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SubjectRe: ide write cache issue? [Re: Something corrupts raid5 disks slightly during reboot]
On Sun, 2 Nov 2003, Ville Herva wrote:

> On Sat, Nov 01, 2003 at 10:05:31PM -0800, you [Andre Hedrick] wrote:
> >
> > I added the flush code to flush a drive in several places but it got
> > pulled and munged.
> >
> > The original model was to flush each time a device was closed, when any
> > partition mount point was released, and called by notifier.
> >
> > In a minimal partition count of 1, you had at least two flush before
> > shutdown or reboot.
> >
> > So it was not the code because I fixed it, but then again I am retiring
> > from formal maintainership.
>
> Thanks, Andre :(.
>
> As an^Wthe IDE expert, can you clarify a few points:
>
> - How long can the unwritten data linger in the drive cache if the drive
> is otherwise idle? (Without an explicit flush and with write caching
> enabled.)

Basically forever, until a read is issued to a range of lba's which starts
smaller than the uncommitted contents's lba, and includes the content in
question. Or if a flush cache or disable write-back cache is issued.

> I had unmounted the fs an raidstopped the md minutes before the boot.

The problem imho, is a break down of fundamental cascading callers.

Unmount MD -> flush MD

MD is a fakie device :-/

MD fakie calls for flush of R_DEV's

Likewise unloading or stopping MD operations should repeat regardless of
mount or not.

> - Can this corruption happen on warmboot or only on poweroff?

Given POST (assume x86 for only a brief moment) will issue execute
diagnositics to hunt for signatures on the ribbon, that basically wacks
the content. Cool cycle obviously wacks the buffer.

> - What kind of corruption can one see the if boot takes place "too fast"
> and drive hasn't got enough time to flush its cache?

erm, I am lost with the above.
Flush Cache is a hold and wait on completion, period.
However, a cache error at this point is a wasted effort to attempt
recovery.

Not sure I helped or not ...

Cheers,

Andre

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