[Gluster-users] Add single server

[Pranith Kumar Karampuri]

On Mon, Apr 24, 2017 at 1:24 PM, Gandalf Corvotempesta <
gandalf.corvotempesta at gmail.com> wrote:

> Il 24 apr 2017 9:40 AM, "Ashish Pandey" <aspandey at redhat.com> ha scritto:
>
>
> There is  difference between server and bricks which we should understand.
> When we say  m+n = 6+2, then we are talking about the bricks.
> Total number of bricks are m+n = 8.
>
> Now, these bricks could be anywhere on any server. The only thing is that
> the server should be a part of cluster.
> You can have all the 8 bricks on one server or on 8 different servers.
> So, there is no *restriction* on number of servers when you add bricks.
> However, the number of bricks which you want to add should be in multiple
> of the
> configuration you have.
>
>
> This is clear but it doesn't change the result
> As no one is using gluster to replicate data by loosing redundancy (it's
> nonsense), adding bricks means adding servers
> If our server are already full with no more available slots for adding
> disks, the only solution is to add 8 servers more (at least 1 brick per
> server)
>
>
>
> In you case it should be 8, 16, 24....
>
> "can I add a single node moving from 6:2 to 7:2 and so on ?"
> You can not make 6+2 config volume  to 7+2 volume. You can not change the
> *configuration* of an existing volume.
> You can just add bricks in multiple to increase the storage capacity.
>
>
> Yes and this is the worst thing in gluster: the almost zero flexibility
>

> Bigger the cluster, higher the cost to maintain it or expand it.
>

At least in case of EC it is with good reason. If you want to change
volume's configuration from 6+2->7+2 you have to compute the encoding again
and place different data on the resulting 9 bricks. Which has to be done
for all files. It is better to just create a new volume with 7+2 and just
copy the files on to this volume and remove the original files on volume
with 6+2.


> If you start with a 6:2 by using commodity hardware, you are screwed, your
> next upgrade will be 8 servers with 1 disk/brick each.
>

Not true.


>
> Yes, gluster doesn't make use of any metadata server, but I really prefer
> to add 2 metadata server and 1 storage server at once when needed than
> avoid metadata servers but being forced to add a bounch of servers every
> time
>
> More servers means more power cost, more hardware that could fails and so
> on.
>
> Let's assume a replica 3 cluster.
> If I need to add 2tb more, I have to add 3 servers with 2tb on each server.
> Ceph, Lizard, Moose and others allow adding a single server/disk and then
> they rebalance data aroud by freeing up the used space adding the new disk.
>

Didn't understand this math. If you want to add 2TB capacity to a volume
that is 3-way replicated, you essentially need to add 6TB in whatever
solution you have. At least 6TB with a single server. Which you can do even
with Gluster.


>
> I thought that this lack of flexibility was addressed is some way in
> latest version...
>

I think we had this discussion last July[1] with you that we can simulate
the same things other storage solutions with metadata do by doing
replace-bricks and rebalance. If you have a new server with 8 bricks then
we can add a single server and make sure things are rebalanced with 6+2.
Please note it is better to use data-bricks that is power of 2 like
4+2/8+2/16+4 etc than 6+2.

Are you suggesting this process to be easier through commands, rather than
for administrators to figure out how to place the data?

[1] http://lists.gluster.org/pipermail/gluster-users/2016-July/027431.html


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-- 
Pranith
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