Date: Sun, 6 May 2001 12:06:55 +1200 From: "Juha Saarinen" <juha@saarinen.org> To: "Doug Russell" <drussell@saturn-tech.com>, "Matt Dillon" <dillon@earth.backplane.com> Cc: "freebsd-stable" <freebsd-stable@FreeBSD.ORG> Subject: RE: soft update should be default Message-ID: <KPECIILENDDLPCNIMLOFMEKECDAA.juha@saarinen.org> In-Reply-To: <Pine.BSF.4.21.0105051736310.95873-100000@beastie.saturn-tech.com>
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:: Yes, yes, yes! No argument there. People should realize what they are :: doing by enabling the write cache, softupdates or no softupdates. In my experience, very few people even know that there is such a thing as a write-cache setting on hard drives. Also, most hard drives today ship with write-caching enabled, without tools to turn it off. Someone mentioned a sysctl that can be used to turn off drive write-caching... which one is it? I asked my friend Daryl Jay, who as the tech manager of a systems integrator has had plenty of experience with hard drives in various situations, about write caching. Daryl says WCE has always been a problem; Windows 9x and ME can suffer file system corruption with IDE hard drives that have large caches, as they can become corrupted during an APM/ACPI shutdown -- MS issued a patch to for Win98SE and WinME, but not fo the older variants, that increases the delay before power-off to permit the cache contents to be written out. He also confirms that if you have a SCSI RAID controller with onboard cache, and set WCE for the drives / array as a whole, you have to have a UPS or a battery to avoid f/s corruption. However, a transaction / journalled file system such as NTFS, XFS and ReiserFS goes a long way towards mitigating the effects of incomplete cache write-outs. OTOH, FAT16/32 and Linux Ext2fs are very vulnerable in this respect. No idea how UFS w/ or w/o softupdates performs here, unfortunately. Daryl says that IBM drives are less likely to cause corruption, as they shipped with a decently-sized power capacitor which in most cases will have enough power to complete the cache write-out. This doesn't help with caching SCSI controllers, obviously. IBM used to ship all its drives with WCE set to off, and charged a fee to have the drive "AV optimised", which was just a simple procedure to turn WCE on. As of 2-3 years ago, all IBM drives are shipped with WCE on, however. The performance drop you see when you turn off WCE is substantial. Daryl and a customer tested a 36GB IBM Scorpion with and without WCE... without, they got 3.2MBps average sustained DTR across the disk, with, 17.8MBps. Just a few datapoints, in case anyone's interested... I think this should be documented in the manual. -- Juha To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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