[Gluster-users] glusterfs alternative ? :P

[Keith Freedman]

At 12:34 PM 1/20/2009, Jerker Nyberg wrote:
>On Tue, 20 Jan 2009, Stas Oskin wrote:
>
> > Can someone explain in layman terms what is the meaning under the term "
> > shared block storage"? As far as I understand it's a shared storage device
> > (SAN, plain hard-disk, etc...), which provides write/read access 
> to multiple
> > servers/clients, and coordinates all the access operations (i.e. locking
> > files, keeping files shadow copies, i.e.).
>
>I will try. Shared block storage is a block device like /dev/sda
>accessible on more than one server at once. This may be a plain hard-disk
>or a hardware RAID. Coordination among the servers is handled on the file
>system level above that - for example by OCFS or GFS. Using a normal file
>system like Ext3 is not possible for read/write operations to the same
>shared block storage since the servers would get very confused.

As I understand it, OCFS allows both (many) machines to physically 
modify data on the shared block device, but it adds a lock manager to 
insure that 2 machines wont be writing to the same file at the same time.

This  has a performance hit, since all locking can't be in memory and 
can't be cached, but the benefit is a true clustering filesystem but 
on shared storage.

OCFS doesn't replicate data, but I believe relies on the hardware for 
HA features (mirroring/raid/etc), so it's not ideal for use with 
commodity hardware.