[geeks] IBM rant

[Jonathan C. Patschke]

On Thu, 30 Nov 2006, Bryan Fullerton wrote:

> Yeah, it'd be nice to have real hardware RAID mirroring for our
> applications, just for redundancy. From my understanding, HostRAID is
> kindof like a winmodem -- most of the functions are actually
> implemented in the driver, with a bit of hardware assist.

Almost all SATA "RAID" systems work this way.  3Ware makes actual
hardware RAID devices, and every one of the SAS RAID systems I've seen
are true hardware RAID, but Promise, Silicon Image, Intel, Highpoint,
NVIDIA, and friends all push the heavy-lifting crap onto the OS for
their SATA offerings.

The hardware uses a small portion of the disk as a scratchpad for
metadata (member ID, metadevice layout, etc.) so that it can rebuild the
mirror or mark the stripe as bad, but that's about the limit of the
hardware's involvement.  Everything else happens in the driver.

One of the biggest downsides of this is that your I/O performance on a
mirror -halves- for writes, since the write has to be issued twice from
the driver, rather than being...err...mirrored at the controller.

-- 
Jonathan Patschke  ) "Some people grow out of the petty theft of
Elgin, TX         (   childhood.  Others grow up to be CEOs and
USA                )  politicians."              --Forrest Black