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SubjectRe: Should raw I/O be added to the kernel?
Hi,

On 21 Jan 1999 23:38:15 -0500, uixjjji1@umail.furryterror.org (Zygo
Blaxell) said:

>> Yes you can. The way these applications work is to write all of the

> No you can't. Suppose you send a bunch of raw writes to a SCSI disk
> drive. OK, so the SCSI disk drive queues them in its embedded cache
> RAM and tells the host CPU to send more data. Then the power fails
> before the SCSI drive can flush its embedded cache.

> Oops.

Fine. If your disk hardware tells the host that it has completed an
update to oxide and the update is still volatile, you have a broken
disk: send it back. No enterprise-class databases support such
hardware. For writeback caching to be supported, the cache _must_ be
battery-backed or (in advanced multi-controller redundant storage
cabinets) multipowered and multiported. That is a fundamental storage
architecture issue, nothing at all to do with the host O/S.

> There's the possibility of external RAID devices that will undo all that
> work for you by doing buffering and ACK's by themselves, then turning
> around to talk to disks with data in cache.

Not on ANY decent storage systems.

--Stephen

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