Ok guys, I think I found the bug. One or more bugs.
Pool has chunksize 1MB.
In sysfs the thin volume has: queue/discard_max_bytes and
queue/discard_granularity are 1048576 .
And it has discard_alignment = 0, which based on sysfs-block
documentation is correct (a less misleading name would have been
discard_offset imho).
Here is the blktrace from ext4 fstrim:
...
252,9 17 498 0.030466556 841 Q D 19898368 + 2048 [fstrim]
252,9 17 499 0.030467501 841 Q D 19900416 + 2048 [fstrim]
252,9 17 500 0.030468359 841 Q D 19902464 + 2048 [fstrim]
252,9 17 501 0.030469313 841 Q D 19904512 + 2048 [fstrim]
252,9 17 502 0.030470144 841 Q D 19906560 + 2048 [fstrim]
252,9 17 503 0.030471381 841 Q D 19908608 + 2048 [fstrim]
252,9 17 504 0.030472473 841 Q D 19910656 + 2048 [fstrim]
252,9 17 505 0.030473504 841 Q D 19912704 + 2048 [fstrim]
252,9 17 506 0.030474561 841 Q D 19914752 + 2048 [fstrim]
252,9 17 507 0.030475571 841 Q D 19916800 + 2048 [fstrim]
252,9 17 508 0.030476423 841 Q D 19918848 + 2048 [fstrim]
252,9 17 509 0.030477341 841 Q D 19920896 + 2048 [fstrim]
252,9 17 510 0.034299630 841 Q D 19922944 + 2048 [fstrim]
252,9 17 511 0.034306880 841 Q D 19924992 + 2048 [fstrim]
252,9 17 512 0.034307955 841 Q D 19927040 + 2048 [fstrim]
252,9 17 513 0.034308928 841 Q D 19929088 + 2048 [fstrim]
252,9 17 514 0.034309945 841 Q D 19931136 + 2048 [fstrim]
252,9 17 515 0.034311007 841 Q D 19933184 + 2048 [fstrim]
252,9 17 516 0.034312008 841 Q D 19935232 + 2048 [fstrim]
252,9 17 517 0.034313122 841 Q D 19937280 + 2048 [fstrim]
252,9 17 518 0.034314013 841 Q D 19939328 + 2048 [fstrim]
252,9 17 519 0.034314940 841 Q D 19941376 + 2048 [fstrim]
252,9 17 520 0.034315835 841 Q D 19943424 + 2048 [fstrim]
252,9 17 521 0.034316662 841 Q D 19945472 + 2048 [fstrim]
252,9 17 522 0.034317547 841 Q D 19947520 + 2048 [fstrim]
...
Here is the blktrace from xfs fstrim:
252,12 16 1 0.000000000 554 Q D 96 + 2048 [fstrim]
252,12 16 2 0.000010149 554 Q D 2144 + 2048 [fstrim]
252,12 16 3 0.000011349 554 Q D 4192 + 2048 [fstrim]
252,12 16 4 0.000012584 554 Q D 6240 + 2048 [fstrim]
252,12 16 5 0.000013685 554 Q D 8288 + 2048 [fstrim]
252,12 16 6 0.000014660 554 Q D 10336 + 2048 [fstrim]
252,12 16 7 0.000015707 554 Q D 12384 + 2048 [fstrim]
252,12 16 8 0.000016692 554 Q D 14432 + 2048 [fstrim]
252,12 16 9 0.000017594 554 Q D 16480 + 2048 [fstrim]
252,12 16 10 0.000018539 554 Q D 18528 + 2048 [fstrim]
252,12 16 11 0.000019434 554 Q D 20576 + 2048 [fstrim]
252,12 16 12 0.000020879 554 Q D 22624 + 2048 [fstrim]
252,12 16 13 0.000021856 554 Q D 24672 + 2048 [fstrim]
252,12 16 14 0.000022786 554 Q D 26720 + 2048 [fstrim]
252,12 16 15 0.000023699 554 Q D 28768 + 2048 [fstrim]
252,12 16 16 0.000024672 554 Q D 30816 + 2048 [fstrim]
252,12 16 17 0.000025467 554 Q D 32864 + 2048 [fstrim]
252,12 16 18 0.000026374 554 Q D 34912 + 2048 [fstrim]
252,12 16 19 0.000027194 554 Q D 36960 + 2048 [fstrim]
252,12 16 20 0.000028137 554 Q D 39008 + 2048 [fstrim]
252,12 16 21 0.000029524 554 Q D 41056 + 2048 [fstrim]
252,12 16 22 0.000030479 554 Q D 43104 + 2048 [fstrim]
252,12 16 23 0.000031306 554 Q D 45152 + 2048 [fstrim]
252,12 16 24 0.000032134 554 Q D 47200 + 2048 [fstrim]
252,12 16 25 0.000032964 554 Q D 49248 + 2048 [fstrim]
252,12 16 26 0.000033794 554 Q D 51296 + 2048 [fstrim]
As you can see, while ext4 correctly aligns the discards to 1MB, xfs
does not.
It looks like an fstrim or xfs bug: they don't look at discard_alignment
(=0 ... a less misleading name would be discard_offset imho) +
discard_granularity (=1MB) and they don't base alignments on those.
Clearly the dm-thin cannot unmap anything if the 1MB regions are not
fully covered by a single discard. Note that specifying a large -m
option for fstrim does NOT widen the discard messages above 2048, and
this is correct because discard_max_bytes for that device is 1048576 .
If discard_max_bytes could be made much larger these kind of bugs could
be ameliorated, especially in complex situations like layers over
layers, virtualization etc.
Note that also in ext4 there are parts of the discard without the 1MB
alignment as seen with blktrace (out of my snippet), so this also might
need to be fixed, but most of it is aligned to 1MB. In xfs there are no
parts aligned to 1MB.
Now, another problem:
Firstly I wanted to say that in my original post I missed the
conv=notrunc for dd: I complained about the performances because I
expected the zerofiles would have been rewritten in-place without block
re-provisioning by dm-thin, but clearly without conv=notrunc this was
not happening. I confirm that with conv=notrunc performances are high at
the first rewrite, also in ext4, and occupied space in the thin volume
does not increase at every rewrite by dd.
HOWEVER
by NOT specifying conv=notrunc, the behaviour of dd / ext4 / dm-thin is
different if skip_block_zeroing is specified or not. If
skip_block_zeroing is not specified (provisioned blocks are pre-zeroed)
the space occupied by dd truncate + rewrite INCREASES at every rewrite,
while if skip_block_zeroing is NOT specified, dd truncate + rewrite DOES
NOT increase space occupied on the thin volume. Note: try this on ext4,
not xfs.
This looks very strange to me. The only reason I can think of is some
kind of cooperative behaviour of ext4 with the variable
dm-X/queue/discard_zeroes_data
which is different in the two cases. Can anyone give an explanation or
check if this is the intended behaviour?
And still an open question is: why the speed of provisioning new blocks
does not increase with increasing chunk size (64K --> 1MB --> 16MB...),
not even when skip_block_zeroing has been set and there is no CoW?
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