Wietse Venema put forth on 10/22/2009 4:25 PM: > Stan Hoeppner: >> running at 1/4 speed (I'm only getting 3MB/sec whereas with the >> [...] kernel they are getting 14-18MB/sec)" > > I hope you have those numbers mixed up, and that you meant to write > 45MB/s with a good driver and 15MB/s with a bad one. With single-disk > sequential file access of uncached data, 45MB/s is not impressive.
That's not me talking Wietse, it's a quote from an OP with the problem who posted comments on the Gentoo kernel bug a few years ago, and his disks were apparently a little old at that time, SE vs LVD. The last gen of SE drives were Ultra Wide SCSI and topped out at 40 MB/s sync on the SCSI bus. LVD SCSI is still with us (not for long due to SAS) and tops out at 320 MB/s sync on the bus. Obviously we still don't have drives capable of pushing seq I/O anywhere close to 320 MB/s, though most easily eclipse 40 MB/s. Forget the hard numbers for a minute and concentrate on the ratio of performance degradation. He was seeing a 5 to 6 fold decrease in sequential I/O performance after updating his kernel, which contained the new buggy kernel SCSI driver. Ransom I/O throughput would be a little worse yet. The point I was attempting to make is that, even with todays fast disks, on a heavily loaded Postfix server, a 6 fold decrease in disk throughput due to an obscure bug like this would likely wreak havoc for a few hours, if not days, depending on the skill and experience of the OP, before the problem were found and fixed. Ergo, we should never rule out the rare/obscure/unlikely possible causes of problems that pop up. -- Stan
