Date: Mon, 08 Jul 2002 16:37:21 -0700 From: Peter Wemm <peter@wemm.org> To: Julian Elischer <julian@elischer.org> Cc: John Nielsen <hackers@jnielsen.net>, hackers@FreeBSD.ORG Subject: Re: offtopic: low level format of IDE drive. Message-ID: <20020708233721.DC2C33808@overcee.wemm.org> In-Reply-To: <Pine.BSF.4.21.0207081422100.29644-100000@InterJet.elischer.org>
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Julian Elischer wrote: > this is not a 'reformat' > > what I want to do is an old-fashionned refomat/verify where the controller > writes new track headers etc. The thing is, just about all IDE drives more than a few GB or so do 'track writing' and have no fixed sectoring or sector positioning. ie: each time you write a single sector to a track, it does a read-modify-write of *THE ENTIRE TRACK*. This is why we have to have write caching turned on for IDE drives to get decent performance. Without it, it essentially rewrites the entire track over and over and over again because it cannot fill its write buffer in order to write a contiguous block to completely replace what was there before. ie: each track is one giant physical sector with multiple logical sectors inside it. The really annoying thing is that most newer scsi drives do this too. The sad thing is that this makes softdep almost completely useless, because the basic assumption is that sectors that were not explicitly written to will not be touched. The problem is that this isn't the case, even with write caching turned off. Writing a single sector causes the drive to completely rebuild the track and all the sectors on it... in a different relative postition to what was there before. The resulting power off midwrite can cause an absolute mess in sectors *adjacent* to where soft updates was carefully writing to. This means that the 'power off failsafe' file system idea isn't possible with these drives. The only thing that can deal with this sort of failure mode is being willing to resort to 'newfs and restore' or a log structured file system (can you say LFS?). Get a UPS if you value the data. :-] Back to the topic for a moment.. In theory, dd if=/dev/zero of=disk bs=64k is as good as it gets for a low level format, on these drive. With write caching turned on, you are causing every single bit on the disk to be written to, including the metadata. And the dd if=/dev/disk of=/dev/null is the read verify. Some drives can have write verify turned on (I know of certain maxtor models) where the drive will read back the data and rewrite the entire track if necessary. Take the above with a grain of salt, I've never actually worked at a drive manufacturer. The only thing for sure is that all hard drives suck. :-) Cheers, -Peter -- Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com "All of this is for nothing if we don't go to the stars" - JMS/B5 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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