linux-ext4 - Re: [PATCH] e2fsck: Discard free data and inode blocks.

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Date:	Thu, 21 Oct 2010 12:07:22 -0600
From:	Andreas Dilger <adilger.kernel@...ger.ca>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	linux-ext4@...r.kernel.org, tytso@....edu, sandeen@...hat.com
Subject: Re: [PATCH] e2fsck: Discard free data and inode blocks.

On 2010-10-21, at 08:15, Lukas Czerner wrote:
> In Pass 5 when we are checking block and inode bitmaps we have great
> opportunity to discard free space and unused inodes on the device,
> because bitmaps has just been verified as valid. This commit takes
> advantage of this opportunity and discards both, all free space and
> unused inodes.
> 
> I have added new option '-K' which when set, disables discard. Also when
> the underlying device does not support discard, or BLKDISCARD ioctl
> returns any kind of error, or when some errors occurred in bitmaps, the
> discard is disabled.

I'm always a bit nervous with patches like this, that will prevent data recovery after an e2fsck run (which seems like the opposite of what we want from e2fsck).

Two suggestions:
- it probably makes sense to disable this by default, and allow it to be
  specified on the command-line and e2fsck.conf
- should we really have a short option, or a "-E discard" and "-E nodiscard"
  options, which allow us to change the default easily at some later time
  (which we can't do with a single -K flag)

> +static void e2fsck_discard_blocks(e2fsck_t ctx, blk_t start,
> +				  blk_t count)
> +{
> +	fd = open64(fs->device_name, O_RDWR);
> +	if (fd < 0) {
> +		com_err("open", errno,
> +			_("while opening %s for discarding"),
> +			ctx->device_name);
> +		fatal_error(ctx, 0);
> +	}
> +
> +	ret = ioctl(fd, BLKDISCARD, &range);
> +	if (ret)
> +		ctx->options &= ~E2F_OPT_DISCARD;
> +
> +	close(fd);
> +}

If we are calling this ioctl for a lot of small block ranges, doing an open/close for each one could add significant overhead.  The unix struct_io_manager already has an open file descriptor for this block device, maybe it is better to encapsulate this operation there?  The ioctl also doesn't make sense for non-Linux platforms (though they may have a different ioctl that is equivalent) so that may be a better solution.

(defect) It makes sense to start with a blk64_t for this function, instead of a blk_t that needs to be fixed immediately for > 16TB filesystems, or the block number will be truncated and accidentally discard the wrong data.  Oops.

Cheers, Andreas

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