Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
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.