On Jan 9, 2012, at 5:44 AM, Edward Ned Harvey wrote: >> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- >> boun...@opensolaris.org] On Behalf Of Bob Friesenhahn >> >> To put things in proper perspective, with 128K filesystem blocks, the >> worst case file fragmentation as a percentage is 0.39% >> (100*1/((128*1024)/512)). On a Microsoft Windows system, the >> defragger might suggest that defragmentation is not warranted for this >> percentage level. > > I don't think that's correct... > Suppose you write a 1G file to disk. It is a database store. Now you start > running your db server. It starts performing transactions all over the > place. It overwrites the middle 4k of the file, and it overwrites 512b > somewhere else, and so on.
It depends on the database, but many (eg Oracle database) are COW and write fixed block sizes so your example does not apply. > Since this is COW, each one of these little > writes in the middle of the file will actually get mapped to unused sectors > of disk. Depending on how quickly they're happening, they may be aggregated > as writes... But that's not going to help the sequential read speed of the > file, later when you stop your db server and try to sequentially copy your > file for backup purposes. Those who expect sequential to get performance out of HDDs usually end up being sad :-( Interestingly, if you run Oracle database on top of ZFS on top of SSDs, then you have COW over COW over COW. Now all we need is a bull! :-) -- richard -- ZFS and performance consulting http://www.RichardElling.com illumos meetup, Jan 10, 2012, Menlo Park, CA http://www.meetup.com/illumos-User-Group/events/41665962/ _______________________________________________ zfs-discuss mailing list firstname.lastname@example.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss