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Hello all<br>
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).<br>
There are a few problems with ext4 and (different ones with) xfs<br>
<br>
I am doing this:<br>
dd if=/dev/zero of=zeroes bs=1M count=1000 conv=fsync<br>
lvs<br>
rm zeroes #optional<br>
dd if=/dev/zero of=zeroes bs=1M count=1000 conv=fsync #again<br>
lvs<br>
rm zeroes #optional<br>
...<br>
dd if=/dev/zero of=zeroes bs=1M count=1000 conv=fsync #again<br>
lvs<br>
rm zeroes<br>
fstrim /mnt/mountpoint<br>
lvs<br>
<br>
On ext4 the problem is that it always reallocates blocks at
different places, so you can see from lvs that space occupation in
the pool and thinlv increases at each iteration of dd, again and
again, until it has allocated the whole thin device (really 100% of
it). And this is true regardless of me doing rm or not between one
dd and the other.<br>
The other problem is that by doing this, ext4 always gets the worst
performance from thinp, about 140MB/sec on my system, because it is
constantly allocating blocks, instead of 350MB/sec which should have
been with my system if it used already allocated regions (see below
compared to xfs). I am on an MD raid-5 of 5 hdds.<br>
I could suggest to add a "thinp mode" mount option to ext4 affecting
the allocator, so that it tries to reallocate recently used and
freed areas and not constantly new areas. Note that mount -o discard
does work and prevents allocation bloating, but it still always gets
the worst write performances from thinp. Alternatively thinp could
be improved so that block allocation is fast :-P (*) <br>
However, good news is that fstrim works correctly on ext4, and is
able to drop all space allocated by all dd's. Also mount -o discard
works.<br>
<br>
On xfs there is a different problem.<br>
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.]<br>
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
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<a href="http://xfs.org/index.php/FITRIM/discard">http://xfs.org/index.php/FITRIM/discard</a>
which declare fstrim and mount -o discard to be working.<br>
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:<br>
[160508.497879] device-mapper: thin: Discard unsupported by data
device (dm-1): Disabling discard passdown.<br>
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.<br>
<br>
(*) Strange thing is that write performance appears to be roughly
the same for default thin chunksize and for 1MB thin chunksize. I
would have expected thinp allocation to be faster with larger thin
chunksizes but instead it is actually slower (note that there are no
snapshots here and hence no CoW). This is also true if I set the
thinpool to not zero newly allocated blocks: performances are about
240 MB/sec then, but again they don't increase with larger
chunksizes, they actually decrease slightly with very large
chunksizes such as 16MB. Why is that?<br>
<br>
Thanks for your help<br>
S.<br>
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