1:56 a.m.
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Hi oVirt group,
I'm currently trying to set up an oVirt 3.6 self hosed engine on a
replica 2 gluster volume. As you can imagine it fails since it expects a
replica 3 volume.
Since this part is hard coded during the installation this could be
easily remedied but on the other hand I'm not familiar with the oVirt
setup so I'm not sure if I would be disturbing something far a head that
again is puzzled by the lack of a replica 3 volume.
Is there a reason why it has to be exactly replica 3?
Shouldn't the redundancy of the storage back end be the responsibility
of the admin and therefore the option to choose (at least replicated and
distributed replicated) open?
I'm asking because the storage back and I'm using is kind of a 'big'
thing and it's hardware is put together to facilitate the use of replica
2. Sure I could create multiple bricks on those two machines but other
than a negative performance impact I would gain nothing. Since the price
tag of one of those servers is quite high the likelihood of getting
another one is exactly 0. So I'm sure it makes sens to you that I would
rather choose my own replication level than having one predetermined.
All the best
Richard
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3:02 a.m.
On Fri, Sep 4, 2015 at 8:56 AM, Richard Neuboeck <hawk(a)tbi.univie.ac.at>
wrote:
Hi oVirt group,
I'm currently trying to set up an oVirt 3.6 self hosed engine on a
replica 2 gluster volume. As you can imagine it fails since it expects a
replica 3 volume.
Since this part is hard coded during the installation this could be
easily remedied but on the other hand I'm not familiar with the oVirt
setup so I'm not sure if I would be disturbing something far a head that
again is puzzled by the lack of a replica 3 volume.
Is there a reason why it has to be exactly replica 3?
To have a valid quorum having the system being able to decide witch is the
right and safe copy avoiding an issue called split brain.
Under certain circumstances/issues (network issue, hosts down or whatever
could happen) the data on different replica could diverge: if you have two
and just two different hosts that claim each other that its copy is the
right one there is no way to automatically take the right decision. Having
three hosts and setting the quorum according to that solves/mitigates the
issue.
Shouldn't the redundancy of the storage back end be the
responsibility
of the admin and therefore the option to choose (at least replicated and
distributed replicated) open?
I'm asking because the storage back and I'm using is kind of a 'big'
thing and it's hardware is put together to facilitate the use of replica
2. Sure I could create multiple bricks on those two machines but other
than a negative performance impact I would gain nothing. Since the price
tag of one of those servers is quite high the likelihood of getting
another one is exactly 0. So I'm sure it makes sens to you that I would
rather choose my own replication level than having one predetermined.
All the best
Richard
_______________________________________________
Users mailing list
Users(a)ovirt.org
http://lists.ovirt.org/mailman/listinfo/users
3:14 a.m.
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On 04.09.15 10:02, Simone Tiraboschi wrote:
Is there a reason why it has to be exactly replica 3?
To have a valid quorum having the system being able to decide witch is
the right and safe copy avoiding an issue called split brain.
Under certain circumstances/issues (network issue, hosts down or
whatever could happen) the data on different replica could diverge: if
you have two and just two different hosts that claim each other
that its
copy is the right one there is no way to automatically take the
right
decision. Having three hosts and setting the quorum according to that
solves/mitigates the issue.
Thanks for the explanation. I do understand the problem but since
I'm somewhat limited in my hardware options is there a way to
override this requirement? Meaning if I change the checks for
replica 3 in the installation scripts does something else fail on
the way?
In my case coherence checks would come from outside the storage and
vm host setup and fencing would be applied appropriately.
I would very much appreciate it if the particulars of the storage
setup could be either selected from a list of possibilities or be
ignored and just a warning be issued that this setup is not recommended.
Thanks!
Richard
--=20
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3:49 a.m.
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On 09/09/2015 10=3A14 AM=2C Richard Neuboeck wrote=3A
=3E On 04=2E09=2E15 10=3A02=2C Simone Tiraboschi wrote=3A
=3E=3E Is there a reason why it has to be exactly replica 3=3F
=3E=3E
=3E=3E
=3E=3E To have a valid quorum having the system being able to decide witch=
is
=3E=3E the right and safe copy avoiding an issue called split brain=2E
=3E=3E Under certain circumstances/issues =28network issue=2C hosts down or=
=3E=3E whatever could happen=29 the data on different replica could diverge=
=3A if
=3E=3E you have two and just two different hosts that claim each other
=3E that its
=3E=3E copy is the right one there is no way to automatically take the righ=
t
=3E=3E decision=2E Having three hosts and setting the quorum according to t=
hat
=3E=3E solves/mitigates the issue=2E
=3E
=3E Thanks for the explanation=2E I do understand the problem but since
=3E I=27m somewhat limited in my hardware options is there a way to
=3E override this requirement=3F Meaning if I change the checks for
=3E replica 3 in the installation scripts does something else fail on
=3E the way=3F
=3E
=3E In my case coherence checks would come from outside the storage and
=3E vm host setup and fencing would be applied appropriately=2E
=3E
=3E I would very much appreciate it if the particulars of the storage
=3E setup could be either selected from a list of possibilities or be
=3E ignored and just a warning be issued that this setup is not recommended=
=2E
=3E
=3E Thanks!
=3E Richard
=3E
=3E
As a side question=2C in Glusterfs 3=2E7 there is an =22AFR arbiter volume=
=22
=28http=3A//gluster=2Ereadthedocs=2Eorg/en/release-3=2E7=2E0-1/Features/afr=
-arbiter-volumes/=29
that will only contain the metadata=2E
Will ovirt support this in 3=2E6=3F
Kind regards=2C
Jorick Astrego
Met vriendelijke groet=2C With kind regards=2C
Jorick Astrego
Netbulae Virtualization Experts=20
----------------
=09Tel=3A 053 20 30 270 =09info=40netbulae=2Eeu =09Staalsteden 4-3A =09KvK=
08198180
=09Fax=3A 053 20 30 271 =09www=2Enetbulae=2Eeu =097547 TA Enschede =09BTW=
NL821234584B01
----------------
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=3Chtml=3E
=3Cbody=3E
<br <br On 09/09/2015 10:14 AM, Richard Neuboeck
wrote: <br <font
color=3D"#000000">> On 04.09.15 10:02, Simone Tiraboschi
wrote:=
3;</font><br <font
color=3D"#000000">>> Is
there a reason =
why it has to be exactly replica 3? </font><br <font
color=3D"#000000">>> </font><br <font
color=3D"#000000">>> </font><br <font color=3D"#000000">>> To
have a valid quorum having the system b=
eing able to decide witch is </font><br <font
color=3D"#000000">>> the right and safe copy avoiding an issue
=
called split brain. </font><br <font
color=3D"#000000">>> Under certain circumstances/issues
(networ=
k issue, hosts down or </font><br <font
color=3D"#000000">>> whatever could happen) the data on
differe=
nt replica could diverge: if </font><br <font
color=3D"#000000">>> you have two and just two different hosts
=
that claim each other </font><br <font
color=3D"#000000">> that its </font><br <font color=3D"#000000">>> copy
is the right one there is no way to a=
utomatically take the right </font><br <font
color=3D"#000000">>> decision. Having three hosts and setting
t=
he quorum according to that </font><br <font
color=3D"#000000">>> solves/mitigates the
issue. </font><br=
<font
color=3D"#000000">> </font><br <font color=3D"#000000">> Thanks for the
explanation. I do understand th=
e problem but since </font><br <font
color=3D"#000000">> I'm somewhat limited in my hardware options
is=
there a way to </font><br <font
color=3D"#000000">> override this requirement? Meaning if I change=
the checks for </font><br <font
color=3D"#000000">> replica 3 in the installation scripts does som=
ething else fail on </font><br <font
color=3D"#000000">> the way? </font><br <font
color=3D"#000000">> </font><br <font color=3D"#000000">> In my case
coherence checks would come from ou=
tside the storage and </font><br <font
color=3D"#000000">> vm host setup and fencing would be applied app=
ropriately. </font><br <font
color=3D"#000000">> </font><br <font color=3D"#000000">> I would very
much appreciate it if the particu=
lars of the storage </font><br <font
color=3D"#000000">> setup could be either selected from a list of =
possibilities or be </font><br <font
color=3D"#000000">> ignored and just a warning be issued that this=
setup is not recommended. </font><br <font
color=3D"#000000">> </font><br <font color=3D"#000000">>
Thanks! </font><br <font
color=3D"#000000">> Richard </font><br <font
color=3D"#000000">> </font><br <font
color=3D"#000000">> </font><br As a side question, in Glusterfs 3.7 there is an "AFR
arbiter volume&q=
uot; <br (<a
href=3D"http://gluster.readthedocs.org/en/release-3.7.0-1/Features/a...
arbiter-volumes/">http://gluster.readthedocs.org/en/release-3.7.0...
s/afr-arbiter-volumes/</a>) <br that will
only contain the metadata. <br
<br Will ovirt support this in
3.6? <br <br Kind regards, <br
<br Jorick Astrego <br =
=3CBR /=3E
=3CBR /=3E
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3:56 a.m.
On Wed, Sep 9, 2015 at 10:49 AM, Jorick Astrego <j.astrego(a)netbulae.eu>
wrote:
On 09/09/2015 10:14 AM, Richard Neuboeck wrote:
> On 04.09.15 10:02, Simone Tiraboschi wrote:
>> Is there a reason why it has to be exactly replica 3?
>>
>>
>> To have a valid quorum having the system being able to decide witch is
>> the right and safe copy avoiding an issue called split brain.
>> Under certain circumstances/issues (network issue, hosts down or
>> whatever could happen) the data on different replica could diverge: if
>> you have two and just two different hosts that claim each other
> that its
>> copy is the right one there is no way to automatically take the right
>> decision. Having three hosts and setting the quorum according to that
>> solves/mitigates the issue.
>
> Thanks for the explanation. I do understand the problem but since
> I'm somewhat limited in my hardware options is there a way to
> override this requirement? Meaning if I change the checks for
> replica 3 in the installation scripts does something else fail on
> the way?
>
> In my case coherence checks would come from outside the storage and
> vm host setup and fencing would be applied appropriately.
>
> I would very much appreciate it if the particulars of the storage
> setup could be either selected from a list of possibilities or be
> ignored and just a warning be issued that this setup is not recommended.
>
> Thanks!
> Richard
>
>
As a side question, in Glusterfs 3.7 there is an "AFR arbiter volume"
(
http://gluster.readthedocs.org/en/release-3.7.0-1/Features/afr-arbiter-vo...)
that will only contain the metadata.
Will ovirt support this in 3.6?
Not explicitly and I never tried it with hosted-engine but I suspect it
works as far it will report replica level = 3.
By the way you can save disk space on the third host but you still need
three hosts.
Kind regards,
Jorick Astrego
Met vriendelijke groet, With kind regards,
Jorick Astrego
*Netbulae Virtualization Experts *
------------------------------
Tel: 053 20 30 270 info(a)netbulae.eu Staalsteden 4-3A KvK 08198180 Fax:
053 20 30 271 www.netbulae.eu 7547 TA Enschede BTW NL821234584B01
------------------------------
_______________________________________________
Users mailing list
Users(a)ovirt.org
http://lists.ovirt.org/mailman/listinfo/users
3:54 a.m.
On Wed, Sep 9, 2015 at 10:14 AM, Richard Neuboeck <hawk(a)tbi.univie.ac.at>
wrote:
On 04.09.15 10:02, Simone Tiraboschi wrote:
> Is there a reason why it has to be exactly replica 3?
>
>
> To have a valid quorum having the system being able to decide witch is
> the right and safe copy avoiding an issue called split brain.
> Under certain circumstances/issues (network issue, hosts down or
> whatever could happen) the data on different replica could diverge: if
> you have two and just two different hosts that claim each other
that its
> copy is the right one there is no way to automatically take the right
> decision. Having three hosts and setting the quorum according to that
> solves/mitigates the issue.
Thanks for the explanation. I do understand the problem but since
I'm somewhat limited in my hardware options is there a way to
override this requirement? Meaning if I change the checks for
replica 3 in the installation scripts does something else fail on
the way?
I'm advising that it's not a safe configuration so it's not recommended for
a production environment.
Having said that, as far as I know it's enforced only in the setup script
so tweaking it should be enough.
Otherwise, if you have enough disk space, you can also have a different
trick: you could create a replica 3 volume with 2 bricks from a single host.
It's not a safe procedure at all cause you still have only 2 hosts, so it's
basically just replica 2, and in case of split brain the host with two
copies will win by configuration which is not always the right decision.
In my case coherence checks would come from outside the storage and
vm host setup and fencing would be applied appropriately.
Can I ask how?
I would very much appreciate it if the particulars of the storage
setup could be either selected from a list of possibilities or be
ignored and just a warning be issued that this setup is not recommended.
Thanks!
Richard
--
/dev/null
6:17 a.m.
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On 09/09/2015 10:54 AM, Simone Tiraboschi wrote:
=20
=20
On Wed, Sep 9, 2015 at 10:14 AM, Richard Neuboeck
<hawk(a)tbi.univie.ac.at <mailto:hawk@tbi.univie.ac.at>> wrote:
=20
On 04.09.15 10:02, Simone Tiraboschi wrote:
> Is there a reason why it has to be exactly replica 3?
>
>
> To have a valid quorum having the system being able to decide wit=
ch is
> the right and safe copy avoiding an issue called split
brain.
> Under certain circumstances/issues (network issue, hosts down or
> whatever could happen) the data on different replica could diverg=
e: if
> you have two and just two different hosts that claim each
other
that its
> copy is the right one there is no way to automatically take the r=
ight
> decision. Having three hosts and setting the quorum
according to =
that
> solves/mitigates the issue.
=20
=20
Thanks for the explanation. I do understand the problem but since
I'm somewhat limited in my hardware options is there a way to
override this requirement? Meaning if I change the checks for
replica 3 in the installation scripts does something else fail on
the way?
=20
=20
I'm advising that it's not a safe configuration so it's not
recommended for a production environment.
Having said that, as far as I know it's enforced only in the setup
script so tweaking it should be enough.
Otherwise, if you have enough disk space, you can also have a
different trick: you could create a replica 3 volume with 2 bricks
from a single host.
I've thought about that but since that would obviously only help to
fool the installation script there is nothing else in this setup
that would improve the situation. Worse the read/write overhead on
the second machine would be a performance downgrade.
It's not a safe procedure at all cause you still have only 2
hosts,
so it's basically just replica 2, and in case of split brain the
host with two copies will win by configuration which is not always
the right decision.
Right. I'm thinking of trying to add a dummy node as mentioned in
the RHEL documentation. This would (in theory) prevent the read only
state in the split brain scenario and make it possible to access the
storage. But still the installation requirement of replica 3 would
not be satisfied.
=20
In my case coherence checks would come from outside the storage and=
vm host setup and fencing would be applied appropriately.
=20
=20
Can I ask how?
Multiple machines separated from the storage and virtualization
machines that will check communication (in general and of several
services) and try to intervene if there is something going awry
first by accessing the machines directly (if possible) and then by
deactivating those machines by remote management.
Cheers
Richard
=20
I would very much appreciate it if the particulars of the storage
setup could be either selected from a list of possibilities or be
ignored and just a warning be issued that this setup is not
recommended.
=20
Thanks!
Richard
=20
=20
--
/dev/null
=20
=20
--=20
/dev/null
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3151
days inactive
3156
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6 comments
3 participants
participants (3)
-
Jorick Astrego -
Richard Neuboeck -
Simone Tiraboschi