[ale] Fileserver/filesystem replication

[Danny Cox]

All,

On Wed, 2003-12-31 at 04:13, Bob Toxen wrote:
> Ya want real-time replication.  I'll give ya real-time replication.
> 
> 1. Build your ext3 or ReiserFS on top of Linux software mirroring.
> 2. Make the second (mirror) disk a network disk device.
> 3. Ensure that writes are not acknowledged until they percolate through
>    to the remote real hard disk.

	When I was working for Connex (a startup NAS box company absorbed by
Quantum, alas), we had a guy in Germany doing just about what you
describe above, with another wrinkle.  He was using the Network Block
Device (see the kernel docs for NBD) as one 'disk' of the mirror, but he
took it one step further.  As a block came in to be written, he
performed an MD4 (maybe MD5) checksum of it, and sent that across first,
along with the block number.  If the data was already there (I don't
have or recall any stats on it), then he was done.  If not, the NAK
would cause the whole block to be sent.  Higher latency, but as I
remember, it worked pretty well.

	Just FYI.  I don't know if it was ever released, or even if it ever saw
any use outside our "lab".

-- 
kernel, n.: A part of an operating system that preserves the
medieval traditions of sorcery and black art.

Danny