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Date:      Thu, 23 Mar 2000 17:44:38 -0600
From:      Dan Nelson <dnelson@emsphone.com>
To:        Greg Lehey <grog@lemis.com>
Cc:        Poul-Henning Kamp <phk@critter.freebsd.dk>, Alfred Perlstein <bright@wintelcom.net>, Matthew Dillon <dillon@apollo.backplane.com>, current@FreeBSD.ORG
Subject:   Re: patches for test / review
Message-ID:  <20000323174438.B59166@dan.emsphone.com>
In-Reply-To: <20000323152718.C9318@mojave.worldwide.lemis.com>; from "Greg Lehey" on Thu Mar 23 15:27:18 GMT 2000
References:  <20000320115902.C14789@fw.wintelcom.net> <20211.953581241@critter.freebsd.dk> <20000320152330.A48212@dan.emsphone.com> <20000323152718.C9318@mojave.worldwide.lemis.com>

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In the last episode (Mar 23), Greg Lehey said:
> 
> Agreed.  This is on the Vinum wishlist, but it comes at the expense of
> reliability (how long do you wait to cluster?  What happens if the
> system fails in between?).  In addition, for Vinum it needs to be done
> before entering the hardware driver.

For the simplest case, you can choose to optimize only when the user
sends a single huge write().  That way you don't have to worry about
caching dirty pages in vinum.  This is basically what the hardware
RAIDs I have do.  They'll only do the write optimization (they call it
"pipelining") if you actually send a single SCSI write request large
enough to span all the disks.  I don't know what would be required to
get our kernel to even be able to write blocks this big (what's the
upper limit on MAXPHYS)?

-- 
	Dan Nelson
	dnelson@emsphone.com


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