Date: Thu, 6 Dec 2001 18:47:49 -0600 From: Vikram Kulkarni <vkulkarn@brownforces.org> To: BSD Freak <bsd-freak@mbox.com.au> Cc: freebsd-questions@freebsd.org Subject: Re: Journaling File Systems and Soft Updates confusion Message-ID: <20011206184749.H17472@padu.brownforces.org> In-Reply-To: <63ba6e639af8.639af863ba6e@mbox.com.au>; from bsd-freak@mbox.com.au on Fri, Dec 07, 2001 at 10:26:19AM %2B1100 References: <63ba6e639af8.639af863ba6e@mbox.com.au>
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On Fri, Dec 07, 2001 at 10:26:19AM +1100, BSD Freak wrote: > > The whole point of a journaling file system on systems such as > Linux/NT/Solaris etc. is not to increase performance but rather to > avoid an 'fsck' when the server gets shutdown "uncleanly". An fsck is needed when a filesystem is remounted after being uncleanly unmounted. The purpose of the fsck is to ensure that all of the filesystem data structures are in a consistant state. Journaling provides a way to quickly restore a filesystem to a consistant state on mount by maintaining an on disk log (the journal) of filesystem data structure updates (often refered to as meta-data changes). Before any operation modifies filesystem data structures is started, it is entered into the journal. Then all of the data structures are modified. Then the entry is removed from the journal. If this process is interupted at any point by a forced unmounting of the filesystem, the filesystem can be restored to a consistant state by 'replaying' the journal. SoftUpdates work by ensureing that the filesystem is never in an inconsistant state (it can also provide performance boosts) to begin with. All operations that modify filesystem data structures are reordered, and the dependencys tracked, in a way that ensures that the file system is always in a consistant state. (Yes gurus, I've glossed over things a bit...) Read this paper for a detailed explination, and analysis: http://www.usenix.org/publications/library/proceedings/usenix2000/general/full_papers/seltzer/seltzer_html/index.html -Vik -- vikram kulkarni There is probably no more terrible instant vkulkarn@uiuc.edu of enlightenment than the one in which you vkulkarn@brownforces.org discover your father is a man -- with human flesh. -Paul Muad'Dib To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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