Re: What represent 646345728 bytes

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Aneesh Kumar K. V wrote:
> On Mon,  1 Feb 2010 15:08:24 +0100 (CET), <paul.chavent@xxxxxxxx> wrote:
>> 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 guess that would be balance_dirty_pages starting to writeback the
> delayed allocated pages. You can try if that changes by changing
> /proc/sys/vm/dirty_ratio

But he's doing direct IO...

-Eric
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