On Mon, Sep 05, 2011 at 07:54:32PM -0600, Andreas Dilger wrote:
> On 2011-09-05, at 1:20 PM, "Darrick J. Wong" <djwong@xxxxxxxxxx> wrote:
> > On Sun, Sep 04, 2011 at 12:28:24PM -0600, Andreas Dilger wrote:
> >> On 2011-08-31, at 6:36 PM, "Darrick J. Wong" <djwong@xxxxxxxxxx> wrote:
> >>> Write out checksummed inodes even when writing out a zeroed table.
> >>>
> >>> + if (fs->super->s_creator_os == EXT2_OS_LINUX &&
> >>> + fs->super->s_feature_ro_compat &
> >>> + EXT4_FEATURE_RO_COMPAT_METADATA_CSUM) {
> >>
> >> Somehow it doesn't look like this is skipping the zeroing of the inode table
> >> blocks if lazy itable zeroing is set.
> >>
> >> Any measurements on how much this slows down inode table writing (which is
> >> already the slowest part of mke2fs)?
> >
> > Quite a lot, actually. Trouble is, if you're going to write zeroes to the
> > inode table (without using uninit) then I think you need the checksums to
> > match. Maybe the solution is to modify the kernel/e2fsck to ignore the
> > checksum if the inode bitmap says the inode isn't in use?
>
> The kernel is already aware of which inodes are not in use if the uninit_bg
> feature is enabled. Even without uninit_bg, the kernel will not read itable
> blocks from disk if none of the inodes in that block are used.
>
> Also, if the lazy_itable_init is passed to mke2fs it isn't supposed to
> initialize the inode table at all, and the kernel should do it instead.
Ok.
> > A better solution is to zero the buffer, stuff in all the checksums in the
> > correct places, and then write the block out.
>
> Rather, the kernel should do it in the background.
Append "...If the kernel won't do it in the background." to my earlier
statement. :) There seems to be some code that probes around in sysfs to make
sure that the kernel can handle uninit_bg ...
/sys/fs/ext4/features/lazy_itable_init I think?
--D
> >>> + bzero(&inode, sizeof(inode));
> >>> + for (ino = fs->super->s_inodes_per_group * i;
> >>> + ino < fs->super->s_inodes_per_group * (i + 1);
> >>> + ino++) {
> >>
> >> Why recompute "ino" each time through this loop? It should be enough to
> >> simply initialize it at 1 and then increment it for each inode written.
> >
> > Agreed.
> >
> > --D
> >>> + if (!ino)
> >>> + continue;
> >>> + retval = ext2fs_write_inode(fs, ino, &inode);
> >>> + if (retval) {
> >>> + com_err("inode_init", retval,
> >>> + "while writing inode %d\n",
> >>> + ino);
> >>> + exit(1);
> >>> + }
> >>> + }
> >>> + } else {
> >>> + retval = ext2fs_zero_blocks2(fs, blk, num, &blk, &num);
> >>> + if (retval) {
> >>> + fprintf(stderr, _("\nCould not write %d "
> >>> + "blocks in inode table starting "
> >>> + "at %llu: %s\n"),
> >>> + num, blk, error_message(retval));
> >>> + exit(1);
> >>> + }
> >>> }
> >>> if (sync_kludge) {
> >>> if (sync_kludge == 1)
> >>> @@ -829,7 +851,8 @@ static __u32 ok_features[3] = {
> >>> EXT4_FEATURE_RO_COMPAT_EXTRA_ISIZE|
> >>> EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER|
> >>> EXT4_FEATURE_RO_COMPAT_GDT_CSUM|
> >>> - EXT4_FEATURE_RO_COMPAT_BIGALLOC
> >>> + EXT4_FEATURE_RO_COMPAT_BIGALLOC|
> >>> + EXT4_FEATURE_RO_COMPAT_METADATA_CSUM
> >>> };
> >>>
> >>>
> >>>
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