Re: Ext4 and xfs problems in dm-thin on allocation and discard

[Dave Chinner <david@fromorbit.com>]

On Mon, Jun 18, 2012 at 11:33:50PM +0200, Spelic wrote:
> Hello all
> I am doing some testing of dm-thin on kernel 3.4.2 and latest lvm
> from source (the rest is Ubuntu Precise 12.04).
> There are a few problems with ext4 and (different ones with) xfs
> 
> I am doing this:
> dd if=/dev/zero of=zeroes bs=1M count=1000 conv=fsync
> lvs
> rm zeroes #optional
> dd if=/dev/zero of=zeroes bs=1M count=1000 conv=fsync  #again
> lvs
> rm zeroes #optional
> ...
> dd if=/dev/zero of=zeroes bs=1M count=1000 conv=fsync  #again
> lvs
> rm zeroes
> fstrim /mnt/mountpoint
> lvs

[snip ext4 problems]

> On xfs there is a different problem.
> Xfs apparently correctly re-uses the same blocks so that after the
> first write at 140MB/sec, subsequent overwrites of the same file are
> at full speed such as 350MB/sec (same speed as with non-thin lvm),
> and also you don't see space occupation going up at every iteration
> of dd, either with or without rm in-between the dd's. [ok actually
> now retrying it needed 3 rewrites to stabilize allocation...
> probably an AG count thing.]

That's just a characteristic of the allocation algorithm. It's not
something that you see in day-to-day operation of the filesystem,
though, because you rarely remove and rewrite a file like this
repeatedly. So in the real world, performance will be more like ext4
when you are running workloads where you actually store data for
longer than a millisecond...

Expect that the 140MB/s number is the normal performance case,
because as soon as you take a snapshot, the overwrite requires new
blocks to be allocated in dm-thinp. You don't get thinp for nothing
- it has an associated performance cost as you are now finding
out....

> However the problem with XFS is that discard doesn't appear to work.
> Fstrim doesn't work, and neither does "mount -o discard ... + rm
> zeroes" . There is apparently no way to drop the allocated blocks,
> as seen from lvs. This is in contrast to what it is written here
> http://xfs.org/index.php/FITRIM/discard which declare fstrim and
> mount -o discard to be working.

I don't see why it wouldnt be if the underlying device supports it.
Have you looked at a block trace or an xfs event trace to see if
discards are being issued by XFS?

Are you getting messages like:

XFS: (dev) discard failed for extent [0x123,4096], error -5

in dmesg, or is fstrim seeing errors returned from the trim ioctl?

> Please note that since I am above MD raid5 (I believe this is the
> reason), the passdown of discards does not work, as my dmesg says:
> [160508.497879] device-mapper: thin: Discard unsupported by data
> device (dm-1): Disabling discard passdown.
> but AFAIU, unless there is a thinp bug, this should not affect the
> unmapping of thin blocks by fstrimming xfs... and in fact ext4 is
> able to do that.

Does ext4 report that same error?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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