Correct way to clear a free space cache file invalid error

Correct way to clear a free space cache file invalid error

[Martin Steigerwald]

Sorry for incomplete subject, reposting.

Hi!

Kernel 6.1, btrfs-progs 6.1.2. Filesystem on NVME SSD, via LVM and dm-
crypt.

I would like

[   18.657903] BTRFS info (device dm-1): the free space cache file 
(23912775680) is invalid, skip it
[   18.697033] BTRFS info (device dm-1): the free space cache file 
(25254952960) is invalid, skip it

to be gone.

So I tried

mount -o remount,clear_cache,space_cache=v1 /

as well as

mount -o remount,clear_cache /
mount -o remount,space_cache=v1 /

to no avail.

Before that I had space cache v2 on the filesystem as with most of my 
other BTRFS filesystems. But since it really should not be necessary for 
a 50 GiB filesystem for the Linux system (not user data), I thought I 
play it safe this time. At least from what I learned from btrfs manpage.

In a page on BTRFS wiki it is stated that one should run "btrfs check" 
on the filesystem.

So what is the correct way to clear that error?

I did not perceive any malfunctioning due to the error in dmesg. The 
filesystem scrubs without errors.

Best,
-- 
Martin

Re: Correct way to clear a free space cache file invalid error

[Qu Wenruo]

On 2023/1/16 18:22, Martin Steigerwald wrote:
> Sorry for incomplete subject, reposting.
> 
> Hi!
> 
> Kernel 6.1, btrfs-progs 6.1.2. Filesystem on NVME SSD, via LVM and dm-
> crypt.
> 
> I would like
> 
> [   18.657903] BTRFS info (device dm-1): the free space cache file
> (23912775680) is invalid, skip it
> [   18.697033] BTRFS info (device dm-1): the free space cache file
> (25254952960) is invalid, skip it
> 
> to be gone.
> 
> So I tried
> 
> mount -o remount,clear_cache,space_cache=v1 /
> 
> as well as
> 
> mount -o remount,clear_cache /
> mount -o remount,space_cache=v1 /
> 
> to no avail.
> 
> Before that I had space cache v2 on the filesystem as with most of my
> other BTRFS filesystems. But since it really should not be necessary for
> a 50 GiB filesystem for the Linux system (not user data), I thought I
> play it safe this time. At least from what I learned from btrfs manpage.
> 
> In a page on BTRFS wiki it is stated that one should run "btrfs check"
> on the filesystem.
> 
> So what is the correct way to clear that error?

"btrfs check --clear-space-cache v1 <device>".

Of course you need to unmount the fs in the first place.

The clear_cache mount option only clears the old cache of touched bgs.
If the bg is not touched during the runtime, it won't be modified.

Or, you can go v2 cache, which would drop the v1 cache at the first mount.

Thanks,
Qu
> 
> I did not perceive any malfunctioning due to the error in dmesg. The
> filesystem scrubs without errors.
> 
> Best,

[PATCH] btrfs-progs: docs: improve space cache documentation

[Christoph Anton Mitterer]

- "Wipe" in storage terms is often understood as some kind of secure deletion.
  Use "remove" instead in order to indicate that the space cache is fully
  removed (and not just cleared and then e.g. automatically rebuild).

- The --clear-space-cache option for btrfs check actually clears the whole space
  cache, just as documented.
  Thus move any documentation about the clear_cache mount option not doing so
  for v1 to that.
  Instead, refer to the mount option.

- Also note that when clear_cache is used with v1, the free space cache for
  block groups that are modified gets always cleared, but rebuilt only if
  nospace_cache is not used.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
---
 Documentation/btrfs-check.rst      | 12 ++----------
 Documentation/ch-mount-options.rst | 18 +++++++++++++++---
 2 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/Documentation/btrfs-check.rst b/Documentation/btrfs-check.rst
index bca0e7f8..1bd35fe2 100644
--- a/Documentation/btrfs-check.rst
+++ b/Documentation/btrfs-check.rst
@@ -79,17 +79,9 @@ SAFE OR ADVISORY OPTIONS
         superblock is damaged.
 
 --clear-space-cache v1|v2
-        completely wipe all free space cache of given type
+        completely remove the free space cache of the given version
 
-        For free space cache *v1*, the *clear_cache* kernel mount option only rebuilds
-        the free space cache for block groups that are modified while the filesystem is
-        mounted with that option. Thus, using this option with *v1* makes it possible
-        to actually clear the entire free space cache.
-
-        For free space cache *v2*, the *clear_cache* kernel mount option destroys
-        the entire free space cache. This option, with *v2* provides an alternative
-        method of clearing the free space cache that doesn't require mounting the
-        filesystem.
+        See also the *clear_cache* mount option.
 
 --clear-ino-cache
         remove leftover items pertaining to the deprecated inode map feature
diff --git a/Documentation/ch-mount-options.rst b/Documentation/ch-mount-options.rst
index 3536a5bb..c9b64f19 100644
--- a/Documentation/ch-mount-options.rst
+++ b/Documentation/ch-mount-options.rst
@@ -86,8 +86,19 @@ check_int, check_int_data, check_int_print_mask=<value>
         for more information.
 
 clear_cache
-        Force clearing and rebuilding of the disk space cache if something
-        has gone wrong. See also: *space_cache*.
+        Force clearing and rebuilding of the free space cache if something
+        has gone wrong.
+
+        For free space cache *v1*, this only clears (and, unless *nospace_cache* is
+        used, rebuilds) the free space cache for block groups that are modified while
+        the filesystem is mounted with that option. To actually clear an entire free
+        space cache *v1*, see ``btrfs check --clear-space-cache v1``.
+
+        For free space cache *v2*, this clears the entire free space cache.
+        To do so without requiring to mounting the filesystem, see
+         ``btrfs check --clear-space-cache v2``.
+
+        See also: *space_cache*.
 
 commit=<seconds>
         (since: 3.12, default: 30)
@@ -348,7 +359,8 @@ space_cache, space_cache=<version>, nospace_cache
         implementation, which adds a new b-tree called the free space tree, addresses
         this issue. Once enabled, the *v2* space cache will always be used and cannot
         be disabled unless it is cleared. Use *clear_cache,space_cache=v1* or
-        *clear_cache,nospace_cache* to do so. If *v2* is enabled, kernels without *v2*
+        *clear_cache,nospace_cache* to do so. If *v2* is enabled, and *v1* space
+        cache will be cleared (at the first mount) and kernels without *v2*
         support will only be able to mount the filesystem in read-only mode.
 
         The :doc:`btrfs-check(8)<btrfs-check>` and `:doc:`mkfs.btrfs(8)<mkfs.btrfs>` commands have full *v2* free space
-- 
2.39.0

Re: [PATCH] btrfs-progs: docs: improve space cache documentation

[Christoph Anton Mitterer]

Hey Qu, Dave.

Since the space cache clearing options remain a source for confusion,
I'd have written the above patch (there's also a draft PR) which tries
to improve the documentation a bit.


What remains open (which why the patch/PR is just a draft) is:

a) Does --clear-space-cache v1|v2 really remove (or just clear) any
   space cache from the fs? Or is there even a difference?
 
   Especially, when I ran --clear-space-cache v1|v2 and then mount the
   fs (without neither nospace_cache nor space_cache[=*])...
   will it automatically get a new space cache, if so, which?

b) Is my assumption correct, that with v1, clear_cache itself really
   only clears the space cache for written blocks but not automatically
   rebuilds it.
   I.e. when clear_cache,nospace_cache would be used, its just cleared
   but not rebuild.

c) What about clear_cache and v2? Is it only clearing? Or is it,
   unless nospace_cache is used, right after clearing completely
   rebuilt?


Cheers,
Chris.

Re: Correct way to clear a free space cache file invalid error

[Martin Steigerwald]

Qu Wenruo - 16.01.23, 11:41:07 CET:
> On 2023/1/16 18:22, Martin Steigerwald wrote:
[…]
> > So what is the correct way to clear that error?
> 
> "btrfs check --clear-space-cache v1 <device>".
> 
> Of course you need to unmount the fs in the first place.
> 
> The clear_cache mount option only clears the old cache of touched bgs.
> If the bg is not touched during the runtime, it won't be modified.
> 
> Or, you can go v2 cache, which would drop the v1 cache at the first
> mount.

Thanks. I went to v2 cache during rebooting by putting the mount options 
for that in /etc/fstab temporarily. This appears to have worked.

Ciao,
-- 
Martin