On Sunday 31 July 2011 15:17:16 Joshua Murphy wrote: > There probably is a fair chunk of difference in maximum speed the disk > can work at on each end (I've even seen around a 20MB/s difference on > several 160GB drives I've dealt with), but outside of some older > drives that've been heavily abused in their lives, I'm not sure I've > seen a sata drive that I've used my usual drive test (MHDD on a > Hiren's bootable USB) on register below around 60MB/s on the slow end, > and USB2's *theoretical* limit is 480Mb/s (60MB/s) ... real-world > implementations rarely reach, let alone top, around 40MB/s, so disk > speed variation across the disk is an unlikely source of the slowdown.
Sounds entirely reasonable, and I wasn't really trying to blame the slowness on that variation - just mentioning it in passing. > More likely, it's the fact that parted has to start from the end, and > work its way backwards, reading, writing, and verifying in separate > rotations of the disk with no benefit from the drive's ability to > stream a larger block into cache, since the whole process is backwards > compared to the streaming read most drives are optimized for. Perhaps I'm naive here, but I should have thought an intelligent disk copying algorithm would be able to account for that, at least in part. Maybe that's why it ran the speed tests at the beginning. > Of course, this is all off the cuff conjecture on my part, including my > assumptions about how parted approaches the whole task... mixed with a > bit of anecdotal evidence on my end... but, makes for amusing > conversation and contemplation, if nothing more substantial. Indeed. > I will point out that the newer advanced format WD 500GB blue's I've > worked recently with pulled a consistent 120-110MB/s speed from end to > end... when their older 320s usually peaked at around 85 or so. Well, I haven't run any proper tests, but watching gkrellm during an occasional large transfer I don't remember seeing more than half that lower figure. These are two Samsung Spinpoint F3 1TB disks in md-raid with LVM-2, and I haven't fiddled with any of their settings. -- Rgds Peter Linux Counter 5290, 1994-04-23
