Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
On Tue, Dec 18, 2007 at 04:35:21PM +0100, Tomasz Chmielewski wrote: > With RAID-1, a program which writes will generally write with a speed of > the slowest device in the array (it is apparent when one device is > local, and the second is available remotely via iSCSI). > > > In Linux RAID-1 this can be mitigated a bit by setting --write-behind > for a remote/slow device: with this, bursts of writes will find its way > to a local device and a program won't have to wait until everything hits > the remote device as well. > Long term, of course, write speed is still limited to the speed of the > slowest device. > > > Is something similar possible with drbd? we have the protocol setting, try 'A'. but the amount of data "buffered" is basically your tcp-sendbuffer plus in-flight-io (counting everything on the wire and on the way to the remote disk). > The fine drbd.conf manual explains "rate" as: > > To ensure smooth operation of the application on top of DRBD, it is > possible to limit the bandwidth which may be used by background > synchronizations. The default is 250 KB/sec > > So, what happens if I want to write 1 GB with and rate is set to 250 kB/s: > > dd if=/dev/zero of=/dev/drbd1 > > ? > > > 1. dd will write the whole 1 GB with an average ~250 kB/s speed > > 2. dd will write as fast as possible to the primary device, and will > finish with an average speed of 20 MB/s (local/primary HDD limit). > The secondary device will not be synchronized for several minutes, until > all data reach it with ~250 kB/s speed. _syncer_ is the "reconstruction" of a previously degraded drbd. this is what is throttled by the syncer rate configuration. _application_ io (and normal _replication_) does not care for that setting at all, and is affected by that setting only in so far as it has to compete for bandwidth with a parallel running syncer. which is why you should throttle the syncer to something below your available bandwidth. -- : Lars Ellenberg http://www.linbit.com : : DRBD/HA support and consulting sales at linbit.com : : LINBIT Information Technologies GmbH Tel +43-1-8178292-0 : : Vivenotgasse 48, A-1120 Vienna/Europe Fax +43-1-8178292-82 : __ please use the "List-Reply" function of your email client.