Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
On Sun, Mar 09, 2008 at 09:58:14PM +0100, Mario Peschel wrote: > Hi Sam, > > I tried the newest kernel 2.6.24 with the kernel module mptsas, without > any success. > > Then I found the following solution: > > http://lists.us.dell.com/pipermail/linux-precision/2007-June/001188.html which boils down to "enable write cache" on the disks. I don't know about your controller, but I do hope that cache is backed by a good battery. really. otherwise you risk your data in face of power outage. > Now my performance is much better: > > # dd if=/dev/zero bs=4096 count=10000 of=/tmp/testfile oflag=dsync WCE=on from main memory to disk cache. basically you measure the bus latency here. > 40960000 bytes (41 MB) copied, 7.02387 seconds, 5.8 MB/s ~latency per request: 0.7 ms WCE=off this is the latency of getting 4kB from main memory all the way down to rotating rust. >>> 40960000 bytes (41 MB) copied, 263.415 seconds, 155 kB/s ~latency per request: 26 ms without cache, mostly depending on rotation per minute, disks should theoretically have some latency of 3 to 10 ms. so unless you have very slow disks, there is aparently some logic with timers involved, arbitrarily delaying single requests, "anticipating" more. all tuned for maximum bandwidth, not minimum latency. -- : commercial DRBD/HA support and consulting: sales at linbit.com : : Lars Ellenberg Tel +43-1-8178292-0 : : LINBIT Information Technologies GmbH Fax +43-1-8178292-82 : : Vivenotgasse 48, A-1120 Vienna/Europe http://www.linbit.com : __ please use the "List-Reply" function of your email client.