Date: Thu, 08 Apr 1999 20:33:57 +0100 (BST) From: Duncan Barclay <dmlb@ragnet.demon.co.uk> To: Alfred Perlstein <bright@rush.net> Cc: hackers@FreeBSD.ORG, Peter Jeremy <peter.jeremy@auss2.alcatel.com.au> Subject: Re: Volume managers Message-ID: <XFMail.990408203357.dmlb@computer.my.domain> In-Reply-To: <Pine.BSF.3.96.990408125020.4169i-100000@cygnus.rush.net>
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On Thu, 8 Apr 1999, Peter Jeremy wrote: > Whilst we're all making up wishlists for things we'd like to see > (someone else implement): The `clone filesystem' command supported by > Digital UNIX ADVfs is _very_ nice for `point-in-time' backups. > (Basically clonefset makes a read-only snapshot of the filesystem - > changes to the `active' filesystem are done using copy-on-write). > > Unfortunately, I suspect it couldn't be implemented within UFS > (because UFS relies on blocks being at particular physical locations > within a CG - ie superblock, array of inode blocks, data blocks - > which would make creating the copy-on-write blocks difficult). Well NetApps file system does a similar thing and it is based on UFS. The main trick they use is similar to that described in http://www.pdos.lcs.mit.edu/papers/cffs.html where inodes are allowed to "float" - the above paper tries to group inodes near directories so there isn't a seek between reading a directory and reading the inode and there is a high probability that the read-ahead will get the directory and inode together. It seems to work well for directories with a number of small files in them. Once you have removed the inode position constraint snap shots can be taken by using COW for directories, inodes and blocks and creating at least one directory which points to the old inodes. There were/are some papers at the NetApp site that go into how they do it in more detail. Duncan --- ________________________________________________________________________ Duncan Barclay | God smiles upon the little children, dmlb@ragnet.demon.co.uk | the alcoholics, and the permanently stoned. ________________________________________________________________________ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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