[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

Re: [dm-devel] A thin-p over 256 GiB fails with I/O errors with non-power-of-two chunk



Dne 21.1.2013 19:49, Mike Snitzer napsal(a):
On Fri, Jan 18 2013 at  5:19am -0500,
Daniel Browning <db kavod com> wrote:

Why do I get the following error, and what should I do about it? When I
create a raid0 md with a non-power-of-two chunk size (e.g. 1152K instead of
512K), then create a thinly-provisioned volume that is over 256 GiB, I get
the following dmesg error when I try to create a file system on it:

     "make_request bug: can't convert block across chunks or bigger than 1152k 4384 127"

This bubbles up to mkfs.xfs as

     "libxfs_device_zero write failed: Input/output error"

What I find interesting is that it seems to require all three conditions
(chunk size, thin-p, and >256 GiB) in order to fail. Without those, it seems
to work fine:

     * Power-of-two chunk (e.g. 512K), thin-p vol, >256 GiB? Works.
     * Non-power-of-two chunk (e.g. 1152K), thin-p vol, <256 GiB? Works.
     * Non-power-of-two chunk (e.g. 1152K), regular vol, >256 GiB? Works.
     * Non-power-of-two chunk (e.g. 1152K), thin-p vol, >256 GiB? FAIL.

Attached is a self-contained test case to reproduce the error, version
numbers, and an strace. Thank you in advance,
--
Daniel Browning
Kavod Technologies

Appendix A. Self-contained reproduce script
===========================================================
dd if=/dev/zero of=loop0.img bs=1G count=150; losetup /dev/loop0 loop0.img
dd if=/dev/zero of=loop1.img bs=1G count=150; losetup /dev/loop1 loop1.img
mdadm --create /dev/md99 --verbose --level=0 --raid-devices=2 \
       --chunk=1152K /dev/loop0 /dev/loop1
pvcreate /dev/md99
vgcreate test_vg /dev/md99
lvcreate --size 257G --type thin-pool --thinpool test_thin_pool test_vg
lvcreate --virtualsize 257G --thin test_vg/test_thin_pool --name test_lv
mkfs.xfs /dev/test_vg/test_lv

# That is where the error occurs. Next is cleanup.
lvremove -f /dev/test_vg/test_lv
lvremove -f /dev/mapper/test_vg-test_thin_pool
vgremove -f test_vg
pvremove /dev/md99
mdadm --stop /dev/md99
mdadm --zero-superblock /dev/loop0 /dev/loop1
losetup -d /dev/loop0 /dev/loop1
rm loop*.img

Limits of the raid0 device (/dev/md99):
cat /sys/block/md99/queue/minimum_io_size
1179648
cat /sys/block/md99/queue/optimal_io_size
2359296

Limits of the thin-pool device (/dev/test_vg/test_thin_pool):
cat /sys/block/dm-9/queue/minimum_io_size
512
cat /sys/block/dm-9/queue/optimal_io_size
262144

Limits of the thin-device device (/dev/test_vg/test_lv):
cat /sys/block/dm-10/queue/minimum_io_size
512
cat /sys/block/dm-10/queue/optimal_io_size
262144

I notice that lvcreate is not using a thin-pool chunksize that matches
the raid0's chunksize (just uses the lvm2 default of 256K).

Switching the thin-pool lvcreate to use --chunksize 1152K at least
enables me to format the filesystem.

And both the thin-pool and thin device have an optimal_io_size that
matches the chunk_size of the underlying raid volume:

cat /sys/block/dm-9/queue/optimal_io_size
1179648
cat /sys/block/dm-10/queue/optimal_io_size
1179648

I'm still investigating the limits issue when --chunksize 1152K isn't
used for the thin-pool lvcreate.

Just a comment for the selection of thin chunksize here -

I think it has couple aspects here - by default (unless changed via
lvm.conf {allocation/thin_pool_chunk_size}) it is targeting for 64K
and scales chunksize up to fit thin metadata within 128MB.
(compiled in as DEFAULT_THIN_POOL_OPTIMAL_SIZE)
So lvm2 here scaled from 64k to 256k in multiTB case.

lvcreate currently doesn't look out for geometry of underlying PV(s) during its allocation (somewhat chicken-egg problem) - yet there are possible ways to try to put this into equation - thought it might not be actually wanted by the user - since for snapshots the smaller chunksize is more usable
(>1MB is quite a lot here IMHO) - but it probably worth some thinking.

Zdenek


[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]