Gluster users - May 2017 - Add single server

2017-05-01 18:57 GMT+02:00 Pranith Kumar Karampuri <pkarampu at
redhat.com>:
> Yes this is precisely what all the other SDS with metadata servers kind of
> do. They kind of keep a map of on what all servers a particular file/blob
is
> stored in a metadata server.
Not exactly. Other SDS has some servers dedicated to metadata and,
personally, I don't like that approach.
> GlusterFS doesn't do that. In GlusterFS what
> bricks need to be replicated is always given and distribute layer on top of
> these replication layer will do the job of distributing and fetching the
> data. Because replication happens at a brick level and not at a file level
> and distribute happens on top of replication and not at file level. There
> isn't too much metadata that needs to be stored per file. Hence no need
for
> separate metadata servers.
And this is great, that's why i'm talking about embedding a sort of
database
to be stored on all nodes. no metadata servers, only a mapping between files
and servers.
> If you know path of the file, you can always know where the file is stored
> using pathinfo:
> Method-2 in the following link:
> https://gluster.readthedocs.io/en/latest/Troubleshooting/gfid-to-path/
>
> You don't need any db.
For the current gluster yes.
I'm talking about a different thing.

In a RAID, you have data stored somewhere on the array, with metadata
defining how this data should
be wrote or read. obviously, raid metadata must be stored in a fixed
position, or you won't be able to read
that.

Something similiar could be added in gluster (i don't know if it would
be hard): you store a file mapping in a fixed
position in gluster, then all gluster clients will be able to know
where a file is by looking at this "metadata" stored in
the fixed position.

Like ".gluster" directory. Gluster is using some "internal"
directories for internal operations (".shards", ".gluster",
".trash")
A ".metadata" with file mapping would be hard to add ?
> Basically what you want, if I understood correctly is:
> If we add a 3rd node with just one disk, the data should automatically
> arrange itself splitting itself to 3 categories(Assuming replica-2)
> 1) Files that are present in Node1, Node2
> 2) Files that are present in Node2, Node3
> 3) Files that are present in Node1, Node3
>
> As you can see we arrived at a contradiction where all the nodes should
have
> at least 2 bricks but there is only 1 disk. Hence the contradiction. We
> can't do what you are asking without brick splitting. i.e. we need to
split
> the disk into 2 bricks.
I don't think so.
Let's assume a replica 2.

S1B1 + S2B1

1TB each, thus 1TB available (2TB/2)

Adding a third 1TB disks should increase available space to 1.5TB (3TB/2)

On Mon, May 1, 2017 at 10:39 PM, Gandalf Corvotempesta <
gandalf.corvotempesta at gmail.com> wrote:
> 2017-05-01 18:57 GMT+02:00 Pranith Kumar Karampuri <pkarampu at
redhat.com>:
> > Yes this is precisely what all the other SDS with metadata servers
kind
> of
> > do. They kind of keep a map of on what all servers a particular
> file/blob is
> > stored in a metadata server.
>
> Not exactly. Other SDS has some servers dedicated to metadata and,
> personally, I don't like that approach.
>
> > GlusterFS doesn't do that. In GlusterFS what
> > bricks need to be replicated is always given and distribute layer on
top
> of
> > these replication layer will do the job of distributing and fetching
the
> > data. Because replication happens at a brick level and not at a file
> level
> > and distribute happens on top of replication and not at file level.
There
> > isn't too much metadata that needs to be stored per file. Hence no
need
> for
> > separate metadata servers.
>
> And this is great, that's why i'm talking about embedding a sort of
> database
> to be stored on all nodes. no metadata servers, only a mapping between
> files
> and servers.
>
> > If you know path of the file, you can always know where the file is
> stored
> > using pathinfo:
> > Method-2 in the following link:
> > https://gluster.readthedocs.io/en/latest/Troubleshooting/gfid-to-path/
> >
> > You don't need any db.
>
> For the current gluster yes.
> I'm talking about a different thing.
>
> In a RAID, you have data stored somewhere on the array, with metadata
> defining how this data should
> be wrote or read. obviously, raid metadata must be stored in a fixed
> position, or you won't be able to read
> that.
>
> Something similiar could be added in gluster (i don't know if it would
> be hard): you store a file mapping in a fixed
> position in gluster, then all gluster clients will be able to know
> where a file is by looking at this "metadata" stored in
> the fixed position.
>
> Like ".gluster" directory. Gluster is using some
"internal"
> directories for internal operations (".shards",
".gluster", ".trash")
> A ".metadata" with file mapping would be hard to add ?
>
> > Basically what you want, if I understood correctly is:
> > If we add a 3rd node with just one disk, the data should automatically
> > arrange itself splitting itself to 3 categories(Assuming replica-2)
> > 1) Files that are present in Node1, Node2
> > 2) Files that are present in Node2, Node3
> > 3) Files that are present in Node1, Node3
> >
> > As you can see we arrived at a contradiction where all the nodes
should
> have
> > at least 2 bricks but there is only 1 disk. Hence the contradiction.
We
> > can't do what you are asking without brick splitting. i.e. we need
to
> split
> > the disk into 2 bricks.
>
> I don't think so.
> Let's assume a replica 2.
>
> S1B1 + S2B1
>
> 1TB each, thus 1TB available (2TB/2)
>
> Adding a third 1TB disks should increase available space to 1.5TB (3TB/2)
>
I agree it should. Question is how? What will be the resulting brick-map?


-- 
Pranith
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