Re: Accepting discard to free space from disk images

[Roman Mamedov <rm@romanrm.net>]

On Mon, 09 Feb 2015 10:26:33 -0500
"Devon B." <devon.b@virtualcomplete.com> wrote:

> If you don't mind me asking, what version kernel are you running and are 
> you using any special mount options?

Well actually I did not claim I have working discard through 'loop', but your
post made me curious.

$ sudo dd if=/dev/zero of=100g bs=1M seek=100000 count=1
1+0 records in
1+0 records out
1048576 bytes (1.0 MB) copied, 0.00221052 s, 474 MB/s

$ sudo mkfs.ext4 100g
[...]

$ du -hsc 100g 
133M	100g
133M	total

$ sudo mount -o loop 100g /mnt/tmp1/

(then in a new terminal window):
$ cd /mnt/tmp1/
$ df -h .
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0       96G   60M   92G   1% /mnt/tmp1
$ sudo dd if=/dev/zero of=zerofile bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 0.944377 s, 1.1 GB/s
$ sync

(back to the original one):
$ du -hsc 100g 
1.2G	100g
1.2G	total

(2nd window):
$ sudo fstrim .

(back to the original one):
$ du -hsc 100g 
133M	100g
133M	total

So it does work for me just fine even with 'loop'.
Kernel version 3.14.32, mount options
rw,noatime,nodiratime,compress=zlib,space_cache,inode_cache.

-- 
With respect,
Roman