Slow and predictable vs. fast and bursty

ayende Blog

This is a tale of two options that we took for an exhaustive test. Amazon recently came out with a new disk type on the cloud. As a database vendor, that is of immediate interest to me, so we took a deep look into that.

GP3 disks are about 20% cheaper than their GP2 equivalent. What is more, they come with a guarantee level of performance even before you purchase additional IOPS. Consider the following two disks:

Size IOPS MB/SPrice
GP2 512GB 1,536 25051.2 USD
GB3 512GB 3,000 12540.9 USD
GB3512GB4,075 25051.2 USD

In other words, for the same disk, we can get a much better baseline performance at a cheaper price. What isn’t there not to like?

The major difference between GP2 and GP3, however, is their latency. In practice, we see an additional 1 – 2 milliseconds in response times from the GP3 disk vs. the GP2 disk. In other words, GP3 disks are somewhat slower, even if they are able to run more IOPS, their latency is higher.

A really key observation from us, however, is that GP3 does not offer burst I/O capabilities. And that means that I can breath a huge sigh of relief.

RavenDB as a database is meant to run on anything from an SD card to HDD to SSD to NVMe drives. We are used to account for the I/O being the slowest thing around and have already mostly coded around that. An additional millisecond in disk latency doesn’t matter that much in the grand scheme of things.

However… the fact that this doesn’t provide I/O burst is a huge plus for us. RavenDB can easily deal with slow I/O, what it find it very hard to deal with is an environment that very rapidly change its operational characteristics.

Let’s assume that we have a 100 GB GP2 disk, which means that we have a baseline of around 300 IOPS and 75MB / sec of throughput. RavenDB is under some high load, and it is using the maximum capabilities of the hardware. However, because of burstiness, we are actually able to utilize 3,000 IOPS and 250MB/sec for a while.

Until all the I/O credits are gone and we are forced into a screeching halt. That means, for example, that we read from the network at a rate of 250MB/sec, but we are unable to write to the disk at this level. There is a negative balance of 125MB/sec that needs to be stored some where. We can buffer that in memory, of course, but that only work for so long. That means that we have to put a huge break all of a sudden, which the rest of the eco system isn’t happy with. For example, the other side that is sending us data at 250MB /sec, they are likely not going to be able to respond in time to the shift is our behavior. It is very likely that the network connection would congest and break in this case.

All of the internal optimizations inside of RavenDB will also be skewed for a while, until we are used to the new level of speed. If this was gradual, we could adjust a lot more easily, but this is basically like hitting the brakes at speed. You will slow down, sure, but you are also likely to cause an accident.

As a simple example, RavenDB can compress the data that it writes to disk, and it balances the compression ratio vs. the cost to write to the disk. If we know that the disk is slow, we can spend more time trying to reduce the amount of data we write. If this changes rapidly, we are operating under the old assumptions and may create a true traffic jam

The fact that GP3 disks have a predictable performance profile means that we are much better suited to run on them. A more predictable platform from which to operate gives me a much better opportunity to handle optimizations.

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