default mount options 3.10

default mount options 3.10

[David Kofler]

Hi,
can someone tell me which mount options are included in "defaults" mount 
option? Couldn't find this in BTRFS Wiki. I'm using Debian Wheezy 7.1 
and Linux kernel 3.10.6.
Thanks in advance.

Best Regards
David

Re: default mount options 3.10

[Xavier Bassery]

On Fri, 23 Aug 2013 11:38:56 +0200
David Kofler <dkofler92@googlemail.com> wrote:

> Hi,
> can someone tell me which mount options are included in "defaults"
> mount option? Couldn't find this in BTRFS Wiki. I'm using Debian
> Wheezy 7.1 and Linux kernel 3.10.6.
> Thanks in advance.

Hi,
you've looked at the wrong place.
>From mount man page:

FILESYSTEM INDEPENDENT MOUNT OPTIONS
defaults : Use default options: rw, suid, dev, exec, auto, nouser, async

You can also have a look here [1].

Best regards,
Xavier

[1] https://wiki.debian.org/fstab#Field_definitions

Re: default mount options 3.10

[Martin Steigerwald]

Am Freitag, 23. August 2013, 12:29:42 schrieb Xavier Bassery:
> On Fri, 23 Aug 2013 11:38:56 +0200
> 
> David Kofler <dkofler92@googlemail.com> wrote:
> > Hi,
> > can someone tell me which mount options are included in "defaults"
> > mount option? Couldn't find this in BTRFS Wiki. I'm using Debian
> > Wheezy 7.1 and Linux kernel 3.10.6.
> > Thanks in advance.
> 
> Hi,
> you've looked at the wrong place.
> From mount man page:
> 
> FILESYSTEM INDEPENDENT MOUNT OPTIONS
> defaults : Use default options: rw, suid, dev, exec, auto, nouser, async
> 
> You can also have a look here [1].

That are just VFS options, no BTRFS specific ones.

A good way is to look at output of "mount" and "cat /proc/mounts". It can 
differ from situation as well, for example SSD or not.

But some hints at BTRFS default options are also on (search for "default"):

https://btrfs.wiki.kernel.org/index.php/Mount_options

I don´t know wether there is a complete documentation on the defaults.

> [1] https://wiki.debian.org/fstab#Field_definitions
-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7

Re: default mount options 3.10

[Duncan]

Martin Steigerwald posted on Fri, 23 Aug 2013 14:58:07 +0200 as excerpted:

> Am Freitag, 23. August 2013, 12:29:42 schrieb Xavier Bassery:
>> On Fri, 23 Aug 2013 11:38:56 +0200
>> 
>> David Kofler <dkofler92@googlemail.com> wrote:
>> > Hi,
>> > can someone tell me which mount options are included in "defaults"
>> > mount option? Couldn't find this in BTRFS Wiki. I'm using Debian
>> > Wheezy 7.1 and Linux kernel 3.10.6.
>> > Thanks in advance.
>> 
>> Hi,
>> you've looked at the wrong place.
>> From mount man page:
>> 
>> FILESYSTEM INDEPENDENT MOUNT OPTIONS defaults : Use default options:
>> rw, suid, dev, exec, auto, nouser, async
> 
> That are just VFS options, no BTRFS specific ones.
> 
> A good way is to look at output of "mount" and "cat /proc/mounts". It
> can differ from situation as well, for example SSD or not.
> 
> But some hints at BTRFS default options are also on (search for
> "default"):
> 
> https://btrfs.wiki.kernel.org/index.php/Mount_options

FWIW, space_cache seems to be the default now, and for ssds (detected if 
sysfs has a device rotation value of zero), ssd.  There's some others 
that are the normal default now, but aren't listed (flushoncommit is 
one), as running with those features disabled can increase performance 
but also dramatically increases chance of data loss, so it's not 
recommended.

Compression options are the big ones you may want to add, but they aren't 
default as usage is too varied for them to be a sane default.  And of 
course ssd if it applies and isn't detected, and possibly ssd_spread.

I recommend autodefrag for general use, as well, but you'll want to have 
it enabled when you first start copying data to the filesystem, and some 
distros don't enable it by default and setup a system on btrfs, which 
means there's fragmentation to begin with, and performance will probably 
be bad for awhile (even on newly setup systems!) when the option is first 
enabled as a result.  You can readup on defrag on the wiki for a defrag 
command that should fix the problem, but unfortunately the command isn't 
as straightforward as one might expect.  (Also note that if compression 
is enabled, a compressed file over a certain size, 128 MiB IIRC, but I'm 
not sure if that's compressed or uncompressed size, will appear 
fragmented to filefrag no matter what.  That's an artifact of the way 
btrfs handles compression.)

Inode_cache LOOKS good, but beware; people have reported problems with it 
and unsafe shutdowns or the like.  So it's probably best to avoid it 
unless you really need it for your workload and judge it to be worth the 
risk.  That's why it's not the default, yet.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

Re: default mount options 3.10

[Chris Murphy]

On Aug 24, 2013, at 6:23 AM, Duncan <1i5t5.duncan@cox.net> wrote:

> I recommend autodefrag for general use, as well, but you'll want to have 
> it enabled when you first start copying data to the filesystem, and some 
> distros don't enable it by default and setup a system on btrfs, which 
> means there's fragmentation to begin with, and performance will probably 
> be bad for awhile (even on newly setup systems!) when the option is first 
> enabled as a result.  You can readup on defrag on the wiki for a defrag 
> command that should fix the problem, but unfortunately the command isn't 
> as straightforward as one might expect.

At least Fedora's installer developers have made it pretty clear they don't intend to use non-default mount options like this. So if autodefrag is really that important for general use, it needs to be part of the default mount options. And quite honestly, with as many changes that happen with btrfs, I think it's better if btrfs developers are making the decisions to add/remove what they think should be current default mount options for general purpose, rather than installer developers or users. Especially when the command isn't straightforward.


Chris Murphy

Re: default mount options 3.10

[Duncan]

Duncan posted on Sat, 24 Aug 2013 12:23:18 +0000 as excerpted:

> Martin Steigerwald posted on Fri, 23 Aug 2013 14:58:07 +0200 as
> excerpted:
> 
>> Am Freitag, 23. August 2013, 12:29:42 schrieb Xavier Bassery:
>>> On Fri, 23 Aug 2013 11:38:56 +0200
>>> 
>>> David Kofler <dkofler92@googlemail.com> wrote:

[mount-option discussion]

One that I omitted in my previous discussion because it's not btrfs 
specific, but that makes a rather larger difference than normal on btrfs, 
especially with lots of snapshots, is noatime.  Unfortunately, for legacy 
reasons, that can't be the default, but unless you're running mutt (which 
needs atime to track read messages except with mbox) or something 
similar, there really are very few even half-modern apps that require 
atime, and it really is best to run with noatime, saving the constant 
access-time update writes.

The general kernel default is now relatime, which is a compromise that 
works reasonably well on most filesystems, but even that tends to trigger 
way more atime updates and thus de-dupped metadata when dealing with 
btrfs snapshots than would be optimal.

Similarly, limited-write-cycle SSDs will do best with noatime.  If you 
happen to be running btrfs on ssd as I am, well...

So just run with noatime unless you are running something (like mutt) 
that is known to need atimes, and then, preferably limit it to a 
particular dedicated filesystem, perhaps choosing a filesystem other than 
btrfs or at least a limited-snapshot btrfs.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman