[Gluster-devel] Question

[Anand Babu Periasamy]

> Hello,
> 
> I was reading the Glusterfs and it's really a nice concept for
> shared storage but is it possible to inverse the layers? What I mean
> is that Glusterfs is on top of a ext3/xfs fs, how about the oposite?

To do reverse, GlusterFS has to provide a raw block level access, by
just aggregating the underlying block devices. Even though it is
easier to achieve, benefits are not attractive. 

* RAID-0/1/10:
  - Scaling in capacity leads to painfully longer fsck downtime and
    possibilities for corruption.
  - Scaling in I/O bandwidth is bottle-necked by client's NIC. Because
    block-devices cannot be shared, a single client cannot take
    advantage of aggregated I/O bandwidth.
* RAID-3/4/5/6/XXX 
  - Inherits the above problems.
  - Checksum calculation will consume serious CPU cycles at the client
    side and cause slow down.

They are the same scaling problems SAN technology is facing
currently. To build a scalable powerful file system, clustering has to
be done at file I/O level.

> I was thinking about it because I don't know how Glusterfs reacts if
> it was built on top of a MD+LVM+XFS and then with resizing and
> snapshotting behind Glusterfs back.

MD+LVM+XFS should be transparent to GlusterFS. I don't see any issues
in doing so.

Actually, instead of MD/LVM, GlusterFS..
 * can cluster across systems.
 * can scale on demand both in terms of capacity and I/O performance
   by adding more storage bricks(servers).
 * can snapshot distributedly (coming in 1.4). GlusterFS's snapshot
   will allow you even roll back selected files. 

-- 
Anand Babu
GPG Key ID: 0x62E15A31
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