Iustin Pop wrote:
On Sun, Aug 12, 2007 at 07:03:44PM +0200, Jan Engelhardt wrote:
On Aug 12 2007 09:39, [email protected] 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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