enable `discard` mount option by default in `/etc/fstab`

[adrelanos]

This is for easier trim support in VMs to release the free space on the host operating system after deleting files inside the VM image.

[adrelanos]

Do you know if this causes any issues for any of the file systems? I am only using ext4.

 ext*|exfat|fat|jfs|nilfs2|vfat)

Ideally file systems not supporting it would simply ignore it.

Otherwise this would need a different implementation doing this only where known to be functional, for ext4 with others happy to send patches to opt this in for other file systems.

[mika]

AFAICS all those file systems seem to support discard, so I'd give this a try.

@zeha @jkirk what do you think?

[zeha]

I'm generally ok with enabling trim by default, but:

1) IIRC adding discard to mount options is somewhat deprecated and instead regular fstrim is suggested and iirc enabled by default nowadays 2) it shouldn't be done in the block that is responsible for errors=remount-ro, 3) xfs should also get it, if done at all

[jkirk]

I am not so sure about changing the default mount options.

ext4(5) states:

   discard/nodiscard
          Controls whether ext4 should issue discard/TRIM commands to the underlying block device when blocks are freed. 
          This is useful for SSD devices and sparse/thinly-provisioned LUNs,
          but it is off by default until sufficient testing has been done.

xfs(5) states:

   discard|nodiscard
          Enable/disable the issuing of commands to let the block
          device reclaim space freed by the filesystem.  This is
          useful for SSD devices, thinly provisioned LUNs and
          virtual machine images, but may have a performance impact.

          Note: It is currently recommended that you use the fstrim
          application to discard unused blocks rather than the
          discard mount option because the performance impact of
          this option is quite severe.  For this reason, nodiscard
          is the default.

I could not find a man page for mount.exfat. mount.exfat(8) from exfat-fuse does not mention discard and mount(8) does not mention exfat at all.

mount(8) does not mention discard for jfs.

mount.nilfs2(8) mentions discard, but AFAICS nilfs-tools is needed for mkfs.nilfs2; shouldn't it be installed via install_fs_tools()?

And finally fstrim(8)

Running fstrim frequently, or even using mount -o discard, might negatively affect the lifetime of poor-quality SSD devices. For most desktop and server systems a sufficient trimming frequency is once a week. Note that not all devices support a queued trim, so each trim command incurs a performance penalty on whatever else might be trying to use the disk at the time.

If at all, I'd vote to make discard optional.

[mika]

Re filesystems: I looked into the linux source itself, the manpages don't seem to be fully up2dated, AFAICS.

But as @zeha wrote:

IIRC adding discard to mount options is somewhat deprecated and instead regular fstrim is suggested and iirc enabled by default nowadays

And as stated by @jkirk:

If at all, I'd vote to make discard optional.

I agree, I don't think that's a sane default we should enable by default. And if anyone wants that, this should be taken care of through a custom hook script or alike.

ACK?

[adrelanos]

Yes. Thank you for your consideration!

Closing.