From mboxrd@z Thu Jan  1 00:00:00 1970
From: Randy Dunlap <rdunlap@infradead.org>
Subject: Re: dummy file read periodically for external USB Hard Disk
Date: Fri, 18 Jul 2014 11:13:25 -0700
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Cc: NeilBrown <neilb@suse.de>,
	Linux-FSDevel <linux-fsdevel@vger.kernel.org>
To: loody <miloody@gmail.com>
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On 07/18/2014 10:31 AM, loody wrote:
> hi Randy and All:
> 
>>
>> It means that the <buffer> still is not aligned to a 512-byte boundary.
>> Try
>>    char buffer[1024] __attribute__ ((aligned(1024)));
>>
>> Well, that is aligned to 1024 bytes, not 512, but whatever.
> the suggestion you gave to me works ^^
> is there any alignment check in kernel FS such that it will directly
> output to device instead of getting data from page cache?

Not sure that I understand your question, but I think that using
O_DIRECT in the open() call attempts to do what you are asking.

Please read 'man open' and search for O_DIRECT.

-- 
~Randy