On 05/01/2017 01:13 PM, Pranith Kumar Karampuri wrote:
On Mon, May 1, 2017 at 10:42 PM, Pranith Kumar Karampuri
<[email protected] <mailto:[email protected]>> wrote:
On Mon, May 1, 2017 at 10:39 PM, Gandalf Corvotempesta
<[email protected]
<mailto:[email protected]>> wrote:
2017-05-01 18:57 GMT+02:00 Pranith Kumar Karampuri
<[email protected] <mailto:[email protected]>>:
> 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/
<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.
Splitting the bricks need not be a post factum decision, we can start
with larger brick counts, on a given node/disk count, and hence spread
these bricks to newer nodes/bricks as they are added.
If I understand the ceph PG count, it works on a similar notion, till
the cluster grows beyond the initial PG count (set for the pool) at
which point there is a lot more data movement (as the pg count has to be
increased, and hence existing PGs need to be further partitioned) .
(just using ceph as an example, a similar approach exists for openstack
swift with their partition power settings).
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?
I don't see any solution that we can do without at least 2 bricks on
each of the 3 servers.
--
Pranith
--
Pranith
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