On Wed, Sep 26, 2012 at 12:54 PM, Edward Ned Harvey (opensolarisisdeadlongliveopensolaris) < opensolarisisdeadlongliveopensola...@nedharvey.com> wrote:
> Here's another one.**** > > ** ** > > Two identical servers are sitting side by side. They could be connected > to each other via anything (presently using crossover ethernet cable.) And > obviously they both connect to the regular LAN. You want to serve VM's > from at least one of them, and even if the VM's aren't fault tolerant, you > want at least the storage to be live synced. The first obvious thing to > do is simply cron a zfs send | zfs receive at a very frequent interval. But > there are a lot of downsides to that - besides the fact that you have to > settle for some granularity, you also have a script on one system that will > clobber the other system. So in the event of a failure, you might > promote the backup into production, and you have to be careful not to let > it get clobbered when the main server comes up again.**** > > ** ** > > I like much better, the idea of using a zfs mirror between the two > systems. Even if it comes with a performance penalty, as a result of > bottlenecking the storage onto Ethernet. But there are several ways to > possibly do that, and I'm wondering which will be best.**** > > ** ** > > Option 1: Each system creates a big zpool of the local storage. Then, > create a zvol within the zpool, and export it iscsi to the other system. Now > both systems can see a local zvol, and a remote zvol, which it can use to > create a zpool mirror. The reasons I don't like this idea are because > it's a zpool within a zpool, including the double-checksumming and > everything. But the double-checksumming isn't such a concern to me - I'm > mostly afraid some horrible performance or reliability problem might be > resultant. Naturally, you would only zpool import the nested zpool on > one system. The other system would basically just ignore it. But in the > event of a primary failure, you could force import the nested zpool on > the secondary system.**** > > ** ** > > Option 2: At present, both systems are using local mirroring ,3 mirror > pairs of 6 disks. I could break these mirrors, and export one side over > to the other system... And vice versa. So neither server will be doing > local mirroring; they will both be mirroring across iscsi to targets on > the other host. Once again, each zpool will only be imported on one > host, but in the event of a failure, you could force import it on the other > host.**** > > ** ** > > Can anybody think of a reason why Option 2 would be stupid, or can you > think of a better solution?**** > > > I would suggest if you're doing a crossover between systems, you use infiniband rather than ethernet. You can eBay a 40Gb IB card for under $300. Quite frankly the performance issues should become almost a non-factor at that point. --Tim
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