![]() The seek latency is how long it takes to move the mechanical arm the disk head is mounted on to the correct track on disk. In conventional spinning rust disks, there are two major sources of latency: rotational latency, and seek latency. ![]() Most of the worst storage bottlenecks are latency issues that affect throughput, not the other way around. Where throughput refers to how many bytes of data per second you can move on or off the disk, latency-most commonly measured in milliseconds-refers to the amount of time it takes to read or write a single block. ![]() Latency is the flip side of the same performance coin. It's also worth noting that most SATA controllers won't move much more data than a single link can manage, even with many disks connected to the controller-so it's common to see even lots of very fast solid state drives in an array bottlenecking at around 700 MB/sec. You can't actually move data across the SATA or SAS bus at the full theoretical link speed, but you can get fairly close. So a SATA-3 6Gbps link can theoretically move up to 768MB/sec. You divide Gbps by 8 to get GB/sec, then multiply by 1024 to get MB/sec. That's the difference between bytes and bits. Things get a little extra complicated here, because we're mixing units-notice the big B in MB/sec, and the small b in Gbps. ![]() You're also limited by the bandwidth of your controller and cabling-for example, modern SATA links typically operate at 6Gbps, while modern SAS links can operate up to 22.5Gbps. If you've got a single head on a conventional rust disk spinning at 7200RPM, the rate you can get data on or off that disk will be limited by the number of physical sectors/blocks passing beneath the head. There are several choke points in a storage system for throughput-first and foremost, there's the speed of the physical medium itself. Throughput, measured most commonly in storage systems in MB/sec, is the most commonly used way to talk about storage performance. Throughput, latency, IOPS and cache Throughput We're going to walk you through some simple but effective uses of fio on Windows, Mac, and Linux computers-but before we do that, let's talk a little bit about storage systems from a more basic perspective. ![]() Fortunately, we don't have to invent such a tool-there's already a free and open source software tool called fio, and it's even cross-platform! So we do want an artificial benchmarking tool-but we want one that we can use intelligently to test storage systems across realistic scenarios that model our day-to-day usage well. Unfortunately, that's neither very repeatable, nor is it simple to analyze. The most realistic way to test and benchmark disks is, of course, to just use them and see what happens. Unfortunately, the actual workload of a typical disk doesn't look like that-and that "simple speed test" doesn't reproduce a lot of the bottlenecks that slow down disk access in real-world systems. Admins and enthusiasts have for decades been tempted to just "get the big number" by reading or writing a large amount of data to a disk, getting a figure in MB/sec, and calling it a day. Further Reading How Ars tests Wi-Fi gear (and you can, too)Storage benchmarking-much like Wi-Fi benchmarking-is a widely misunderstood black art. ![]()
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