Are random writes optimized sequentially by Linux kernel?

From: "Dmitry Koterov" <dmitry(at)koterov(dot)ru>
To: pgsql-performance(at)postgresql(dot)org
Subject: Are random writes optimized sequentially by Linux kernel?
Date: 2009-01-07 18:39:12
Message-ID: d7df81620901071039s3c8cafd8x79a0886a8bca605@mail.gmail.com
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Hello.

Suppose I perform 1000 RANDOM writes into a file. These writes are saved
into Linux writeback buffer and are flushed to the disc asynchronously,
that's OK.

The question is: will physical writes be performed later in the sequence of
physical SECTOR position on the disc (minimizing head seeking)? Or Linux
background writer knows nothing about physical on-disc placement and flushes
data in order it is saved in the RAM?

E.g., if I write in the application:

a) block 835
b) block 136
c) block 956
d) block 549
e) block 942

dows the Linux background writer save flush them e.g. in physical order "136
- 549 - 835 - 942 - 956" or not?

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