Monitoring SSD Performance::www.brentozar.com

https://www.brentozar.com/archive/2013/05/monitoring-ssd-performance/

May 16, 2013Jeremiah PeschkaSQL Server, Storage2 Comments

Everyone wants to make sure they’re getting the best performance out of their solid state storage. If you’re like a lot of people, you want to make sure you’re getting what you paid for, but how do you know for sure that the drive is performing well?

Watch that Average

The first way to monitor performance it to use some perfmon counters. Although there are a lot of perfmon counters that seem helpful, we’re only going to look at two:

• PhysicalDisk\Avg. Disk Sec/Read
• PhysicalDisk\Avg. Disk Sec/Write

As soon as you get a solid state drive in your server, start monitoring these numbers. Over time you’ll be able to trend performance over time and watch for poor performance. When the SSDs pass out of your valid performance guidelines (and they probably will), you can pull them out of the storage one at a time and reformat them before adding them back into the RAID array. Note it isn’t necessary to do this

Although it’s risky, this approach can work well for detecting performance problems while they’re happening. The downside is that we don’t have any idea that the drives are about to fail – we can only observe the side effects of writing to the SSDs. As SSD health gets worse, this average is going to trend upwards. Of course, you could also be doing something incredibly dumb with your hardware, so we can’t really use average performance as a potential indicator of impending hardware failure.

Which SMART Attributes Work for SSDs?

What if we could watch SSD wear in real time? It turns out that we’ve been able to do this for a while. Many vendors offer SMART status codes to return detailed information about the status of the drive. Rotational drives can tell you how hot the drive is, provide bad sector counts, and a host of other information about drive health.

SSDs are opaque, right? Think again.

SSD vendors started putting information in SMART counters to give users a better idea of SSD performance, wear, and overall health. Although the SMART counters will vary from vendor to vendor (based on the disk controller), Intel publish documentation on the counters available with their SSDs – check out the “SMART Attributes” section of theIntel 910 documentation. These are pretty esoteric documents, you wouldn’t want to have to parse that information yourself. Thankfully, there are easier ways to get to this information; we’ll get to that in a minute.

Which SMART Attributes Should I Watch?

There are a few things to watch in the SMART status of your SSDS:

• Write Amplification
• Media Wear-out Indicator
• Available Reserved Space

Write Amplification, roughly, is a measure of the ratio of writes issued by your OS compared to the number of writes performed by the SSD. A lower score is better – this can even drop below 1 when the SSD is able to compress your data. Although the Write Amplification doesn’t help you monitor drive health directly, it provides a view of how your use pattern will change the SSD’s lifespan.

The Media Wear-Out Indicator gives us a scale from 100 to 0 of the remaining flash memory life. This starts at 100 and drifts toward 0. It’s important to note that your drive will keep functioning after Media Wear-Out Indicator reports 0. This is, however, a good value to watch.

Available Reserved Space measures the original spare capacity in the drive. SSD vendors provide additional storage capacity to make sure wear leveling and garbage collection can happen appropriately. Like Media Wear-Out Indicator, this starts at 100 and will drift toward 0 over time.

It’s worth noting that each drive can supply additional information. The Intel 910 also monitors battery backup failure and provides two reserved space monitors – one at 10% reserved space available and a second at 1% reserved space available. If you’re going to monitor the SMART attributes of your SSDs, it’s worth doing a quick search to find out what your SSD controllers support.

How do I Watch the SMART Attributes of my SSD?

This is where things could get ugly. Thankfully, we’ve got smartmontools. There are two pieces of smartmontools and we’re only interested in one: smartctl. Smartctl is a utility to view the SMART attributes of a drive. On my (OS X) laptop, I can run smartctl -a disk1 to view the SMART attributes of the drive. On Windows you can either use the drive letter for a basic disk, like this:

smartctl -a X:

Things get trickier, though, for certain PCI-Express SSDs. Many of these drives, the Intel 910 included, present one physical disk per controller on the PCI-Express card. In the case of the Intel 910, there are four. In these scenarios you’ll need to look at each controller’s storage individually. Even if you have configured a larger storage volume using Windows RAID, you can still read the SMART attributes by looking at the physical devices underneath the logical disk.

The first step is to get a list of physical devices using WMI:

wmic diskdrive list brief

The physical device name will be in the DeviceID column. Once you have the physical device name, you can view the SMART attributes with smartctl like this:

smartctl -a /dev/pd0 -q noserial

Run against my virtual machine, it looks like this:

C:\Windows\system32> smartctl -a /dev/pd0 -q noserial
smartctl 6.1 2013-03-16 r3800 [x86_64-w64-mingw32-win8] (sf-6.1-1)
Copyright (C) 2002-13, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Device Model:     Windows 8-0 SSD
Serial Number:    0RETRD4FE6AMF823QE7R
Firmware Version: F.2FKG1C
User Capacity:    68,719,476,736 bytes [68.7 GB]
Sector Size:      512 bytes logical/physical
Rotation Rate:    Solid State Device
Device is:        Not in smartctl database [for details use: -P showall]
ATA Version is:   ATA8-ACS, ATA/ATAPI-5 T13/1321D revision 1
SATA Version is:  SATA 2.6, 3.0 Gb/s
Local Time is:    Sat Apr 27 08:35:03 2013 PDT
SMART support is: Unavailable - device lacks SMART capability.

Unsurprisingly, my virtual drive doesn’t display much information. But a real drive looks something like this:

Intel 910 smartctl output

Holy cow, that’s a lot of information. The Intel 910 clearly has a lot going on. There are two important criteria to watch, simply because they can mean the difference between a successful warranty claim and an unsuccessful one

• SS Media used endurance indicator
• Current Drive Temperature

The Intel 910 actually provides more information via SMART, but to get to it, we have to use Intel’s command line tools. By using the included isdct.exe, we can get some very helpful information about battery backup failure (yup, your SSD is protected by a battery), reserve space in the SSD, and the drive wear indicator. Battery backup failure is a simple boolean value – 0 for working and 1 for failure. The other numbers are stored internally as a hexadecimal number, but the isdct.exe program translates them from hex to decimal. These numbers start at zero and work toward 100.

If you’re enterprising, you can take a look at the vendor specification and figure out how to read this data in the SMART payload. Or, if you’re truly lazy, you can parse the text coming out of smartcl or isdct (or the appropriate vendor tool) and use that to fuel your reports. Some monitoring packages even include all SMART counters by default.

The Bad News

The bad news is that if you’re using a hardware RAID controller, you may not be able to see any of the SMART attributes of your SSDs. If you can’t get accurate readings from the drives and you’ll have to resort to using the Performance Monitor counters I mentioned at the beginning of the article. RAID controllers that support smartmontools are listed in the smartctl documentation.

Special thanks go out to a helpful friend who let us abuse their QA Intel 910 cards for a little while in order to get these screenshots.