Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
Thanks Lars, that really helped! I totally get what you're saying here now. I've sent off a request to the sales linbit folks to ask about a proxy trial. More out of experimental curiosity than anything I'd be interested to give that a shot to see what happens with our SSD/spindle combination... realistically we'll just need to go fork over the money and buy a 2nd SSD for our secondary box and call it a day. On Fri, Sep 21, 2012 at 9:39 AM, Lars Ellenberg <lars.ellenberg at linbit.com>wrote: > On Fri, Sep 21, 2012 at 07:11:53AM -0300, Andrew Eross wrote: > > Hi guys, > > > > I've been doing the pre-requisite Google research and I haven't reached a > > conclusion, so thought I'd ask here. I have an experimental pair of > > identical XenServers setup with DRBD running over a Gigabit cross-over > > cable. The only difference is that the primary has a SSD and the > secondary > > is a normal spindle drive. > > > > dd tests on the underlying hardware show: > > * the spindle server is capable of writing at ~70MB/s > > * the SSD server at ~250MB/s > > > > If I put the primary into drbd standalone mode, I also get about ~250MB/s > > when writing to the DRBD device. > > > > When running in primary/secondary mode, however, we only get around the > > ~65MB/s mark, which makes perfect sense with protocol C. > > > > I was expecting that if I switched to protocol A, I would be able to let > > the SSD drive write at it's full speed (e.g. 250MB/s) only at the price > of > > the secondary potentially falling a little bit behind, however > performance > > is almost exactly the same with protocol A, B, or C at around 60-70MB/s. > > Throughput != Latency. > > > > (thanks, ascii-art.de) > ___________ > /=//==//=/ \ > |=||==||=| | > |=||==||=|~-, | > |=||==||=|^.`;| > jgs \=\\==\\=\`=.: > `"""""""`^-,`. > `.~,' > ',~^:, > `.^;`. > ^-.~=;. > `.^.:`. > \ / > \ funnel/ > \ / > \ / > \ / > `---- pipe ---- > > > > Ok, so if that funnel is big enough for one bucket, > you can pour out one bucket quasi instantaneoulsly. > > During the time it takes you to fetch the next bucket, > the funnel asynchronously drains through the (thin) pipe. > > "Feels" like a "fat pipe", but is not. > > Now, if you fetch the new bucket faster than the funnel can drain, > you reach congestion, and you have to pour more slowly. > > Unless spilling is allowed ;-) > > > I then tried combining that with "on-congestion pull-ahead;" to see if > that > > would allow the primary to write at full speed, but still, same result. > > > > Is this simply not do-able for some reason to let the primary write at a > > faster speed than the secondary? > > For a short peak period, yes, see above. > To extend that peak period (increase the size of that funnel), > we have the drbd-proxy (contact LINBIT). > > But even with massive buffers (funnels), > the sustained mid term/long term average write rate > obviously cannot exceed the minimum bandwith within the whole system. > > -- > : Lars Ellenberg > : LINBIT | Your Way to High Availability > : DRBD/HA support and consulting http://www.linbit.com > > DRBD® and LINBIT® are registered trademarks of LINBIT, Austria. > __ > please don't Cc me, but send to list -- I'm subscribed > _______________________________________________ > drbd-user mailing list > drbd-user at lists.linbit.com > http://lists.linbit.com/mailman/listinfo/drbd-user > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.linbit.com/pipermail/drbd-user/attachments/20120921/c92c5d34/attachment.htm>