[DRBD-user] is drbd synchronization non-blocking (or: what "rate" really means)?

Tomasz Chmielewski mangoo at wpkg.org
Tue Dec 18 16:35:21 CET 2007

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

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?

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.

Tomasz Chmielewski

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