Understanding your RAID Configuration

For any production MySQL Database system, running RAID is a given these days. Do you know what RAID your database is? Are you sure?. Ask for quantifiable reproducible output from your systems provider or your System Administrator.

As a consultant I don’t always know the specific tools for the clients deployed H/W, but I ask the question. On more the one occasion the actual result differed from the clients’ perspective or what they were told, and twice I’ve discovered that clients when asked if their RAID was running in a degraded mode, it actually was and they didn’t know.

You can read about various benchmarks at MySQL blogs such as BigDBAHead and MySQL Performance Blog however getting first hand experience of your actually RAID configuration, the H/W and S/W variables is critical to knowing how your technology works. You can then build on this to run your own benchmarks.

Over 50% of my clients run on DELL equipment, most using local storage or shared storage options such as Dell MD1000, Dell MD3000, NetApps or EMC. I’ve had the opportunity to spend a few days looking into the more details of RAID, specifically the DELL PERC 5/i Raid Controllers, and I’ve started a few MySQL Cheatsheets for my own reference that others may also benefit from.

Understanding PERC RAID Controllers gives an overview of using the MegaCLI tools to retrieve valuable information on the Adapter, Physical Drives, Logical Drives and the all important Battery Backed Cache.

There are several Google search results out there about finding the MegaCLI tools, I found them to be all outdated. There is of course other tools including Dell OpenManage Server Administrator (GUI and CLI) and an Open Source project called megactl.

Here is just a summary of a few lines of each that yields valuable information:

Adapter Details

$ /opt/MegaRAID/MegaCli/MegaCli64 -AdpAllInfo -aALL

RAID Level Supported             : RAID0, RAID1, RAID5, RAID10, RAID50
Max Stripe Size         : 128kB
Stripe Size                    : 64kB

Physical Details

$ /opt/MegaRAID/MegaCli/MegaCli64 -LDPDInfo -aall

Adapter #0
Number of Virtual Disks: 1

RAID Level: Primary-5, Secondary-0, RAID Level Qualifier-3
Stripe Size: 64kB
Number Of Drives:4
Default Cache Policy: WriteBack, ReadAheadNone, Direct, No Write Cache if Bad BBU
Current Cache Policy: WriteBack, ReadAheadNone, Direct, No Write Cache if Bad BBU

Raw Size: 70007MB [0x88bb93a Sectors]
Inquiry Data: FUJITSU MAY2073RC       D108B363P7305KAU
Inquiry Data: FUJITSU MAY2073RC       D108B363P7305KAJ
Inquiry Data: FUJITSU MAY2073RC       D108B363P7305JSW
Inquiry Data: FUJITSU MAY2073RC       D108B363P7305KB1


$ MegaCli -AdpBbuCmd -aALL
  Fully Charged           : Yes
  Discharging             : Yes

BBU Capacity Info for Adapter: 0
Relative State of Charge: 100 %
Absolute State of charge: 88 %
Run time to empty: 65535 Min

BBU Properties for Adapter: 0

Auto Learn Period: 7776000 Sec
Next Learn time: 304978518 Sec

The big detail that was missing was the details in this ouput of the drive speed, such as 7.2K, 10K, 15K. What is the impact? Well that’s the purpose of the next step.

Following this investigation, testing of the RAID configuration with Bonnie++ was performed to determine the likely performance of various configurations, and to test RAID0, RAID1, RAID5 and RAID10.

Further testing that would be nice would include for example RAID 5 with 3 drives verses 4 drives. The speed of the drives, the performance in a degraded situation, and the performance during a disk rebuild.

This still leaves the question about how to test the performance with and without the Battery Backed Cache. You can easily disable this via CLI tools, but testing an actually database test, and pulling the power plug for example with and without would yield some interesting results. More concerning is when Dell specifically discharges the batters, and it takes like 8 hours to recharge. In your production environment you are then running in degraded mode. Disaster always happens at the worse time.


  1. Xaprb says

    I usually look at the Inquiry data for the model of the disk drive and then Google that to find out exactly what’s physically behind the card :)

  2. Peter Radavich says

    A few comments:

    The x950 hardware is now released in 2 generations: The x950mkIII and the x950. The mkIII hardware comes with the sas6/ir or perc6/ir controller. Anyone running a database on local disk will end up buying the PERC6. The PERC6 is pretty different from the PERC5.

    That said, under FreeBSD, the device changes a bit, and a new controller util is used. I’d be unsurprised to find that MegaCli is ported, but keep an eye out for a tool called mfi_control.

    Anyway, a few other comments about the PERC5/6:

    Dell recently released a firmware upgrade for the PERC5/i which fixes a problem in which a patrol read is happening under heavy io and the controller locks hard. I’ve seen this with several of my systems in production, and cant recommend this more strongly. There have been a few disk firmware upgrades released as well, but these have not affected my databases. You should let your readers know about these upgrades, though. Linux and Windows users can use the dell utilities. Other OSes can netboot DOS via ISOLinux and install them remotely, assuming they have DRAC or IPMI interfaces set up for remote access.

    Good luck!