Re: How to reliably measure fs usage with reflinks enabled?

[Dave Chinner <david@fromorbit.com>]

On Tue, May 15, 2018 at 01:37:32AM +0200, Tarik Ceylan wrote:
> Am 2018-05-15 00:57, schrieb Dave Chinner:
> >On Mon, May 14, 2018 at 05:02:53PM -0500, Eric Sandeen wrote:
> >>
> >>
> >>On 5/14/18 3:02 PM, Tarik Ceylan wrote:
> >>> How can one reliably measure filesystem usage on partitions that were compiled with -m reflink=1 ?
> >>> Here are some numbers i am measuring with df -h (on different partitions holding the same data):
> >>> 7.7G of 36G  (-b size=512  -m crc=0 )
> >>> 8.6G of 36G  (-b size=4096 -m crc=1 )
> >>
> >>8x larger inodes will take 8x more space, but you didn't say how many
> >>inodes you have allocated.
> >>
> >>> 11G  of 36G  (-b size=1024 -m crc=1,reflink=1,rmapbt=1 -i sparse=1 )
> >>> 32G  of 864G (-b size=4096 -m crc=1,reflink=1 )
> >>
> >>In that last case, you have a wildly different total fs size, so
> >>probably
> >>no fair comparison here either.
> >>
> >>The reverse mapping btree also takes up space.  You're turning
> >>too many
> >>knobs at once.  ;)
> 
> Thanks,
> here's a test in which i only compare reflink=0 to reflink=1, all other
> variables being the same:
> 
> mkfs.xfs -f -m reflink=0 /dev/sdc4
> meta-data=/dev/sdc4              isize=512    agcount=4,
> agsize=58687982 blks
>          =                       sectsz=512   attr=2, projid32bit=1
>          =                       crc=1        finobt=1, sparse=0,
> rmapbt=0, reflink=0
> data     =                       bsize=4096   blocks=234751926,
> imaxpct=25
>          =                       sunit=0      swidth=0 blks
> naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
> log      =internal log           bsize=4096   blocks=114624, version=2
>          =                       sectsz=512   sunit=0 blks, lazy-count=1
> realtime =none                   extsz=4096   blocks=0, rtextents=0
> 
> "df -h" shows a usage of 8.8G of 896G
> 
> mkfs.xfs -f -m reflink=1 /dev/sdc4
> [output same as before except the reflink parameter]
> 15G of 896G

So the reflink code reserved ~7GB of space in the filesystem (less
than 1%) for it's own reflink related metadata if it ever needs it.
It hasn't used it yet but we need to make sure that it's available
when the filesystem is near ENOSPC. Hence it's considered used space
because users cannot store user data in that space.

The change I plan to make is to reduce the user reported filesystem
size rather than account for it as used space. IOWs, you'd see a
filesystem size of 889G instead of 896G, but have only 8.8GB used.
It means exactly the same thingi and will behave exactly the same
way, it's just a different space accounting technique....

> >Also, we reserve a lot of space for reflink/rmapbt metadata that
> >isn't actually used, so you're not actually using any more space
> >than the "-b size=4096 -m crc=1" case. I have plans for hiding that
> >reservation from users so that we don't get questions like this....
> 
> That should resolve my confusion. Sorry to have bothered, but it's
> kind of an obvious question.

It's the sort of "obvious question" which almost no-one has asked us
about... :)

> To get back to my original question - can i assume  "df" to be a
> reliable
> way of measuring fs usage going forward (after the change you mention),

df is reliable now, regardless of any change we make in the future.

> or will specialized tools be necessary as is the case with btrfs?

No - df works and it should always work. We try to learn from other
people's mistakes, not just our own... :)

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com