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Subject: RE: Disk write cache (Was: Hyper-Threading Vulnerability)
Date: Wed, 18 May 2005 17:45:23 +0800
Message-ID: <26A66BC731DAB741837AF6B2E29C10171E5590@xmb-hkg-413.apac.cisco.com>
Thread-Topic: Disk write cache (Was: Hyper-Threading Vulnerability)
Thread-Index: AcVbX/qJtcC50EGCS8qqNYFr+wUDZgALbTYw
From: "Lincoln Dale \(ltd\)" <ltd@cisco.com>
To: "Eric D. Mudama" <edmudama@gmail.com>, "Robert Hancock" <hancockr@shaw.ca>
Cc: "linux-kernel" <linux-kernel@vger.kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1905
Lines: 46

Eric,

> On 5/16/05, Robert Hancock <hancockr@shaw.ca> wrote:
> > If the power to the drive is truly just cut, then this is basically 
> > what will happen. However, I have heard, for what it's 
> worth, that in 
> > many cases if you pull the AC power from a typical PC, the 
> Power Good 
> > signal from the PSU will be de-asserted, which triggers the 
> Reset line 
> > on all the buses, which triggers the ATA reset line, which triggers 
> > the drive to finish writing out the sector it is doing. There is 
> > likely enough capacitance in the power supply to do that 
> before the voltage drops off.
> 
> Yes, but as you said this isn't a power loss event.  It is a 
> hard reset with a full write cache, which all drives on the 
> market today respond to by flushing the cache.
> 
> According to the spec the time to flush can exceed 30s, so 
> your PSU better have some honkin caps on it to ensure data 
> integrity when you yank the power cord out of the wall.

why don't drive vendors create firmware which reserved a cache-sized
(e.g. 2MB) hole of internal drive space somewhere for such an event, and
a "cache flush caused by hard-reset" simply caused it to write the cache
to a fixed (contiguous) area of disk.

the same drive firmware on power-on could check that area and 'write
back' the data to the correct locations.

all said and done, why wouldn't a vendor (lets just say "Maxtor" :) )
implement something like this and market it as a feature?
i'd happily spend a few extra bucks for something that given a modern
PSU providing a few Hz of power (e.g. 50msec) provided higher data
reliability in case of power failure..


cheers,

lincoln.
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