Date: Wed, 19 Feb 1997 11:22:39 +0000 From: James Mansion <james@wgold.demon.co.uk> To: hackers@FreeBSD.ORG Subject: [Fwd: Failed mail: unknown user] Message-ID: <330AE27F.7159@wgold.demon.co.uk>
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(Damn, finger trouble)
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To: "james@wgold.demon.co.uk" <james@wgold.demon.co.uk>
Subject: Failed mail: unknown user
Date: Wed, 19 Feb 1997 11:21:09 +0000
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Message-ID: <330AE225.77E7@wgold.demon.co.uk>
Date: Wed, 19 Feb 1997 11:21:09 +0000
From: James Mansion <james@wgold.demon.co.uk>
Organization: Westongold Ltd
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To: hackers:FreeBSD.ORG
Subject: Disk flush
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X-Info: Westongold Ltd: +44 1992 620025 www.westongold.com
I'm trying to build a transactional data manager.
Assume that I'm using a raw device and I set it up so that
I flush through the caches. (Yeah, this is imprecise, but
the precise details are not important, and besides are
different for some platforms)
My question (admitedly not totally BSD related):
- if the OS tells me it has a successful write through the
disk device, can I rely on:
a) the data is on the platter
b) the data is in the drives buffer and may be on the
platter
- if the system crashes during the write (literally, while
the drive is writing to the platter) can I:
a) rely on a sector being there, or not (I doubt this on most
hardware)
b) rely on the data that was not yet written (perhaps a part-
sector) being uncorrupted?
My plan is to flush everyting out, then perform a second write
in which just one bit is changed - and I'm hoping that this single
change will be atomic.
Anyone got any insights?
James
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