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, Brian R. Hellman wrote: > > Holger Kiehl wrote: >> 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? > > The formula for calculating the meta-data can be found at the link > below, I would assume just use external meta-data, which can also be > found in the guide below :) > > http://www.drbd.org/users-guide/ch-internals.html#s-meta-data-size > Thanks! With DRBD metadata on a ramdisk I get up to 120 MB/s when the secondary is disabled. When I enable the secondary the best I get is 70 MB/s. Is there maybe some bad interacion with DRBD and MD? Where does MD keep its meta data? Would hardware raid give better performance with DRBD? Holger