-------- 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15日 19:21
On Tue, Apr 15, 2014 at 10:15:01AM +0800, Qu Wenruo wrote:
When using the blkid_get_cache(), things will be cached to reduce the probe
frequency.
The question is if you really want to use the cache at all :-)
It's pretty common that we have udevd that maintain all necessary
information about devices, including information from libblkid. The
udev db is the primary source for many system actions and tools.
All you need is to use libudev udev_enumerate API to scan for all
block devices and udev_device_get_property_value(ID_FS_TYPE). Maybe
you can add a new method BTRFS_SCAN_UDEV and use it as default. It
would be better to keep libblkid as a fallback solution only. (For
very unusual situations where there is no udev or so.)
Thanks for the suggestion to use libudev.
It sounds solid and I'll try to use it instead.
But filesystem which supports device management like btrfs can remove device
and if fast enough, libblkid will not invalidate the old cache and cause to get
the wrong filesystem type.
Well, since commit 6c2f2b9d (year 2010, v2.18), the libblkid uses
microsecond resolution to check for device modifications.
So the problem is not that btrfs is fast enough ;-)
According to your explaination, it seems that if there is any
modification to the block device, libblkid will rescan it and not
to use the old cache, do I understand it right?
The bug is describe in the following btrfs patch:
https://patchwork.kernel.org/patch/3960191/
The patch does not make sense at all, blkid without a cache scans
/proc/partitions -- so it's almost the same like BTRFS_SCAN_PROC
btrfs_scan_block_devices().
So is there any idea about forcing given cached entry invalidated?
It seems you're trying workaround the *btrfs problem*. The command
btrfs device delete
does not open and modify the device! It uses btrfs ioctl and kernel
does the changes to the device. The problem is that kernel does not
update atime and mtime of the device. So the changes are completely
invisible for libblkid (or another mtime based solutions). See
my example below.
Thanks for pointing out the problem a lot !
IMHO it's strange manner to modify files in kernel, but no update
mtime.
Fortunately kernel at least sends event to udev.
Maybe possible workaround is to add utime() call to "btrfs device
delete". Something like:
This sounds good, much better than my poor patch.
I will also try to fix it in kernel codes if possible.
diff --git a/cmds-device.c b/cmds-device.c
index 309b49e..5c438bf 100644
--- a/cmds-device.c
+++ b/cmds-device.c
@@ -20,6 +20,8 @@
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>
+#include <sys/types.h>
+#include <utime.h>
#include <errno.h>
#include <sys/stat.h>
#include <getopt.h>
@@ -179,7 +181,8 @@ static int cmd_rm_dev(int argc, char **argv)
"ERROR: error removing the device '%s' - %s\n",
argv[i], strerror(e));
ret++;
- }
+ } else
+ utime(argv[i], NULL);
}
close_file_or_dir(fdmnt, dirstream);
BTW, it would be nice to have --verbose option for btrfs-scan, now it
seems like black box.
I'd like to add it, but as you know, 'btrfs dev scan' just calls a btrfs
ioctl, so even '--verbose' is added,
nothing useful will be printed out. :(
Karel
See mtime:
# stat /dev/sdb2
File: ‘/dev/sdb2’
Size: 0 Blocks: 0 IO Block: 4096 block special file
Device: 5h/5d Inode: 392701 Links: 1 Device type: 8,12
Access: (0660/brw-rw----) Uid: ( 0/ root) Gid: ( 6/ disk)
Access: 2014-04-15 11:58:58.338758155 +0200
Modify: 2014-04-15 11:58:58.338758155 +0200
Change: 2014-04-15 11:58:58.338758155 +0200
Birth: -
# btrfs device add /dev/sdb2 /mnt/test
SMALL VOLUME: forcing mixed metadata/data groups
# stat /dev/sdb2
File: ‘/dev/sdb2’
Size: 0 Blocks: 0 IO Block: 4096 block special file
Device: 5h/5d Inode: 392701 Links: 1 Device type: 8,12
Access: (0660/brw-rw----) Uid: ( 0/ root) Gid: ( 6/ disk)
Access: 2014-04-15 11:59:34.930143021 +0200
Modify: 2014-04-15 11:59:34.930143021 +0200
Change: 2014-04-15 11:59:34.930143021 +0200
^^^^^^^^
# btrfs device delete /dev/sdb2 /mnt/test
# stat /dev/sdb2
File: ‘/dev/sdb2’
Size: 0 Blocks: 0 IO Block: 4096 block special file
Device: 5h/5d Inode: 392701 Links: 1 Device type: 8,12
Access: (0660/brw-rw----) Uid: ( 0/ root) Gid: ( 6/ disk)
Access: 2014-04-15 11:59:34.930143021 +0200
Modify: 2014-04-15 11:59:34.930143021 +0200
Change: 2014-04-15 11:59:34.930143021 +0200
^^^^^^^^^
Great thanks for all your advise! It helps a lot!!
Thanks,
Qu
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