Re: [PATCH 1/4] mkfs: stop zeroing old superblocks excessively

[Brian Foster <bfoster@redhat.com>]

On Wed, Sep 05, 2018 at 06:19:29PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> When making a new filesystem, don't zero superblocks way beyond the
> end of the new filesystem. If the old filesystem was an EB scale
> filesytsem, then this zeroing requires millions of IOs to complete.
> We don't want to do this if the new filesystem on the device is only
> going to be 100TB. Sure, zeroing old superblocks a good distance
> beyond the new size is a good idea, as is zeroing the ones in the
> middle and end, but the other 7,999,000 superblocks? Not so much.
> 
> Make a sane cut-off decision - zero out to 10x the size of the new
> filesystem, then zero the middle AGs in the old filesystem, then
> zero the last ones.
> 
> The initial zeroing out to 10x the new fs size means that this code
> will only ever trigger in rare corner cases outside a testing
> environment - there are very few production workloads where a huge
> block device is reused immediately and permanently for a tiny much
> smaller filesystem. Those that do this (e.g. on thing provisioned
> devices) discard the in use blocks anyway and so the zeroing won't
> actually do anything useful.
> 
> Time to mkfs a 1TB filsystem on a big device after it held another
> larger filesystem:
> 
> previous FS size	10PB	100PB	 1EB
> old mkfs time		1.95s	8.9s	81.3s
> patched			0.95s	1.2s	 1.2s
> 

Certainly a nice speedup...

> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  mkfs/xfs_mkfs.c | 64 +++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 59 insertions(+), 5 deletions(-)
> 
> diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
> index 2e53c1e83b6a..c153592c705e 100644
> --- a/mkfs/xfs_mkfs.c
> +++ b/mkfs/xfs_mkfs.c
> @@ -1155,14 +1155,15 @@ validate_ag_geometry(
>  
>  static void
>  zero_old_xfs_structures(
> -	libxfs_init_t		*xi,
> -	xfs_sb_t		*new_sb)
> +	struct libxfs_xinit	*xi,
> +	struct xfs_sb		*new_sb)
>  {
> -	void 			*buf;
> -	xfs_sb_t 		sb;
> +	void			*buf;
> +	struct xfs_sb		sb;
>  	uint32_t		bsize;
>  	int			i;
>  	xfs_off_t		off;
> +	xfs_off_t		end;
>  
>  	/*
>  	 * We open regular files with O_TRUNC|O_CREAT. Nothing to do here...
> @@ -1220,15 +1221,68 @@ zero_old_xfs_structures(
>  
>  	/*
>  	 * block size and basic geometry seems alright, zero the secondaries.
> +	 *
> +	 * Don't be insane when it comes to overwriting really large filesystems
> +	 * as it could take millions of IOs to zero every secondary
> +	 * superblock. If we are remaking a huge filesystem, then do the
> +	 * zeroing, but if we are replacing it with a small one (typically done
> +	 * in test environments, limit the zeroing to:
> +	 *
> +	 *	- around the range of the new filesystem
> +	 *	- the middle of the old filesystem
> +	 *	- the end of the old filesystem
> +	 *
> +	 * Killing the middle and end of the old filesystem will prevent repair
> +	 * from finding it with it's fast secondary sb scan algorithm. The slow
> +	 * scan algorithm will then confirm the small filesystem geometry by
> +	 * brute force scans.
>  	 */
>  	memset(buf, 0, new_sb->sb_sectsize);
> +
> +	/* this carefully avoids integer overflows */
> +	end = sb.sb_dblocks;
> +	if (sb.sb_agcount > 10000 &&
> +	    new_sb->sb_dblocks < end / 10)
> +		end = new_sb->sb_dblocks * 10;

... but what's with the 10k agcount cutoff? Just a number out of a hat
to demonstrate the improvement..?

Given that you've done the work to rough in an AIO buffered write
mechanism for mkfs, have you considered whether we can find a way to
apply that mechanism here? I'm guessing that the result of using AIO
wouldn't be quite as impressive of not doing I/O at all, but as you've
already noted, this algorithmic optimization is more targeted at test
environments than production ones. The AIO approach sounds like it could
be more broadly beneficial, even if not quite as fast in those
particular test cases.

>  	off = 0;
> -	for (i = 1; i < sb.sb_agcount; i++)  {
> +	for (i = 1; i < sb.sb_agcount && off < end; i++)  {
> +		off += sb.sb_agblocks;
> +		if (pwrite(xi->dfd, buf, new_sb->sb_sectsize,
> +					off << sb.sb_blocklog) == -1)
> +			break;
> +	}
> +
> +	if (end == sb.sb_dblocks)
> +		return;
> +
> +	/*
> +	 * Trash the middle 1000 AGs of the old fs, which we know has at least
> +	 * 10000 AGs at this point. Cast to make sure we are doing 64bit
> +	 * multiplies, otherwise off gets truncated to 32 bit. I hate C.
> +	 */
> +	i = (sb.sb_agcount / 2) - 500;
> +	off = (xfs_off_t)sb.sb_agblocks * i;
> +	off = (xfs_off_t)sb.sb_agblocks * ((sb.sb_agcount / 2) - 500);

Looks like a couple lines of dead code there.

Brian

> +	end = off + 1000 * sb.sb_agblocks;
> +	while (off < end) {
> +		if (pwrite(xi->dfd, buf, new_sb->sb_sectsize,
> +					off << sb.sb_blocklog) == -1)
> +			break;
>  		off += sb.sb_agblocks;
> +	}
> +
> +	/*
> +	 * Trash the last 1000 AGs of the old fs
> +	 */
> +	off = (xfs_off_t)sb.sb_agblocks * (sb.sb_agcount - 1000);
> +	end = sb.sb_dblocks;
> +	while (off < end) {
>  		if (pwrite(xi->dfd, buf, new_sb->sb_sectsize,
>  					off << sb.sb_blocklog) == -1)
>  			break;
> +		off += sb.sb_agblocks;
>  	}
> +
>  done:
>  	free(buf);
>  }
> -- 
> 2.17.0
>