[DRBD-user] slow sync speed of 12TB over 10GE

Tomasz Chmielewski mangoo at wpkg.org
Mon Jan 19 10:54:12 CET 2009


Lars Täuber schrieb:

> 6553600000 bytes (6.6 GB) copied, 25.4397 s, 258 MB/s

> md4           72164,00         0,00    288656,00          0     288656
> 
> The numbers are floating between ~18900 and ~23400.
> 
> Should I really mount the device with a filesystem on it? The last tests were made without a filesystem too.

Linear writes to a raw device are different to writes to a filesystem 
because:
- you don't have to update metadata, journal etc., so you can write faster
- raw device can not be fragmented, where a filesystem can (heads in 
your HDD won't jump here and there)


Technically speaking, such writes:

          [==============>.....] sync'ed: 76.4% (242356/1023804)K
          finish: 0:00:01 speed: 130,240 (130,240) K/sec

can be also fragmented as well - some area can be out of sync here, some 
area there - the head will jump.

Also, note that at the same time there can be real writes to that device 
(i.e., something writes to your /dev/drbdXY device) - it will impact the 
speed of that sync because those writes have to be synchronized as well; 
also HDD head will jump more often.


It's hard to tell if such difference (258 MB/s dd write speed versus 130 
MB/s DRBD sync) can be explained by only those items I mentioned.



-- 
Tomasz Chmielewski
http://wpkg.org



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