ENOSPC on almost empty FS

[Stephane CHAZELAS <stephane_chazelas@yahoo.fr>]

Hiya,

trying to restore a FS from a backup (tgz) on a freshly made
btrfs this morning, I got ENOSPCs after about 100MB out of 4GB
have been extracted. strace indicates that the ENOSPC are upon
the open(O_WRONLY).

Restoring with:

mkfs.btrfs /dev/mapper/VG_USB-root
mount -o compress-force,ssd $_ /mnt
cd /mnt
pv ~/backup.tgz | gunzip | sudo bsdtar -xpSf - --numeric-owner


That's on a LVM LV with the PV on a USB key.

If I supspend the job and resume it, then the ENOSPCs go away.

The only way I could restore the backup was via rate limiting
the untar:

zcat ~/backup.tgz | pv -L 3000000 | sudo bsdtar -xpSf - --numeric-owner

That 3MB/s wasn't even enough, as 3 files triggered a ENOSPC,
but I did untar them separately afterwards.

That's with debian's 3.0.0-1 amd64 kernel.

Is that expected behavior due to the way allocation works in
btrfs?

-- 
Stephane