configuration choices with Dell CERC (adaptec 2610SA)

[user]

On Sat, 26 Nov 2005, Herve Boulouis wrote:

> Le 26/11/2005  14:17, user a =E9crit:
> >=20
> > Read caching: yes/no (default is yes)
> >=20
> > (read caching does not rely on a battery, and is completely "safe",
> > right?  I am going to set this to "yes", however I wonder - why would
> > anyone _ever_ set it to "no" ?)
> =20
> You usually want to disable read caching on all aac based adaptec raid
> controllers because it decreases performance.


Thanks - I did not know that, and that is good to know.


> > Write caching: enable/disable
> >
> > (this is the one that can get you into trouble if the system loses powe=
r
> > before a write is committed to disk, and the battery is dead, right?  I=
 am
> > going to set this to "disable" because I do not think the 2610SA has a
> > battery ... right ?)
>=20
> If your server is plugged on a UPS you can safely enable it, it's quite
> a performance gain.


Ok.  It is.  However, my experience has been that no matter how rock solid
the datacenter provided, UPS backed power is, there is a power
interruption about every 1-2 years (and this experience comes from very
very good datacenters - from Level3 to UUnet, etc.) ... so ... let's say I
bank on this and enable write caching, how big of a deal is it when that
power interruption does come ?

Assuming freebsd 6.0 and UFS2 ... is it just an fsck, or is there a
potential for some real problems if the kernel thinks something is totally
committed, but it never makes it to the drive ?


> > Rebuild rate: high/medium/low
> >=20
> > (I have no idea what this means - I never saw this in adaptec bios befo=
re
> > ... can anyone define exactly what this means ?)
>=20
> This controls how much ressources the card will put in rebuilding a volum=
e
> that got a failed disk. A high rebuild rate will mean a short rebuild tim=
e
> but slower disk accesses during the rebuild.


I am going to set it to "low" then ... my experience is that a failed disk
often rebuilds itself only to re-fail shortly thereafter, thus making
possible a continuous loop of rebuilding over and over ... which would
essentially take the system offline if all resources were dedicated to the
rebuild.  Comments ?

thanks very much.