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Path: sserve!manuel.anu.edu.au!munnari.oz.au!spool.mu.edu!agate!tfs.com!tfs.com!julian
From: julian@tfs.com (Julian Elischer)
Subject: Re: Adding Swapspace ??
Message-ID: <1992Oct25.183500.12790@tfs.com>
Organization: TRW Financial Systems
References: <1992Oct18.082017.22382@fcom.cc.utah.edu> <BwLLxp.7Bt@flatlin.ka.sub.org> <1992Oct25.111525.25782@fcom.cc.utah.edu>
Date: Sun, 25 Oct 1992 18:35:00 GMT
Lines: 34
In article <1992Oct25.111525.25782@fcom.cc.utah.edu> terry@cs.weber.edu (A Wizard of Earth C) writes:
>In article <BwLLxp.7Bt@flatlin.ka.sub.org>, bad@flatlin.ka.sub.org (Christoph Badura) writes:
>|> In <1992Oct18.082017.22382@fcom.cc.utah.edu> terry@cs.weber.edu (A Wizard of Earth C) writes:
>|> >I think this was the pre 4.3 code, before the introduction of cluster groups;
>|> >in any case, the write to the disk is done through a write-through cache.
>
>|> The UNIX FS buffer cache has since it's invention been write-behind
>|> and not write-through.
>
>I tend to use these terms synonymously. When can a cache be write through
>but not write behind? To "write through" a cache requires that the data be
>placed in the cache on it's way to the disk. On the other hand, a write behind
>requires that the data be placed in the cache prior to being put on the disk.
>Is the distinction drawn on the idea that there may be a few microseconds more
>delay wherein the data is in the cache but not yet on the disk?
>
>Either situation requires the data to be in cache before it is on the disk.
>Just curious as to why you draw such a sharp distinction, the point being that
>there is negligible overhead in a cached writes for swap no matter how you
>slice the pie.
the difference is in that a write-behind cache allows the 'writer' to proceed
before the data has actually hit the device. Write behind and 'asynchronous
writing' can be considered similar. Write behind may or may not also imply
I/O re-ordering.
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