Date: Thu, 18 Oct 2001 10:15:16 -0500 (CDT) From: Chris Dillon <cdillon@wolves.k12.mo.us> To: <carter@pair.com> Cc: <freebsd-hardware@FreeBSD.ORG> Subject: Re: soft updates, write cache Message-ID: <Pine.BSF.4.32.0110181003130.93846-100000@mail.wolves.k12.mo.us> In-Reply-To: <20011017180318.66984.qmail@smx.pair.com>
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On Wed, 17 Oct 2001 carter@pair.com wrote: > >From what I understand, softupdates allows the filesystem to > always be in a 'clean' state, so that fsck is not necessary after > a crash except to reclaim free space from deleted files. If > softupdates depends on the files having actually been written, > their being stored in a hardware write cache can lead to the OS > believing a filesystem is clean when it is not. As well, I have > seen reference to IDE devices lieing about if their cache has been > written or not. I'm no expert, but.... The problem is not that data in the cache might not make it to disk, it is that it might make it to disk out of order. A write cache is OK as long as it is always FIFO (unfortunately most drives try to reorder whatever is in their cache for optimal performance). I can't imagine any case where this would not be true when you are using a single instance of non-volatile storage or if all instances are completely independant of each other. There could be problems if you depend on two or more pieces of non-volatile storage to be "syncronized" in some way where a volatile write cache on any of the devices could pose a problem, such as if you kept metadata on one drive and data on another. -- Chris Dillon - cdillon@wolves.k12.mo.us - cdillon@inter-linc.net FreeBSD: The fastest and most stable server OS on the planet - Available for IA32 (Intel x86) and Alpha architectures - IA64, PowerPC, UltraSPARC, and ARM architectures under development - http://www.freebsd.org To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hardware" in the body of the message
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