Bug 1368595

Description of problem:

This commit adds 'e2fsck -E discard' which doesn't do what it's intended to do except on SSD's and LVM thin p.

https://github.com/rhinstaller/lorax/pull/82/commits/ba4218ea982790b116926b869bf114fbd6c42668

Eric Sandeen and I tested it today, and it definitely does not punch out blocks on loop mounted files. Whereas fstrim on the mounted volume does work.

The following images were fallocated, formatted ext4, and a single ~500MiB file was copied to each fs and then deleted. And then as the name suggests discard.img = e2fsck -E discard, and fstrim.img = fstrim <mountpoint>, and nodiscardortrim.img has had neither e2fsck or fstrim applied.


[root@f24s 0]# ls -lsh /mnt/0
total 1.3G
571M -rw-r--r--. 1 root root 5.0G Aug 19 10:57 discard.img
130M -rw-r--r--. 1 root root 5.0G Aug 19 10:58 fstrim.img
571M -rw-r--r--. 1 root root 5.0G Aug 19 10:57 nodiscardortrim.img
[root@f24s 0]# gzip discard.img
[root@f24s 0]# gzip fstrim.img
[root@f24s 0]# gzip nodiscardortrim.img

# Then compress everything with gzip.

[root@f24s 0]# ls -lsh
total 863M
429M -rw-r--r--. 1 root root 429M Aug 19 10:57 discard.img.gz
5.0M -rw-r--r--. 1 root root 5.0M Aug 19 10:58 fstrim.img.gz
429M -rw-r--r--. 1 root root 429M Aug 19 10:57 nodiscardortrim.img.gz


So pretty clearly e2fsck -E discard isn't doing what's intended. Use fstrim instead.



Version-Release number of selected component (if applicable):


How reproducible:
Always


Steps to Reproduce:
1. e2fsck -E discard on anything other than LVM thinp or SSD.
2.
3.

Actual results:

Nothing.

Expected results:

Maybe it should punch out those blocks so the backing file gets sparse fragments, and thus saves space. But it doesn't and I guess the e2fsck code is insane, unclear whether this will be fixed especially since fstrim does work. So just use fstrim instead, plus this command is file system agnostic should some other file system end up being used for the resulting rootfs.img.


Additional info:


The main difference is that fstrim needs to happen before umount.