Re: general thoughts and questions + general and RAID5/6 stability?

["Stefan G. Weichinger" <lists@xunil.at>]

Am 23.09.2014 um 15:38 schrieb Austin S Hemmelgarn:
> On 2014-09-23 09:06, Stefan G. Weichinger wrote:
>> What features for example?
> Well, running 'mkfs.btrfs -O list-all' with 3.16 btrfs-progs gives the
> following list of features:
> mixed-bg        - mixed data and metadata block groups
> extref            - increased hard-link limit per file to 65536
> raid56            - raid56 extended format
> skinny-metadata        - reduced size metadata extent refs
> no-holes        - no explicit hole extents for files
> 
> mixed-bg is something that you generally wouldn't want to change after
> mkfs.
> extref can be enabled online, and the filesystem metadata gets updated
> as-needed, and dosen't provide any real performance improvement (but is
> needed for some mail servers that have HUGE mail-queues)

ok, not needed here

> I don't know anything about the raid56 option, but there isn't any way
> to change it after mkfs.

not needed in my systems.

> skinyy-metadata can be changed online, and the format gets updated on
> rewrite of each metadata block.  This one does provide a performance
> improvement (stat() in particular runs noticeably faster).  You should
> probably enable this if it isn't already enabled, even if you don't
> recreate your filesystem.

So this is done via btrfstune, right?

I will give that a try, for my rootfs it doesn't allow me right now as
it is obviously mounted (live-cd, right?).

> no-holes cannot currently be changed online, and is a very recent
> addition (post v3.14 btrfs-progs I believe) that provides improved
> performance for sparse files (which is particularly useful if you are
> doing things with fixed size virtual machine disk images).

Yes, I have some of those!

> AFAIK there isn't really any 'version number' that has any meaning in
> the superblock (except for telling the kernel that it uses the stable
> disk layout), however, there are flag bits that you can look for
> (compat_flags, compat_ro_flags, and incompat_flags).  I'm not 100%
> certain what each bit means, but on my system with a only 1 month old
> BTRFS filesystem, with extref, skinny-metadata, and no-holes turned on,
> i have compat_flags: 0x0, compat_ro_flags: 0x0, and incompat_flags: 0x16b.
> 
> The other potentially significant thing is that the default
> nodesize/leafsize has changed recently from 4096 to 16384, as that gives
> somewhat better performance for most use cases.

I have the 16k for both already.

Thanks for your explanations, I will dig into it as soon as I find the
time. Seems I have to backup/restore quite some stuff ;-)

Stefan