On Wednesday 02 of May 2012 16:28:43 David Sterba wrote:
> On Tue, May 01, 2012 at 02:40:29PM +0200, Hubert Kario wrote:
> > +static const char * const cmd_zero_dev_usage[] = {
> > + "btrfs device zero-superblock <device> [<device> ...]",
>
> FYI, this step is named 'clear superblock' in kernel code as done after the
> device is removed, and I suggest to consider to name the command like
> 'clear-superblock' or 'clear-sb'.
A similar function in mdadm is called zero superblock so I just re used the
name (according to the principle of least surprise). Users, even admins,
generally don't read kernel code...
> Also, kernel clears only the "superblock magic" string, ie. the
> "_BHRfS_M" bytes. This leaves the rest of the superblock intact, for
> possible recovery when it's cleared accidentally.
That's when a device is removed from the filesystem, not when a filesystem is
just not used any more and you want to re-purpose the devices.
> I had prototyped a similar utility (in perl, so nothing for progs
> inclusion for now) and rewrote the magic string with _BHRf$_M ie. the
> S -> $ for visual similarity with the action performed. This allows to
> detect cleared superblocks and activate them back eventually.
>
> I'm not sure if this is useful and sensible usecase, clearing superblock
> is a one-time action anyway, so it's more for the sake of tool
> flexibility.
Clearing superblock is not a light decision and should generally be performed
just before formatting the partition with some other fs or physical volume for
LVM. IMHO recoverability of "cleared" superblock is a function hardly anyone
would use.
> To your implementation: I think adding a function doing the superblock
> reset would be enough here. Something like this (in pseudocode):
>
> for (i = 0 ; i < BTRFS_SUPER_MIRROR_MAX; i++) {
> bytenr = btrfs_sb_offset(i);
> "break if bytenr > device size"
> memset(superblock buffer, CLEARPATTERN, sizeof(...))
> }
> write_all_supers(root);
That's exactly what btrfs_prepare_device does. And it's a function run by btfs
just before btrfs dev add and by mkfs. Duplicating its code would be a bad
idea.
Regards,
--
Hubert Kario
QBS - Quality Business Software
02-656 Warszawa, ul. Ksawerów 30/85
tel. +48 (22) 646-61-51, 646-74-24
www.qbs.com.pl
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
