Default boot/root filesystem

Ric Wheeler rwheeler at
Wed Sep 25 12:12:28 UTC 2013

On 09/24/2013 10:25 PM, Chris Murphy wrote:
> On Sep 24, 2013, at 7:34 PM, William Brown <william at> wrote:
>> Additionally, with the concerns re device shrink. Yes, XFS won't let you
>> shrink, but with thin provision LVM that isn't so much an issue: You
>> just shrink the pv and leave it alone. I would also argue that anyone
>> who is smart enough to shrink their devices, is smart enough to lookup a
>> little bit about lvm thinp.
> I think LVM thinp commands are a lot more esoteric than ext4, btrfs, or xfs resize. But really I think the issue is making it easy for users at install time, so as long as the installer doesn't hit a brick wall being unable to shrink with the default file system, it probably doesn't matter if it's ext4, or XFS on thinp, or btrfs. But right now the installer only supports ext4 resize.
>> I think a really good feature that would go alongside this, would be
>> discards by default so that lvm thinp can relinquish free blocks also.
>> But that's really another topic :)
> So as it turns out some devices get really busy and slow down when TRIM is used, so not all devices are well suited for discard and therefore it's probably not a good idea for it to be set by default. It's also not enabled by default for ssds with btrfs either. And with dm-crypt there's a security concern in that it exposes massive zero'd holes on the device instead of current data being obscured by stale data.
We should not confuse TRIM that gets handled at the device layer (and is a slow, 
non-queued S-ATA command for example) and a dm-thin parsing of that same command 
in software which just updates the metadata that dm-thin maintains.

Using dm-thin is not for free, but I don't know that we have looked carefully at 
the performance profile.


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