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space_cache=v2 on root fs

DISCUSSION Markus Trippelsdorf 2 participants 3 messages Jan 18, 2016 Btrfs file system

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Markus Trippelsdorf

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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

markus@x4 tmp % mount
/dev/sdc3 on / type btrfs (rw,noatime,compress=lzo,ssd,noacl,space_cache=v2,subvolid=5,subvol=/) devtmpfs on /dev type devtmpfs (rw,relatime,size=4080208k,nr_inodes=1020052,mode=755) proc on /proc type proc (rw,relatime)
sys on /sys type sysfs (rw,relatime)
devpts on /dev/pts type devpts (rw,relatime,gid=5,mode=620)
none on /dev/shm type tmpfs (rw,relatime,size=409600k)
/dev/sdb2 on /var type btrfs (rw,relatime,compress=lzo,noacl,space_cache=v2,subvolid=5,subvol=/) /dev/sda3 on /var/tmp type btrfs (rw,relatime,compress=lzo,ssd,noacl,space_cache=v2,subvolid=5,subvol=/)

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Markus
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#1 lore raw
Quoted 29 lines · Markus Trippelsdorf

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
Quoted 7 lines

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...

Signature · 1 line
Markus
#2 lore raw

Am Mon, 18 Jan 2016 20:12:43 +0100
schrieb Markus Trippelsdorf <[email protected]>:

Quoted 37 lines · Markus Trippelsdorf
Quoted 36 lines

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.

Quoted 8 lines
Quoted 5 lines
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.

Signature · 4 lines
Regards,
Kai

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