Le jeudi 28 août 2008 à 09:34 -0400, Theodore Tso a écrit :
> On Thu, Aug 28, 2008 at 11:55:21AM +0200, Frédéric Bohé wrote:
> > With 512 groups by flex group, meta-datas for a single flex-group are 8
> > groups long ! If we have no luck and there are a bunch of groups
> > occupied by meta-datas at the middle of the filesystem, we should
> > slightly increase the number of groups scanned to find a completely free
> > group.
>
> I'm not sure it ever makes sense to use such a huge -G setting, but
> yes, you're right. It actually wasn't a major tragedy, since this
> just specifies the goal block, and so the block allocator would just
> search forward to find the first free block. But it is better to move
> forward to the next free block group, so we leave space for interior
> nodes of the extent tree to be allocated.
>
> The following patch takes into account the flex_bg size, and will
> stash the journal in the first free block group after metadata; we do
> by starting at a flex_bg boundary, and then searching forward until
> bg_free_blocks_count is non-zero. However, if the number of block
> groups is less than half of the flex_bg size, we'll just give up and
> throw it at the mid-point of the filesystem, since that (plus using
> extents instead of indirect blocks) is really the major optimization
> here.
>
> One or two discontinuities in the journal file really isn't a big
> deal, since we're normally seaking back and forth between the rest of
> the filesystem data blocks and the journal anyway. The best benchmark
> to see a problem isn't going to be bonnie, but something that which is
> extremely fsync-intensive.
>
> - Ted
>
> diff --git a/lib/ext2fs/mkjournal.c b/lib/ext2fs/mkjournal.c
> index 96b574e..f5a9dba 100644
> --- a/lib/ext2fs/mkjournal.c
> +++ b/lib/ext2fs/mkjournal.c
> @@ -275,7 +275,7 @@ static errcode_t write_journal_inode(ext2_filsys fs, ext2_ino_t journal_ino,
> blk_t size, int flags)
> {
> char *buf;
> - dgrp_t group, start, end, i;
> + dgrp_t group, start, end, i, log_flex;
> errcode_t retval;
> struct ext2_inode inode;
> struct mkjournal_struct es;
> @@ -311,7 +311,17 @@ static errcode_t write_journal_inode(ext2_filsys fs, ext2_ino_t journal_ino,
> */
> group = ext2fs_group_of_blk(fs, (fs->super->s_blocks_count -
> fs->super->s_first_data_block) / 2);
> - start = (group > 0) ? group-1 : group;
> + log_flex = 1 << fs->super->s_log_groups_per_flex;
> + if (fs->super->s_log_groups_per_flex && (group > log_flex)) {
> + group = group & ~(log_flex - 1);
> + while ((group < fs->group_desc_count) &&
> + fs->group_desc[group].bg_free_blocks_count == 0)
I would have preferred this to test if a group is free :
fs->group_desc[group].bg_free_blocks_count !=
fs->super->s_blocks_per_group)
That's because there could be "holes" with free blocks in flex_bg
meta-datas when they cross backups superblocks and GDT. Very rare case
and not a big issue, I admit.
> + group++;
> + if (group == fs->group_desc_count)
> + group = 0;
> + start = group;
> + } else
> + start = (group > 0) ? group-1 : group;
> end = ((group+1) < fs->group_desc_count) ? group+1 : group;
> group = start;
> for (i=start+1; i <= end; i++)
>
>
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