-------- Original Message --------
Subject: Re: libblkid: Idea to force given cached entry to be invalidated?
From: Karel Zak <kzak@xxxxxxxxxx>
To: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
Date: 2014年04月17日 16:21
On Thu, Apr 17, 2014 at 09:17:03AM +0800, Qu Wenruo wrote:
But after some tests, it seems that the ctime/mtime based cache seems not
perfect due to the fact that multiple
Yes, the cache is not perfect -- it was originally introduced by Ted
in time when things was less complicated :-) The current goal is to
avoid the cache usage, so we have low-level API in libblkid to bypass
the cache at all, and in userspace we use this API or udev db. I guess
that many users have the cache empty, because it's unnecessary for
fsck, mount, lsblk, findmnt, systemd etc.
device file can be created in different fs with same major/minor number.
especially each of them has individual ctime/mtime timestamp.
So if someone (maybe insane) use mknod to create as block device file and
use 'btrfs dev del' on the newly created
block device file, libblkid will still be unable to detect the change since
the ctime/mtime of block device file in /dev/
does not change.
Not sure if I understand, it would be nice to have complete example.
Karel
Sorry for my poor English.
Use /dev/sda6(major 8, minor 6) as an example.
------
# stat /dev/sda6
File: '/dev/sda6'
Size: 0 Blocks: 0 IO Block: 4096 block special
file
Device: 5h/5d Inode: 7289 Links: 1 Device type: 8,6
Access: (0660/brw-rw----) Uid: ( 0/ root) Gid: ( 6/ disk)
Access: 2014-04-17 12:30:08.981732698 +0800
Modify: 2014-04-17 12:30:08.981732698 +0800
Change: 2014-04-17 12:30:08.981732698 +0800
^^^^^^^^^^^^^^^^^
Birth: -
# mknod sda6 b 8 6
# mkfs.ext4 ./sda6 <<< Not /dev/sda6
......
# stat /dev/sda6
File: '/dev/sda6'
Size: 0 Blocks: 0 IO Block: 4096 block special
file
Device: 5h/5d Inode: 7289 Links: 1 Device type: 8,6
Access: (0660/brw-rw----) Uid: ( 0/ root) Gid: ( 6/ disk)
Access: 2014-04-17 12:30:08.981732698 +0800
Modify: 2014-04-17 12:30:08.981732698 +0800
Change: 2014-04-17 12:30:08.981732698 +0800
^^^^^^^^^^^^^^^^ unchanged
Birth: -
# stat ./sda6
File: './sda6'
Size: 0 Blocks: 0 IO Block: 4096 block special
file
Device: 805h/2053d Inode: 8889848 Links: 1 Device type: 8,6
Access: (0644/brw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2014-04-17 16:26:30.312439395 +0800
Modify: 2014-04-17 16:26:40.869106587 +0800
Change: 2014-04-17 16:26:40.869106587 +0800 <<< Only the created block
device file is updated
Birth: -
------
Thanks,
Qu
--
To unsubscribe from this list: send the line "unsubscribe util-linux" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
