No, there is no opened file which is deleted, I did umount and mounted
again and reboot also.
I think I am hitting the below issue, lot of random writes were
happening and the file is not fully written and its sparse file.
Let me try with disabling COW.
file offset 0 offset 302g
[-------------------------prealloced 302g extent----------------------]
(man it's impressive I got all that lined up right)
On disk you have 2 things. First your file which has file extents which says
inode 256, file offset 0, size 302g, offset0, disk bytenr 123, disklen 302g
and then in the extent tree, who keeps track of actual allocated space has this
extent bytenr 123, len 302g, refs 1
Now say you boot up your virt image and it writes 1 4k block to offset
0. Now you have this
[4k][--------------------302g-4k--------------------------------------]
And for your inode you now have this
inode 256, file offset 0, size 4k, offset 0, diskebytenr (123+302g),
disklen 4k inode 256, file offset 4k, size 302g-4k, offset 4k,
diskbytenr 123, disklen 302g
and in your extent tree you have
extent bytenr 123, len 302g, refs 1
extent bytenr whatever, len 4k, refs 1
See that? Your file is still the same size, it is still 302g. If you
cp'ed it right now it would copy 302g of information. But what you
have actually allocated on disk? Well that's now 302g + 4k. Now lets
say your virt thing decides to write to the middle, lets say at offset
12k, now you have thisinode 256, file offset 0, size 4k, offset 0,
diskebytenr (123+302g), disklen 4k
inode 256, file offset 4k, size 3k, offset 4k, diskbytenr 123, disklen 302g
inode 256, file offset 12k, size 4k, offset 0, diskebytenr whatever,
disklen 4k inode 256, file offset 16k, size 302g - 16k, offset 16k,
diskbytenr 123, disklen 302g
and in the extent tree you have this
extent bytenr 123, len 302g, refs 2
extent bytenr whatever, len 4k, refs 1
extent bytenr notimportant, len 4k, refs 1
See that refs 2 change? We split the original extent, so we have 2
file extents pointing to the same physical extents, so we bumped the
ref count. This will happen over and over again until we have
completely overwritten the original extent, at which point your space
usage will go back down to ~302g.
