linux-ext4 - What represent 646345728 bytes

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Date:	Mon,  1 Feb 2010 15:08:24 +0100 (CET)
From:	<paul.chavent@...c.net>
To:	linux-ext4@...r.kernel.org
Subject: What represent 646345728 bytes

Hi

I'am writing an application that write a stream of pictures of fixed size on a disk.

My app run on a self integrated gnu/linux (based on a 2.6.31.6-rt19 kernel).

My media is formated with

# mke2fs -t ext4 -L DATA -O large_file,^has_journal,extent -v /dev/sda3
[...]

And it is mounted with 

# mount -t ext4 /dev/sda3 /var/data/
EXT4-fs (sda3): no journal
EXT4-fs (sda3): delayed allocation enabled
EXT4-fs: file extents enabled
EXT4-fs: mballoc enabled
EXT4-fs (sda3): mounted filesystem without journal

My app opens the file with "O_WRONLY | O_CREAT | O_TRUNC | O_SYNC | O_DIRECT" flags.

Each write takes ~4.2ms for 304K (it is very good since it is the write bandwidth of my hard drive). There is a write every 100ms.

But every exactly 646345728 bytes, the write takes ~46ms.

I had the same problem with ext2 but every ~620M (the amount wasn't so constant).

Also i tryed to "posix_fallocate" with (eg 2G), and the first write overhead comes at this limit. I would like to avoid to preallocate.

I suppose it is a kind of block allocation issue. But i would like to have your opinion :
 - what is exatcly this amount of bytes ?
 - can i do something for having a "constant" write time from the user space point of view ?
 - is it a "probem" only for me ?

Thank you for your reading.

Paul.

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