Fwd: Re: [DRBD-user] lvm2, striping, and drbd

kbyrd-drbd at memcpy.com kbyrd-drbd at memcpy.com
Mon Aug 27 16:58:47 CEST 2007



On Mon, 27 Aug 2007 07:37:42 +0200, Lars Ellenberg
<lars.ellenberg at linbit.com> wrote:
> On Sun, Aug 26, 2007 at 06:00:16PM -0700, kbyrd-drbd wrote:
>>
>> I haven't used drbd yet, I'm getting ready to test and deploy it.
>> I've searched the archives I've seen various posts about md and
>> lvm2 and I know some of this has been covered. But, I'm confused
>> about the current state of things with 0.8
>>
>> I'd like something that feels like clustered RAID1+0 (that's
>> striping on top of drbd). My ideal plan: lvm2 striping on top of
>> four drbd pairs active/active pairs. I'd run GFS on top of this.
>> Do I need cLVM instead of LVM2 for this. Does LVM striping even
>> work with drbd?
>
> don't.
>
> DRBD does not (yet) support consistency bundling of several drbd.
> so whenever you have a connection loss, your four devices will
> disconnect in a slightly different order.
> consistency of your stripe set cannot be guaranteed.
>
>


Thanks for trudging though this again, Is this on "network urrp,
followed by full box failure"? I saw your earlier post in the
archives about disconnection at  slightly different times and
somehow I read into it more than two nodes (drbd+ or something).
You mean some writes will complete on  /dev/drbd3 but not on
/dev/drbd1 (as an example). Does protocol C help this at all?


Based on my own instinct and readings in this archive, RAID1+0
(striping on top of drbd) jsut seems more "right'. because it could
withstand more single drive failures (as long as no single drbd pair
went out) and drbd re-syncing from failure would be faster. That is,
when I replace a down drive, drbd only has to sync from the pair vs.
re-syncing the entire stripe set from the other side. I thought I
read that "md" would have problems but that LVM2 or cLVM would be
ok. It appears I just wanted it to be so, but it is not.



> I also think you'd get better performance out of drbd on top of raid.
>

I suppose that's because we're sending less "writes" across the wire?
That is drbd is sending a 16K write instead of 4 4K stripe chunks
with all their acks?

So RAID0+1 then. Any difference using LVM striping vs md? I suppose
There's no problem with running md on each node because as far as
each md knows, there's just data coming from drbd. It's ignorant of
the other side? Am I right in saying that in this situation if a
drive  goes down, the whole local stripe set goes down and I'm doing
all I/O across the network? Then when I replace one drive, doesn't
the whole stripe set (all 4 drives worth of data) have to be synced
across the network? I guess I'd like to hear from people that run
md or lvm underneath drbd. When I searched the archives, I filtered
these out, looking specifially for the other case.

In this RADI0+1 case, I don't see how LVM buys me anything
underneath drbd. Wouldn't adding capacity later underneath drbd
cause problems?

On the re-sync issue, I guess I could run two separate drbd sets,
dividing my data across the two so that any resync only had 2 drives
to deal with.




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