I remember SEEK_HOLE and SEEK_DATA being proposed by the creators of ZFS. ZFS is the first of various file systems allowing Copy on Write. In order to preserve the space savings after COW, it be a good thing for utilities to have a way of detecting block of zeroes.
David describes how the AST utilities already were aware of holes in data when these were proposed. So much better if sfio supports this out-of-the-box. Cheers, Henk On Mon, Mar 26, 2012, at 11:08 AM, David Korn wrote: > cc: [email protected] > Subject: Re: [ast-users] Implementing SEEK_HOLE, SEEK_DATA in AST cp, mv, > pax > -------- > > > are there plans to implement support for SEEK_HOLE (let lseek() seek > > to the next hole in a sparse file) and SEEK_DATA (let lseek() seek to > > the next place with real data, usually after a hole) in AST cp, mv and > > pax in the next 2-3 months? This has become VERY important to > > enterprise customers now that Linux+btrfs, GNU coreutils, Solaris, > > FreeBSD and others support this feature and that it is going to be > > included in the next iteration of the POSIX standard > > (http://man7.org/linux/man-pages/man2/lseek.2.html) > > > > > > The AST tools do not use read/write/lseek directly but use SFIO for all > input and output. Currently when writing a file and there are more than > a block full of 0 bytes, SFIO seeks to the next non-zero byte so that > the file can be created with holes. The result is that cp foo bar will > create the file bar with the same or less space than bar. I don't > understand how mv is effected by SEEK_HOLE and SEEK_DATA since this is > just a rename operation. pax will create holes whenever the input > file contain large numbers of 0 bytes. > > In order to take advantage of SEEK_HOLE and SEEK_DATA, we will have to > allow these to be passed into sfseek(). I don't know what gain will > be achieved over the current method except possibly on read. > > David Korn > [email protected] > _______________________________________________ > ast-users mailing list > [email protected] > https://mailman.research.att.com/mailman/listinfo/ast-users _______________________________________________ ast-users mailing list [email protected] https://mailman.research.att.com/mailman/listinfo/ast-users
