sata raid & write cache state

[Roisin Murphy]

hi

I'm thinking of setting up a raid array with sata disks
so far this intel controller seems to be winning :) 
http://www.intel.com/design/servers/RAID/SRCS16/

the handbook says: 11.12.1.5 hw.ata.wc
FreeBSD 4.3 flirted with turning off IDE write caching. This reduced
write bandwidth to IDE disks but was considered necessary due to
serious data consistency issues introduced by hard drive vendors. The
problem is that IDE drives lie about when a write completes. With IDE
write caching turned on, IDE hard drives not only write data to disk
out of order, but will sometimes delay writing some blocks
indefinitely when under heavy disk loads. A crash or power failure may
cause serious file system corruption. FreeBSD's default was changed to
be safe. Unfortunately, the result was such a huge performance loss
that we changed write caching back to on by default after the release.
You should check the default on your system by observing the hw.ata.wc
sysctl variable. If IDE write caching is turned off, you can turn it
back on by setting the kernel variable back to 1. This must be done
from the boot loader at boot time. Attempting to do it after the
kernel boots will have no effect.

well, this is what i also heard from a friend of mine, that he has
seen too many dead ide raid setups, because of that ata command set,
that has no way to tell the state of the write cache content.

1. now, the question is, is this the same with the SATA command set?
or is sata more like scsi in this respect?

2. i haven't read much about raid controllers yet, but i would think
that with a proper hardware raid5 controller, there's no need for
write cache being enabled on the actual disks, as the controller with
its cache could optimize the disk writes. Is this so? does a proper
hardware raid controller switch the cache off on its disks?

3. Is this the case with scsi also? if the disk could fully report
write cache state, the array couldn't mess up/die like they report it
with ide raid setups, right? is the disk write cache enabled on scsi
raid5 setups?

4. with that intel SRCS16 controller, would hw.ata.wc sysctl work?
could i turn the cache off on my sata disks like that? or do i need
that manufacturer DOS floppy with utilities to turn the defaults
on/off on my disks? (ideally the controller would take care of this)

5. also that intel SRCS16 controller should support 'online capacity
expansion' < that means if i start with 3 disks, i can add more disk
if i need more storage, without having to recreate the array from
scratch?


thanks