When people buy SANs, it’s often quite a large investment. Whether it’s all SSD, all flash, or there are tiers of storage that different types of data live on, those disks aren’t cheap. When people visualize their SAN, it’s usually just the server and the pool of drives.
But there’s some important stuff in between the Server and the SAN — SAN doesn’t stand for Storage Abstraction Nerglefwomp.
It’s Storage Area Network, and the “Network” part is what causes a lot of problems.
Those wires, especially when they’re attached to a VM Host, have to manage traffic for a lot of different things at once.
It’s really easy to overwhelm even Fibre Channel wiring, when it’s either single-pathed, or when enough concurrent activity hits a multi-pathed setup.
Round The Way SAN
Talk to most people setting up a new SAN, and they’re (hopefully) going to be using 8-10GFC (or Ethernet).
But if you’re moving hundreds of gigs, or terabytes across those wires, you better be darn sure you’ve got plenty of bandwidth. Let’s take a best case scenario, where you’re moving 1 GB across 10 GFC unhindered. It’ll take 800 milliseconds.
But there are 1000 GB in 1 TB, which’ll take around 13 MINUTES.
Let’s use my new hard drive as an example. I’ve got a VM with SQL Server running on there.
I’m doing around the 4GFC mark for sequential reads and writes.
But if I run a workload that generates a lot of I/O for various reasons, and re-run the benchmark, things tank significantly.
Sharing Ain’t Caring
When multiple requests all had to go across my USB 3 connection, my speeds got cut in half, or much worse.
Now take some time to think about how many things you’re asking your storage networking to handle concurrently.
- Think about when you have stuff like backups, checkdb, and index maintenance scheduled.
- Think about when you’re doing ETL, or any bulk data activity.
- Think about if you’ve got AGs or Mirroring set up.
When you ask the SAN admin to take a look at the storage dashboard to see why things are slow, they’re not gonna see a blip. Data simply isn’t arriving at the disks fast enough for them to be burdened.
If you had a bar with 100 people in it, 1 waiter, and 10 bartenders, that one waiter wouldn’t be able to take orders to the bartenders fast enough to keep them busy. Your bartenders would look bored, but your waiter would be a wreck.
What’s The Name Of His Other Leg?
I might even be bar tending.
Thanks for reading!