When is disk latency a problem

As you move to all-flash, though, 20 ms is a lifetime. A good all flash array may have spikes to 3 or 4 milliseconds, but, in general, you should see less than that. And, in many cases, you will see latency figures in the microseconds. Violin Memory, for example, publishes an average latency value of just microseconds, which is particularly impressive. Bear in mind, though, that there may be additional latency imposed by non-storage systems.

However, beyond a most awesome breakfast, no money exchanged hands in return for this post. This entire post — good or bad — is of my own doing. Garnock12 This person is a verified professional.

Verify your account to enable IT peers to see that you are a professional. If so, how many VMs are on each physical disk, it's often writes that are the issue, if you have a VM that writes a lot, it might be bringing down the others, however to get it perfect you'd need more spindles, more LUNS or more SSDs There are a lot of factors to consider, but your post looks like a statement rather than a question at this point I guess it was a statement.

Thank you - it means a lot. Thai Pepper. Andre the Giant This person is a verified professional. We are going to need names here. A full investigation will be conducted Rod-IT Pure Capsaicin. Garnock12 Cayenne. RIAD10 or 6 should be the only options in a disk array that size. Ok - maybe, but not with your drive count, with 3, 4 or even 5 and 6 perhaps. Wish I could figure out how to do that with Veeam You dont have vMotion? Denis Kelley This person is a verified professional.

It was SAM. You dont have vMotion? No Sir. No I'm not sure. Still trying to get my head around Veeam and still learning a lot. I'm talking about outside of Veeam now. Warning: this metric is meaningless without a latency figure. We will discuss latency shortly. We state storage bandwidth mostly in Megabytes and Gigabytes per second. We start our measurement from the moment the request is issued to the storage layer and stop measuring when either we get the requested data, or get confirmation that the data is stored on disk.

Latency is the single most important metric to focus on when it comes to storage performance, under most circumstances. For hard drives, an average latency somewhere between 10 to 20 ms is considered acceptable 20 ms is the upper limit. For solid state drives, depending on the workload it should never reach higher than ms. In most cases, workloads will experience less than 1ms latency numbers.

This is a very important concept to understand. The IOPS metric is meaningless without a statement about latency.

If a storage solution can reach 10, IOPS but only at an average latency of 50 ms that could result in very bad application performance. If we want to hit an upper latency target of 10 ms the storage solution may only be capable of 2, IOPS. For more details on this topic I would recommend this blog and this blog.

An example of a sequential data transfer is copying a large file from one hard drive to another. A large number of sequential often adjacent datablocks is read from the source drive and written to another drive. Backup jobs also cause sequential access patterns. The data could be stored all over various regions on the storage media.

An example of such an access pattern is a heavy utilised database server or a virtualisation host running a lot of virtual machines all operating simultaneously. Both throughput and IOPS will plummet as compared to a sequential access pattern. In practice, most common workloads, such as running databases or virtual machines, cause random access patterns on the storage system. Having a queue is beneficial as the requests in the queue can be submitted to the storage subsystem in an optimised manner and often in parallel.

A queue improves performance at the cost of latency. If you have some kind of storage performance monitoring solution in place, a high queue depth could be an indication that the storage subsystem cannot handle the workload. You may also observe higher than normal latency figures. As long as latency figures are still within tolerable limits, there may be no problem.

Hard drives HDDs are mechanical devices that resemble a record player. This mechanical nature makes them relatively slow as compared to solid state drives which we will cover shortly. The most important thing to know about hard drives is that from a performance perspective focussing on latency higher spindle speeds reduce the average latency. That's why the IOPS figure also increases.