On 10/27/12 7:45 AM, Nix wrote:

[nfs people purged from Cc]

On 27 Oct 2012, Theodore Ts'o verbalised:

quoted

Huh?  It's not turned on by default.  If you mount with no mount
options, journal checksums are *not* turned on.

?! it's turned on for me, and though I use weird mount options I don't
use that one:

journal_async_commit implies journal_checksum:

        {Opt_journal_async_commit, (EXT4_MOUNT_JOURNAL_ASYNC_COMMIT |
                                    EXT4_MOUNT_JOURNAL_CHECKSUM), MOPT_SET},

journal_checksum seems to have broken, at least for me, between 3.3 & 3.4, I think I've narrowed down the commit but not sure yet what the flaw is, will investigate & report back later.  Busy Saturday.

So anyway, turning on journal_async_commit (notionally unsafe) enables journal_checksum (apparently broken).

-Eric

/dev/main/var   /var        ext4        defaults,nobarrier,usrquota,grpquota,nosuid,nodev,relatime,journal_async_commit,commit=30,user_xattr,acl 1  2
Default mount options:    (none)
/dev/mapper/main-var /var ext4 rw,nosuid,nodev,relatime,journal_checksum,journal_async_commit,nobarrier,quota,usrquota,grpquota,commit=30,stripe=16,data=ordered,usrquota,grpquota 0 0

...

Ah! it's turned on by journal_async_commit. OK, that alone argues
against use of journal_async_commit, tested or not, and I'd not have
turned it on if I'd noticed that.

(So, the combinations I'll be trying for effect on this bug are:

 journal_async_commit (as now)
 journal_checksum
 none

Technically to investigate all possibilities we should try
journal_async_commit,no_journal_checksum, but this seems so unlikely to
have ever been tested by anyone that it's not worth looking into...)