tune2fs can't be used on a mounted ext4, or...?

[Roman Mamedov <rm@romanrm.ru>]

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Hello,

Here's what happened. I reshaped my RAID5 to a RAID6 and a larger disk count.
Then I ran "resize2fs /dev/md0". It successfully enlarged the filesystem
online (that took a couple of hours, I think).

Since my count of disks changed, I thought I should change the array stripe
width. Then, while still not unmounting the filesystem, I did:

  tune2fs -E stripe_width=48 /dev/md0

Then I started copying files from another array to the one on which the above
operations were conducted. I did:

  cp -Rp /mnt/array1/data/* /mnt/array2/new-data/

This completed successfully. No errors on the console, silence in dmesg.

Then I thought I'd verify the destination, just to be sure. Luckily, I have
checksums stored in almost every directory as .SFV files (created with "cfv").

And some checksums did not match. On a closer investigation, it appears like
some files deep down in the directory tree (and with a rare occurence,
something like one file in a thousand) were truncated during copying. E.g.
they'd have size of 188 MB instead of 349 MB, or 128 MB instead of 170 MB.
Some files (originally less than 1 MB in size) just had zero-length on the
destination.

Other than these truncations, there is NO corruption of data inside any of the
files. Which kinda rules out CRC-style errors in controller/disk/cable.

So.. this is completely puzzling to me, and I suspect either a kernel bug, or
my mistake, and tune2fs cannot modify a mounted FS (however it did NOT show any
warning that the FS was mounted, and if this is the case, it should've
absolutely refused to operate on a mounted FS).

Any ideas? :)

-- 
With respect,
Roman

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