linux-kernel - Re: Should a block device enforce block atomicity?

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Message-ID: <ac8af0be0806300147i1970a6f7mb705c0ce590326@mail.gmail.com>
Date:	Mon, 30 Jun 2008 16:47:23 +0800
From:	"Zhao Forrest" <forrest.zhao@...il.com>
To:	"Boaz Harrosh" <bharrosh@...asas.com>
Cc:	"Erez Zilber" <erezzi.list@...il.com>,
	"Jens Axboe" <jens.axboe@...cle.com>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Should a block device enforce block atomicity?

>
> Don't forget that all IO requests are queued on the device. With a
> modern HW and disk you usually have NCQ and most drives will throw
> away write request to the same sector if they see a later write to
> the same sector in the queue.
>
> That said. There is nothing wrong with writing again and again to
> the same sector on disk. File/record locking is done at the FileSystem
> level. An application that wants exclusive write need to open the file
> that way. Other wise it could even be written from another machine not
> even another thread.
>
> What is it you are concerned with?
>
I happen to read the email and have a question, that may not be Erez's
real question :)
Let's suppose the following example:
1 pdflush find a dirty inode and decides to flush a set of dirty pages
to harddrive
2 while this set of dirty pages is being committed to
harddrive(dma_mapping is done, but dirty pages are not really written
to disk media), application/FS is trying to update some pages in this
set of dirty pages.

Then what happens? Will application be put into sleep until page
flushing to disk media is done?

Thanks,
Forrest
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