Thursday, November 21, 2013

EBS, the killer app for Amazon EC2

So I gave Ceph a try. I set up a Ceph cluster in my lab with three storage servers connected via 10 gigabit Ethernet. The data store on each machine is capable of approximately 300 megabytes per second streaming throughput. So I created a Ceph block device and ran streaming I/O with large writes to it. As in, 1 megabyte writes. If writing to the raw data store, I get roughly 300 megabytes per second throughput. If writing through ceph, I get roughly 30 megabytes per second throughput.

"But wait!" the folks on the Ceph mailing list said. "We promise good *aggregate* throughput, not *individual* throughput." So I created a second Ceph block device and striped it with the first. And did my write again. And got... 30 megabytes per second. Apparently Ceph aggregated all I/O coming from my virtual machine and serialized it. And this is what its limit was.

This seems to be a hard limit with Ceph if you're not logging to a SSD. And not any old SSD. One optimized for small writes. There seems to be some fundamental architectural issues with Ceph that keep it from performing at anywhere near hardware speed. The decision to log everything, regardless of whether your application needs that level of reliability, appears to be one of those decisions. So Ceph simply doesn't solve a problem that's interesting to me. My users are not going to tolerate running at 1/10th the speed they're accustomed to running, and my management is not going to appreciate me telling them that we need to buy some very pricy and expensive enterprise SSD hardware when the current hardware with the stock Linux storage stack on it runs like a scalded cat without said SSD hardware. There's no "there" there. It just won't work for me.

So that's Ceph. Looks great on paper, but the reality for me is that it's 1/10th the speed without any real benefits for me over just creating iSCSI volumes on my appliances, pointing my virtual machines at them, and then using MD RAID on the virtual machine to do replication between the iSCSI servers. Yes that's annoying to manage, but at least it works, works *fast*, and my users are happy.

At which point let's talk about Amazon EC2. Frankly, EC2 sucks. First, it isn't very elastic. I have autoscaling alerts set up to spin up new virtual machines when load on my cloud reaches a certain point. Okay, fine. That's elastic, right? But: While an image is booting up and configuring, it's using 100% CPU. Which means that your alarm goes off *again* and spin up more instances, ad infinitum, until you hit the upper limit you configured for the autoscaling group, *unless* you put hysteresis in there to wait until the instance is up before you spin up another instance. So: It took five minutes to spin up a new instance. That's not acceptable. If the load has kept going up in the meantime, that means you might never catch up. I then created a new AMI that had Tomcat and other necessary software already pre-loaded. That brought the spin up time to three minutes -- one minute for Amazon to actually create the instance, one minute for the instance to boot and run Puppet to pull in the application .war file payload from the puppetmaster, and one minute for Tomcat to actually start up. Acceptable... barely. This is elastic... if elastic is eons in computer time. The net result is to encourage you to spin up new instances before you need them, spin up *multiple* instances at a time when spinning up instances, and then take a while before tearing them back down again once load goes down. Not the best way to handle things by any means, unless you're Amazon and making money by forcing people to keep excess instances hanging around.

Then let's not talk about the fact that CloudFormation is configured via JSON. A format deliberately designed without comments for data interchange between computers, *not* for configuring application stacks. Whoever specified JSON as the configuration language needs to be taken out behind Amazon HQ and beat with a clue stick until well and bloody. XML is painful enough as a configuration language. JSON is pure torture. Waterboarding is too good for that unknown programmer.

And then there's expense. Amazon EC2 is amazingly expensive compared to somebody like, say, Digital Ocean or Linode. My little 15 virtual machine cloud would cost roughly $300/month at Linode, less at Digital Ocean. You can figure four times that amount for EC2.

So why use EC2? Well, my Ceph experiment should clue you in there: it's all about EBS, the Elastic Block Store. See, I have data that I need to store up in the cloud. A *lot* of data. And if you create EBS-optimized virtual machines and striped MD RAID arrays across multiple EBS volumes, your I/O is wicked fast. With just two EBS volumes I can easily exceed 1,000 IOPS and 100 megabytes per second when doing pg_restore of database files. With more EBS volumes I could do even better.

Digital Ocean has nothing like EBS. Linode has nothing like EBS. Rackspace and HP have something like EBS (in public beta for HP right now, so not sure how much to trust it), but they don't have a good instance size match with what I need and if you go the next size up, their pricing is even more ridiculous than Amazon's. My guess is that as OpenStack matures and budget providers adopt it, you're going to see prices come down for cloud computing and you're going to see more people providing EBS-like functionality. But right now OpenStack is chugging away furiously trying to match Amazon's feature set, and is unstable enough that only providers like HP and Linode who have hundreds of network engineers to throw at it could possibly do it right. Each iteration gets better so hopefully the next iteration will be better. (Note from 10 months later: nope. Still not ready for mere mortals). Finally, there's Microsoft's Azure. I've heard good things about it, oddly enough. But I'm still not trusting it too much for Linux hosting, given that Microsoft only recently started giving grudging support to Linux. Maybe in six months or so I'll return and look at it again. Or maybe not. We'll see.

So Amazon's cloud it is. Alas. We look at the Amazon bill every month and ask ourselves, "surely there is a better alternative?" But the answer to that question has remained the same for each of the past six months that I've asked it, and it remains the same for one reason: EBS, the Elastic Block Store.


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