Re: Recommended sw raid setup

["Stefan /*St0fF*/ Hübner" <stefan.huebner@stud.tu-ilmenau.de>]

Am 02.07.2011 23:30, schrieb Drew:
>> I thought that I had read the TLER was not useful for mdraid, so that
>> enterprise drives were not really advised for mdraid. Am I wrong about
>> this?
> 
> TLER just shortens the firmware's error recovery from something like
> 60 seconds down to 4 seconds. It's mainly useful in hardware RAID but
> I can see it being useful with mdraid in the enterprise where you
> can't afford to wait for the drive to do it's own recovery attempts.
> 
> 
This is not correct.  You may want to read the ATA8-ACS2 draft standard
(see www.t13.org).  There you'll find: SCT-ERC

The internal error recovery procedure includes a vast amount of
algorithms.  If a disk cannot read a sector and starts error recovery,
it may take far more than a minute.
With ERC you can tell the disk beforehand to stop processing the command
(may it be "read" or "write") and return a "uncorrectable error" to the
host.  Some drives (i.E. recent Hitachi Deskstar, which I do recommend!)
do not allow you to set ERC lower than 6.5s.

For a home-box I recommend adding a startup-script and a hotplug-script
(as ERC-settings get reset to firmware defaults upon power-on) which
uses smartmontools >=5.41 to set the read-ERC to 7s and the write ERC to
12s.  We have good experience that this works out well in productive
systems.

/Stefan
P.S.: pseudo-knowledge makes important things go really badly wrong.