Re: space_cache=v2 on root fs

Am Mon, 18 Jan 2016 20:12:43 +0100
schrieb Markus Trippelsdorf [off-list ref]:

On 2016.01.18 at 19:27 +0100, Markus Trippelsdorf wrote:

quoted

Hi,

while testing the new space_cache=v2 mount option it looks like my
root fs doesn't automatically create the new space cache.

I'm using the following /etc/fstab:

UUID=7ef56edc-673e-452e-9e05-3fabf074168b       /
btrfs           noatime,compress=lzo,space_cache=v2             0 0
UUID=6b7d75bf-3d12-409c-88ed-500ac6b78510       /var
btrfs           compress=lzo,space_cache=v2                     0 0
UUID=24cee7e4-9093-4f86-aa73-d8a59d7f3c6c       /var/tmp
btrfs           compress=lzo,space_cache=v2                     0 0

During first mount the space cache was created for /var (/dev/sdb2)
and /var/tmp (/dev/sda3), but apparently not for / (/dev/sdc3):

Jan 18 18:48:18 x4 kernel: BTRFS info (device sdb2): enabling free
space tree Jan 18 18:48:18 x4 kernel: BTRFS info (device sdb2):
using free space tree Jan 18 18:48:18 x4 kernel: BTRFS: creating
free space tree Jan 18 18:48:18 x4 kernel: BTRFS info (device
sdb2): setting 1 ro feature flag Jan 18 18:48:18 x4 kernel: BTRFS:
checking UUID tree Jan 18 18:48:18 x4 kernel: BTRFS info (device
sda3): enabling free space tree Jan 18 18:48:18 x4 kernel: BTRFS
info (device sda3): using free space tree Jan 18 18:48:18 x4
kernel: BTRFS: has skinny extents Jan 18 18:48:18 x4 kernel: BTRFS:
detected SSD devices, enabling SSD mode Jan 18 18:48:18 x4 kernel:
BTRFS: creating free space tree Jan 18 18:48:18 x4 kernel: BTRFS
info (device sda3): setting 1 ro feature flag Jan 18 18:48:18 x4
kernel: BTRFS: checking UUID tree Jan 18 18:48:22 x4 kernel: BTRFS
info (device sdc3): enabling free space tree Jan 18 18:48:22 x4
kernel: BTRFS info (device sdc3): using free space tree

During subsequent boots the "enabling free space tree" message is
repeated every time for the root fs.

Jan 18 18:53:48 x4 kernel: BTRFS info (device sdb2): using free
space tree Jan 18 18:53:48 x4 kernel: BTRFS info (device sda3):
using free space tree Jan 18 18:53:52 x4 kernel: BTRFS info (device
sdc3): enabling free space tree Jan 18 18:53:52 x4 kernel: BTRFS
info (device sdc3): using free space tree  

To answer my own question. Adding "rw rootflags=space_cache=v2" to the
GRUB kernel entry fixes the issue...

This is because the contents of /etc/fstab are not available during
first mount at boot time - for obvious reasons. Actually, the entry for
rootfs in /etc/fstab is more or less a no-op and only there to satisfy
some legacy tools. Some initramfs generators may also use this to
construct a proper grub cmdline. At least dracut seems to rely on other
mechanics, tho. So it's probably obsolete.

There's only one point when it is used: During remount of the fs during
system initialization. But only some mount options can change during
remount, I imagine space_cache is not part of this.

Due to this remount, it is best to keep both in sync: grub cmdline and
fstab.

I think systemd has some efforts of obsoleting the rootfs line from
fstab, however I'm not sure what's the progress here. I still use it
because last time I tried removing it, it had strange effects.

-- 
Regards,
Kai

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