can you guess? wrote: >> For very read intensive and position sensitive >> applications, I guess >> this sort of capability might make a difference? > > No question about it. And sequential table scans in databases > are among the most significant examples, because (unlike things > like streaming video files which just get laid down initially > and non-synchronously in a manner that at least potentially > allows ZFS to accumulate them in large, contiguous chunks - > though ISTR some discussion about just how well ZFS managed > this when it was accommodating multiple such write streams in > parallel) the tables are also subject to fine-grained, > often-random update activity. > > Background defragmentation can help, though it generates a > boatload of additional space overhead in any applicable snapshot.
The reason that this is hard to characterize is that there are really two very different configurations used to address different performance requirements: cheap and fast. It seems that when most people first consider this problem, they do so from the cheap perspective: single disk view. Anyone who strives for database performance will choose the fast perspective: stripes. Note: data redundancy isn't really an issue for this analysis, but consider it done in real life. When you have a striped storage device under a file system, then the database or file system's view of contiguous data is not contiguous on the media. There are many different ways to place the data on the media and we would typically strive for a diverse stochastic spread. Hmm... one could theorize that COW will also result in a diverse stochastic spread. The complexity of the characterization is then caused by the large number of variables which the systems use to spread the data (interlace size, block size, prefetch, caches, cache policies, etc) and the feasibility of understanding the interdependent relationships these will have on performance. Real data would be greatly appreciated. -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
