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https://bugs.launchpad.net/bugs/1850540
Title:
multi-zone raid0 corruption
Status in linux package in Ubuntu:
Incomplete
Status in mdadm package in Ubuntu:
New
Status in linux source package in Precise:
Incomplete
Status in mdadm source package in Precise:
New
Status in linux source package in Trusty:
Incomplete
Status in mdadm source package in Trusty:
New
Status in linux source package in Xenial:
Incomplete
Status in mdadm source package in Xenial:
New
Status in linux source package in Bionic:
Incomplete
Status in mdadm source package in Bionic:
New
Status in linux source package in Disco:
Incomplete
Status in mdadm source package in Disco:
New
Status in linux source package in Eoan:
Incomplete
Status in mdadm source package in Eoan:
New
Status in linux source package in Focal:
Incomplete
Status in mdadm source package in Focal:
New
Bug description:
Bug 1849682 tracks the temporarily revert of the fix for this issue,
while this bug tracks the re-application of that fix once we have a
full solution.
Users of RAID0 arrays are susceptible to a corruption issue if:
- The members of the RAID array are not all the same size[*]
- Data has been written to the array while running kernels < 3.14 *and* >=
3.14.
This is because of an change in v3.14 that accidentally changed how data was
written - as described in the upstream commit message:
https://github.com/torvalds/linux/commit/c84a1372df929033cb1a0441fb57bd3932f39ac9
That change has been applied to stable, but we reverted it to fix
1849682 until we have a full solution ready.
To summarize, upstream is dealing with this by adding a versioned
layout in v5.4, and that is being backported to stable kernels - which
is why we're now seeing it. Layout version 1 is the pre-3.14 layout,
version 2 is post 3.14. Mixing version 1 & version 2 layouts can cause
corruption. However, unless a layout-version-aware kernel *created*
the array, there's no way for the kernel to know which version(s) was
used to write the existing data. This undefined mode is considered
"Version 0", and the kernel will now refuse to start these arrays w/o
user intervention.
The user experience is pretty awful here. A user upgrades to the next
SRU and all of a sudden their system stops at an (initramfs) prompt. A
clueful user can spot something like the following in dmesg:
Here's the message which , as you can see from the log in Comment #1,
is hidden in a ton of other messages:
[ 72.720232] md/raid0:md0: cannot assemble multi-zone RAID0 with
default_layout setting
[ 72.728149] md/raid0: please set raid.default_layout to 1 or 2
[ 72.733979] md: pers->run() failed ...
mdadm: failed to start array /dev/md0: Unknown error 524
What that is trying to say is that you should determine if your data -
specifically the data toward the end of your array - was most likely
written with a pre-3.14 or post-3.14 kernel. Based on that, reboot
with the kernel parameter raid0.default_layout=1 or
raid0.default_layout=2 on the kernel command line. And note it should
be *raid0.default_layout* not *raid.default_layout* as the message
says - a fix for that message is now queued for stable:
https://github.com/torvalds/linux/commit/3874d73e06c9b9dc15de0b7382fc223986d75571)
IMHO, we should work with upstream to create a web page that clearly
walks the user through this process, and update the error message to
point to that page. I'd also like to see if we can detect this problem
*before* the user reboots (debconf?) and help the user fix things.
e.g. "We detected that you have RAID0 arrays that maybe susceptible to
a corruption problem", guide the user to choosing a layout, and update
the mdadm initramfs hook to poke the answer in via sysfs before
starting the array on reboot.
Note that it also seems like we should investigate backporting this to
< 3.14 kernels. Imagine a user switching between the trusty HWE kernel
and the GA kernel.
References from users of other distros:
https://blog.icod.de/2019/10/10/caution-kernel-5-3-4-and-raid0-default_layout/
https://www.linuxquestions.org/questions/linux-general-1/raid-arrays-not-assembling-4175662774/
[*] Which surprisingly is not the case reported in this bug - the user
here had a raid0 of 8 identically-sized devices. I suspect there's a
bug in the detection code somewhere.
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