-------- Original Message --------
Subject: Re: [PATCH] btrfs-progs: Force 'btrfs dev scan' not using old
libblkid cache
From: David Sterba <dste...@suse.cz>
To: Qu Wenruo <quwen...@cn.fujitsu.com>
Date: 2014年04月15日 00:29
On Thu, Apr 10, 2014 at 03:30:21PM +0800, Qu Wenruo wrote:
'btrfs scan' uses libblkid to scan devices by default, and libblkid uses
cache to reduce the probe.
But if operations below is done in less than 2 seconds(BLKID_PROBE_MIN),
'btrfs scan' will still use the uncorrect cache and scan on the deleted device.
0. /dev/sda[1-4] mounted on /mnt using single data/metadata
1. btrfs dev scan
2. btrfs dev del /dev/sda3 /mnt
3. btrfs dev scan
Since the cache made by step 1 is still validated, step 3 will use the
cache and consider /dev/sda3 as a btrfs filesystem and try to scan it.
But the superblock(at least the first one) is wiped and failed to scan,
a error message, which can be avoided and is unneeded, is output.
This patch will force scan_for_btrfs() not to use cache to avoid the
problem.
--- a/utils.c
+++ b/utils.c
@@ -2057,7 +2057,8 @@ int btrfs_scan_lblkid(int update_kernel)
blkid_cache cache = NULL;
char path[PATH_MAX];
- if (blkid_get_cache(&cache, 0) < 0) {
+ /* No to use libblkid cache to avoid old data */
+ if (blkid_get_cache(&cache, "/dev/null") < 0) {
This effectively avoid the blkid cache for all devices but the point of
blkid was to use it so repeated probes are not done.
I think it's not right to skip the whole cache because one entry may be
stale, more that we know which one and when.
I totally agree with that.
We should rather explicitly invalidate the removed device after delete,
I don't what's the right way to do that. Maybe blkid_do_probe() or
blkid_gc_cache()
But the problem is, libblkid does not provide such API to invalidate one
entry of cache.
blkid_gc_cache() will only remove non-exsist device cache, and
blkid_do_probe() seems not using/update the cache,
so I used "/dev/null" as cache to avoid the cache completly.
I'll try to dig the libblkid codes deeper to find a method to invalidate
an entry.
Thanks,
Qu.
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