Re: Tuning read ahead

On May 16, 2013, at 6:01 AM, Shaun Thomas wrote:

> On 05/15/2013 08:04 PM, Ramsey Gurley wrote:
>
>> My question: Is that advice just for the database drive, or should I
>> increase read ahead on the OS/WAL disk as well?
>
> Definitely the database drive, but it doesn't hurt to do both. It doesn't mention it in the book, but if you have a Debian or Ubuntu system, you can set it up to retain these settings through reboots very easily. The udev system can be set with rules that can target whole ranges of devices. Here's one we use:
>
> * In a file named /etc/udev/rules.d/20-pg.rules
>
> ACTION=="add|change", KERNEL=="sd[a-z]",ATTR{queue/read_ahead_kb}="4096"
>
> Our systems are also NVRAM based, so we also throw in a NOOP access scheduler:
>
> ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/scheduler}="noop"
>
> There's really no reason to do it any other way if you have udev installed. You *could* put blockdev calls in /etc/rc.local I suppose, but udev applies rules at device detection, which can be beneficial.

Interesting point. I had not considered whether the setting would be maintained through reboots. I'll have to google for the appropriate settings on Red Hat.

>> I assume both. I should ask the same for noatime advice while I'm at
>> it.
>
> You can probably get away with relatime, which is the default for most modern systems these days.

I will probably go with noatime on the data drive then. I see where that would require lots of reads and should not be writing to the drive. In my mind, WAL should be read much less frequently. Maybe I am wrong about that :-)

Thank you,

Ramsey