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.
