XFS and trim

[Eric Sandeen]

On 4/2/13 9:30 AM, Ric Wheeler wrote:
> On 04/02/2013 10:26 AM, Steven Haigh wrote:
>> On 03/04/13 01:08, Eric Sandeen wrote:
>>> On 4/1/13 5:26 PM, Steven Haigh wrote:
>>>> On 04/02/2013 12:19 AM, Josef Bacik wrote:
>>>>> On Sat, Mar 30, 2013 at 9:38 PM, Steven Haigh <netwiz at crc.id.au> wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> Firstly, Please CC me into replies as I'm not subscribed to this list.
>>>>>>
>>>>>> I'm trying to confirm that Fedora 18 has enabled trim for XFS filesystems. I
>>>>>> have added discard to the mount options in /etc/fstab - however I do not see
>>>>>> it when looking at the output of 'mount':
>>>
>>> ...
>>>
>>>>>> Can anyone share some insight into this?
>>>>>>
>>>>>
>>>>> IIRC you have to rebuild your initrd so it picks up the new fstab
>>>>> mount options for /, the other options is you should be able to do
>>>>> mount -o remount,discard /.  Thanks,
>>>>
>>>> Hi Josef,
>>>>
>>>> This was the basis of my query - even after a remount with the
>>>> discard option, 'discard' still wasn't present in the options - as
>>>> seen by the mount line quoted above.
>>>
>>> Sorry I didn't see this earlier.  Josef is right; discard is not a remountable
>>> option on xfs, unfortunately (only inode64 and barrier options are remountable
>>> today).  So you could either put the option in the rootflags= kernel
>>> commandline parameter, or rebuild the initrd after adding it to fstab
>>> as Josef suggested so that it's initially mounted with the option.
>>
>> Thanks for the reply Eric. I believe it would be helpful if this was documented. I was expecting it to work the same way as ext4 - being a simple remount would enable the discard option. Once that didn't work - and especially as I didn't get an error in either the syslog or returned from the command, I was starting to think it was a bug in the implementation of TRIM in XFS.
>>
>>> I wonder if we should add something to the remount path to printk when
>>> a non-remountable option is encountered; I might look into that, otherwise
>>> it's a little surprising (although semi-obvious when the problem doesn't
>>> show up in /prcoc/mounts...).
>>
>> I think this is probably the best way to handle things. The first thing I did was to look at the return code from mount (which returned 0 - ie success), then dmesg, then /var/log/messages. As I found nothing, I had no hints on where to look.
>>
>> The wiki page[1] for XFS does state that XFS supports TRIM - and it mentions the changes to /etc/fstab to enable it. It doesn't however mention that it needs to be on the initial mount - or that a remount will not enable it.
>>
>> The best of both worlds would be to have it output to the dmesg - or even fail the mount command (if that is an option?) just like ext4 would if you supply a bogus option set.
>>
>> 1: http://xfs.org/index.php/FITRIM/discard
>>
> 
> Is there any reason that discard cannot be enabled on remount like it is for other file systems? And disabled on remount for completeness?

Discard should be one of the easier ones to change on remount; things like quota are tougher.  I was talking w/ Dave about this just yesterday, and he pointed out that we might need to serialize access to some of the things that would get changed on a remount.

BTW Steven - we often recommend against runtime trim, and suggest fstrim instead, for performance reasons.  Just something to maybe keep in mind and experiment with.

-Eric

> Seems like something that we would definitely want to be consistent across file systems on,
> 
> Ric
>