linux-ext4 - Re: [PATCH 10/47] libext2fs: support BLKZEROOUT/FALLOC_FL_ZERO_RANGE in ext2fs_zero

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Date:	Sat, 13 Dec 2014 19:08:40 -0800
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 10/47] libext2fs: support BLKZEROOUT/FALLOC_FL_ZERO_RANGE
 in ext2fs_zero_blocks

On Sat, Dec 13, 2014 at 11:29:17AM -0500, Theodore Ts'o wrote:
> On Fri, Nov 07, 2014 at 01:51:52PM -0800, Darrick J. Wong wrote:
> > +		ret = ioctl(data->dev, BLKZEROOUT, range);
> > +		if (ret)
> > +			goto err;
> > +		if (!(data->flags & IO_FLAG_DIRECT_IO)) {
> > +#ifdef POSIX_FADV_DONTNEED
> > +			ret = posix_fadvise(data->dev, range[0], range[1],
> > +					    POSIX_FADV_DONTNEED);
> > +			if (ret == 0)
> > +				goto err;
> > +#endif
> > +#ifdef BLKFLSBUF
> > +			ret = ioctl(data->dev, BLKFLSBUF, 0);
> > +#endif
> 
> Why do you need BLKFLSBUF after calling POSIX_FADV_DONTNEED?  The

I don't; if the DONTNEED call returns 0, we exit.

(The 'goto err' could be 'return 0' to be more clear, I suppose.)

--D

> problem with BLKFLSBUF is that it has a special meaning for ramdisks,
> dating back to when the ramdisk was stored only in the buffer cache, namely:
> 
> http://www.memegen.com/meme/hotw4i
> 
> We could add yet another special case for the ramdisk, but I'm
> seriously wondering whether the use of BLKZEROOUT is worth it.  Sure,
> it will speed up mke2fs, but with the advent of metadata checksums, we
> can skip clearing the journal and the inode table.
> 
>     	 	      	      	      	    - Ted
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