On Wed, May 30, 2018 at 11:51:41AM +0300, Adrian Hunter wrote:
>
> And discards are not enabled by default by mount so, at least on ext4,
> adding "-o discard" is needed in the mount options.
This is because doing discards right away is not always a win from
performance reasons. There are some flash devices where discards are
super-slow and some devices where issuing discards too quickly would
cause them to trigger internal FTL race conditions and turn them into
paperweights.
There was at least one engineer from a Linux distribution who argued
for making discard not the default because back then, there were a lot
of SSD's floating out there (by a manufacturer who thankfully has
since gone bankrupt :-) for which they didn't want to deal with the
support requests from people who were angry about lost data or
destroyed SSD's --- because guess who they would blame?
Also, please note that for many devices it's much better to
periodically run fstrim (once a day or once a week) out of cron.
If someone wants to do a survey of available hardware and demonstrate:
* there is significant value from enabling -o discard by default
(instead of using fstrim)
* there are no (or at least very, very few) devices for which
enabling -o discard results in a major performance regression,
and
* if there are any devices left that turn into paperweights, they can
be managed using blacklists,
I'm certainly open to changing the default. There was, however, a
really good *reason* why the default was chosen to be the way it is.
- Ted
