On Sun, Aug 12, 2007 at 07:03:44PM +0200, Jan Engelhardt wrote:

On Aug 12 2007 09:39, david@lang.hm wrote:

quoted

now, I am not an expert on either option, but three are a couple things that I
would question about the DRDB+MD option

1. when the remote machine is down, how does MD deal with it for reads and
writes?

I suppose it kicks the drive and you'd have to re-add it by hand unless done by
a cronjob.

From my tests, since NBD doesn't have a timeout option, MD hangs in the
write to that mirror indefinitely, somewhat like when dealing with a
broken IDE driver/chipset/disk.

quoted

2. MD over local drive will alternate reads between mirrors (or so I've been
told), doing so over the network is wrong.

Certainly. In which case you set "write_mostly" (or even write_only, not sure
of its name) on the raid component that is nbd.

quoted

3. when writing, will MD wait for the network I/O to get the data saved on the
backup before returning from the syscall? or can it sync the data out lazily

Can't answer this one - ask Neil :)

MD has the write-mostly/write-behind options - which help in this case
but only up to a certain amount.


In my experience DRBD wins hands-down over MD+NBD because of MD doesn't
know (or handle) a component that never returns from a write, which is
quite different from returning with an error. Furthermore, DRBD was
designed to handle transient errors in the connection to the peer due to
its network-oriented design, whereas MD is mostly designed with local or
at least high-reliability disks (where disk can be SAN, SCSI, etc.) and
a failure is not normal for MD. Thus the need for manual reconnect in MD
case and the automated handling of reconnects in case of DRBD.

I'm just a happy user of both MD over local disks and DRBD for networked
raid.

regards,
iustin