-------- Original Message --------
Subject: Re: [PATCH] btrfs-progs: Force 'btrfs dev scan' not using old
libblkid cache
From: Qu Wenruo <quwen...@cn.fujitsu.com>
To: dste...@suse.cz, linux-btrfs@vger.kernel.org
Date: 2014年04月16日 09:24
-------- 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.
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()
Please ignore this patch, since after consulting with libblkid mail
list, the real problem is that btrfs ioctl for remove device
does not update the mtime/ctime for the device.
I'm sorry that kernel updates the mtime/ctime, but it seems there is
some small latency, which caused libblkd fail to detect the
modification and use old cache.
Will go on investigation.
Thanks,
Qu
Libblkid will only use cache when the device's mtime/ctime stay
unchanged, if mtime/ctime changed libblkid will do the lowprobe.
I'll try to fix it in kernel space.
Thanks,
Qu
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