Date: Sun, 22 Nov 1998 15:54:12 +1030 From: Greg Lehey <grog@lemis.com> To: Stormy Henderson <stormy@futuresouth.com> Cc: FreeBSD Questions <questions@FreeBSD.ORG> Subject: Re: How to use ccd Message-ID: <19981122155412.H1005@freebie.lemis.com> In-Reply-To: <19981121190559.D303@futuresouth.com>; from Stormy Henderson on Sat, Nov 21, 1998 at 07:05:59PM -0600 References: <Pine.BSF.4.05.9811220244590.3324-100000@chain.freebsd.os.org.za> <19981121190559.D303@futuresouth.com>
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On Saturday, 21 November 1998 at 19:05:59 -0600, Stormy Henderson wrote: > A happy camper (Khetan Gajjar, khetan@chain.freebsd.os.org.za) once wrote... >> I've got two SCSI drives I'd like to make into one partition, in order >> to write CD-ROM 660+ MB images to. The drives are not identical. DMesg >> output is listed below. What would the correct interleave number be for >> this scenario ? >> da0: <QUANTUM LPS540S 5900> Fixed Direct Access SCSI2 device >> da0: 516MB (1057616 512 byte sectors: 64H 32S/T 516C) >> da1: <CONNER CFA540S 14B1> Fixed Direct Access SCSI2 device >> da1: 515MB (1056708 512 byte sectors: 64H 32S/T 515C) > > I'm a newbie, but I just did this two days ago, and it worked for me. > > The manpage for ccd says, "For large writes, the optimum interleave factor > is typically the size of a track, while for large reads, it is about a > quarter of a track." Since you won't need as much speed on reads when > writing CDS, use a quarter. Both of your drives are 32 sectors per track > with 512 byte sectors, so try an interleave of 8. The man page is wrong. FreeBSD writes blocks of between 1 and 120 sectors (512 bytes to 60 kB). A typical size is somewhere round 6 to 8 kB. If you have an interleave of 8 sectors, you will cause nearly *all* requests to break down into at least two requests, possibly up to 16. This will cause a significant loss in performance. The theoretical gain by performing the transfers in parallel almost never occurs, because the latency for each transfer usually costs more than the save in transfer time. With a striped organization, you can never guarantee that an I/O can be satisfied from one disk only, but if you have a stripe size noticeably larger than the maximum transfer size, it will minimize the effect. That's why, after some testing, I've come to recommend 256 to 512 kB (512 to 1024 sectors) as a stripe size. Greg -- See complete headers for address, home page and phone numbers finger grog@lemis.com for PGP public key To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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