linux-ext4 - Re: [PATCH 10/37] mke2fs: Allow metadata checksums to be turned on at mkfs time

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Date:	Tue, 6 Sep 2011 10:13:41 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>,
	Theodore Tso <tytso@....edu>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Amir Goldstein <amir73il@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Mingming Cao <cmm@...ibm.com>,
	Joel Becker <jlbec@...lplan.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Coly Li <colyli@...il.com>
Subject: Re: [PATCH 10/37] mke2fs: Allow metadata checksums to be turned on
	at mkfs time

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@...ibm.com> 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@...ibm.com> 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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