Re: Filesystem corruption when adding a new RAID device (delayed-resync, write-mostly)

["Mateusz Jończyk" <mat.jonczyk@o2.pl>]

W dniu 22.07.2024 o 07:39, Mateusz Jończyk pisze:
> W dniu 20.07.2024 o 16:47, Mateusz Jończyk pisze:
>> Hello,
>>
>> In my laptop, I used to have two RAID1 arrays on top of NVMe and SATA SSD
>> drives: /dev/md0 for /boot (not partitioned), /dev/md1 for remaining data (LUKS
>> + LVM + ext4). For performance, I have marked the RAID component device for
>> /dev/md1 on the SATA SSD drive write-mostly, which "means that the 'md' driver
>> will avoid reading from these devices if at all possible" (man mdadm).
>>
>> Recently, the NVMe drive started having problems (PCI AER errors and the
>> controller disappearing), so I removed it from the arrays and wiped it.
>> However, I have reseated the drive in the M.2 socket and this apparently fixed
>> it (verified with tests).
>>
>>     $ cat /proc/mdstat
>>     Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
>>     md1 : active raid1 sdb5[1](W)
>>           471727104 blocks super 1.2 [2/1] [_U]
>>           bitmap: 4/4 pages [16KB], 65536KB chunk
>>
>>     md2 : active (auto-read-only) raid1 sdb6[3](W) sda1[2]
>>           3142656 blocks super 1.2 [2/2] [UU]
>>           bitmap: 0/1 pages [0KB], 65536KB chunk
>>
>>     md0 : active raid1 sdb4[3]
>>           2094080 blocks super 1.2 [2/1] [_U]
>>          
>>     unused devices: <none>
>>
>> (md2 was used just for testing, ignore it).
>>
>> Today, I have tried to add the drive back to the arrays by using a script that
>> executed in quick succession:
>>
>>     mdadm /dev/md0 --add --readwrite /dev/nvme0n1p2
>>     mdadm /dev/md1 --add --readwrite /dev/nvme0n1p3
>>
>> This was on Linux 6.10.0, patched with my previous patch:
>>
>>     https://lore.kernel.org/linux-raid/20240711202316.10775-1-mat.jonczyk@o2.pl/
>>
>> (which fixed a regression in the kernel and allows it to start /dev/md1 with a
>> single drive in write-mostly mode).
>> In the background, I was running "rdiff-backup --compare" that was comparing
>> data between my array contents and a backup attached via USB.
>>
>> This, however resulted in mayhem - I was unable to start any program with an
>> input-output error, etc. I used SysRQ + C to save a kernel log:
>>
> Hello,
>
> It is possible that my second SSD has some problems and high read activity
> during RAID resync triggered it. Reads from that drive are now very slow (between
> 10 - 30 MB/s) and this suggests that something is not OK.

Hello,

Unfortunately, hardware failure seems not to be the case.

I did test it again on 6.10, twice, and in both cases I got filesystem corruption (but not as severe).

On Linux 6.1.96 it seems to be working well (also did two tries).

Please note: in my tests, I was using a RAID component device with
a write-mostly bit set. This setup does not work on 6.9+ out of the
box and requires the following patch:

commit 36a5c03f23271 ("md/raid1: set max_sectors during early return from choose_slow_rdev()")

that is in master now.

It is also heading into stable, which I'm going to interrupt.

Greetings,
Mateusz