linux-ext4 - Re: [PATCH 1/5] Add helper function for blkdev_issue

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Date:	Thu, 9 Sep 2010 13:11:07 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Andreas Dilger <adilger@...ger.ca>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	rwheeler@...hat.com, sandeen@...hat.com, tytso@....edu
Subject: Re: [PATCH 1/5] Add helper function for blkdev_issue_zeroout

On Wed, 8 Sep 2010, Andreas Dilger wrote:

> On 2010-09-08, at 10:59, Lukas Czerner wrote:
> > +static inline int sb_issue_zeroout(struct super_block *sb,
> > +				   sector_t block, sector_t nr_blocks)
> > +{
> > +	block <<= (sb->s_blocksize_bits - 9);
> > +	nr_blocks <<= (sb->s_blocksize_bits - 9);
> > +	return blkdev_issue_zeroout(sb->s_bdev, block, nr_blocks, GFP_KERNEL,
> > +				   BLKDEV_IFL_WAIT | BLKDEV_IFL_BARRIER);
> > +}
> 
> While I can understand that we might need a barrier for this (to avoid
> it being reordered with later writes that are using these blocks), I'm
> not sure it needs to wait for previous IO to complete.

We are waiting for submitted bios to finish. Especially in my patch it
is needed, because I need to know how long it takes to get this IO on
disk do determine next schedule time, depending on the IO load. I am not
sure about this, but when it would not wait it will complete fairly
quickly and my "IO throttling" code would be useless.

And if I am wrong, and you are really convinced that it should not wait,
what about sb_issue_discard(), it is also waiting for completion, is it
ok?

Regards.
-Lukas

> 
> Cheers, Andreas
> 
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