Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
On Thu, 7 May 2009, Lars Ellenberg wrote: > On Thu, May 07, 2009 at 08:13:55PM +0000, Holger Kiehl wrote: >>> local: 160 MB/s >>> drbd disconnected: 70 MB/s >>> AL-transaction latency appears to be: 32142 micro seconds >>> you may want to verify this using e.g. >>> dd if=/dev/zero of=ll_dev bs=512 count=1000 oflag=direct >>> >>> there. now, how long does a synchonous single sector write take on you >>> lower level device? >>> >> dd if=/dev/zero of=/dev/md4 bs=512 count=1000 oflag=direct >> 1000+0 records in >> 1000+0 records out >> 512000 bytes (512 kB) copied, 3.38987 s, 151 kB/s > > right. though, your meta data is "internal", > it is at the very end of the device, > so all the disks in the md4 raid have to seek to the very end, > possibly adjusting rotation speed, do that single sector write, > then seek back (again possibly adjusting rotational speed), > then write the next 4MB, seek back to the end, do the next transaction, > seek to the front again. > > just for the fun of it, and to rule it out as possible cause, > as an experiment, you could place DRBD meta data on a ram disk... > I will try this tommorrow and report what I get. Is there some documentation on how to do this? How do I get the size of DRBD meta data? Many thanks for all the help! Holger