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
http://wpkg.org