Re: [RFD] Layering: Use-Case Composers (was: DRBD - what is it, anyways?[compare with e.g. NBD + MD raid])

[Paul Clements]

Iustin Pop wrote:
> On Sun, Aug 12, 2007 at 07:03:44PM +0200, Jan Engelhardt wrote:
> > On Aug 12 2007 09:39, david@xxxxxxx wrote:
> > > 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.

Yes, and with a bitmap configured on the raid1, you just resync the 
blocks that have been written while the connection was down.


> > 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.

Well, if people would like to see a timeout option, I actually coded up 
a patch a couple of years ago to do just that, but I never got it into 
mainline because you can do almost as well by doing a check at 
user-level (I basically ping the nbd connection periodically and if it 
fails, I kill -9 the nbd-client).


> > > 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.
> >
> > > 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.

You can configure write_behind (aka, asynchronous writes) to buffer as 
much data as you have RAM to hold. At a certain point, presumably, you'd 
want to just break the mirror and take the hit of doing a resync once 
your network leg falls too far behind.

--
Paul
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