Date: Fri, 26 Oct 2001 20:09:36 -0700 (PDT) From: Matthew Jacob <mjacob@feral.com> To: Greg Lehey <grog@FreeBSD.org> Cc: Doug Rabson <dfr@nlsystems.com>, Luigi Rizzo <rizzo@aciri.org>, John Baldwin <jhb@FreeBSD.org>, Jonathan Lemon <jlemon@FreeBSD.org>, cvs-all@FreeBSD.org, cvs-committers@FreeBSD.org Subject: Re: RAID-5 parity calculations (was: cvs commit: src/sys/dev/fxp if_fx) Message-ID: <Pine.BSF.4.21.0110262009050.12956-100000@beppo> In-Reply-To: <20011027123750.D8085@wantadilla.lemis.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sat, 27 Oct 2001, Greg Lehey wrote: > On Friday, 26 October 2001 at 19:46:27 -0700, Matt Jacob wrote: > >>> I guess the real question is: 'can you get the parity calculations > >>> done in time so that the entire stripe can go out together'. > >> > >> Why? > > > > Because it's best if you can push stripes in some configurations out > > together- essentially a software spindle sync. > > I don't see any advantage in this. Spindle sync is good for RAID-3, > but nobody I know uses that on general-purpose machines. It also has > the bad disadvantage that it implies that you always transfer complete > bands, which is bad both for throughput and for memory requirements. Umm.. > > On Friday, 26 October 2001 at 19:49:26 -0700, Matt Jacob wrote: > >>> Instantaneously? Well, how long does it take to checksum 6 kB (the > >>> average request size)? That's 1500 words, say 10000 instructions. At > >>> 1 GHz, that's 10 µs, which is completely negligible compared to the > >>> minimum four I/O transactions needed, which add up to about 25 ms. > >>> > > > > BTW- for the Convex in question, the average request size was 64K. > > Yes, that's the "always transfer whole bands" syndrome. It makes > people choose artificially small stripes, but it does simplify the > calculations. To get an idea how horrendous they are, look at > vinumraid5.c, specifically bre5(). will do To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe cvs-all" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.BSF.4.21.0110262009050.12956-100000>