disk type='block' device='disk'>
<driver name='qemu' type='raw' cache='writethrough'/>
<source dev='/mnt/shared/io-perf-testdisk.img'/>
<target dev='vdb' bus='virtio'/>
<alias name='virtio-disk1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x07'
function='0x0'/>
</disk>
<controller type='usb' index='0'>
<alias name='usb0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01'
function='0x2'/>
</controller>
<interface type='network'>
<mac address='52:54:00:36:5f:dd'/>
<source network='default'/>
<target dev='vnet0'/>
<model type='virtio'/>
<alias name='net0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03'
function='0x0'/>
</interface>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='5900' autoport='yes' listen='127.0.0.1'>
<listen type='address' address='127.0.0.1'/>
</graphics>
<video>
<model type='cirrus' vram='9216' heads='1'/>
<alias name='video0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02'
function='0x0'/>
</video>
<memballoon model='virtio'>
<alias name='balloon0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05'
function='0x0'/>
</memballoon>
</devices>
<seclabel type='none'/>
</domain>
Thank you very much in advance,
Jose Lausuch
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