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List:       drbd-user
Subject:    Re: [DRBD-user] Extremely high latency problem
From:       Philipp Marek <philipp.marek () linbit ! com>
Date:       2014-06-05 10:34:45
Message-ID: 3510505.GE1tmQh67Y () cacao
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Hi Bret,

> Intel(R) Xeon(R) CPU E3-1230 V2 @ 3.30GH
> CentoS 6.5 - 2.6.32-431.17.1.el6.x86_64
> drbd version: 8.3.16 (api:88/proto:86-97)
please use 8.4.4 (or .5).
Eg because of http://blogs.linbit.com/p/469/843-random-writes-faster/


> dd if=/dev/zero of=./testbin  bs=512 count=1000 oflag=direct
> 512000 bytes (512 kB) copied, 32.3254 s, 15.8 kB/s
This means ~32msec per write.

But: you're writing into a _filesystem_, not directly onto the DRBD 
resource.
And that means quite some write amplification - each 512 byte write 
_might_ result (depending on the filesystem) in _multiple_ write-
requests, each with a separate barrier ...


Please try writing into a DRBD device, without any filesystem inbetween; 
and using 4kB blocks might help to avoid read/modify/write cycles, too.


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