Raid1 mismatches maybe associated to free space
Some time ago there was a discussion on how it could happen that sometimes people see mismatches on MD raid1.
Some people reported to see a lot of them, especially in raid1 directly on partitions and without LVM on top.
I had never seen one, but note I usually use LVM.
Today I saw this happening on a machine of mine without lvm so I could do a test that has been on my mind for a while.
Firstly, the machine is a very particular setup:
- It boots from 2 x 4GB usb flash keys in raid1 (2-way raid1) and has no HDD.
- The raid1 was directly on partitions and no LVM on it
- The usb flash keys probably do not honour sync.
- It had a few abrupt poweroffs in its life.
- The MD bitmap was active, and has been active most of its life. But probably it experienced at least 1 abrupt poweroff with bitmap active and 1 abrupt poweroff with bitmap inactive.
When I did the "check" today I saw 1024 mismatch_cnt. So I took the occasion to do a test it was on my mind for a while:
dd if=/dev/zero of=fillitup bs=1M ; sync ; rm fillitup
wait for it, then do again check, wait for it, and then read mismatch_cnt again.
The mismatch_cnt was zero this time!
This means afaiu that the mismatches were on the free space.
Now quoting Goswin Von Bredelow as of 01/22/2010 07:13 PM:
Quoted 9 lines
There is some unknown corruption going on with raid1 that causes mismatches but it is believed that it will never occur on any used block. Swapping is a likely cause.
Any swap device on the raid? Try turning that off.
If that doesn't help try umounting filesystems or remounting RO.MfG
Goswin
...exactly... this seemed to be my case.
Clearly I don't know if it's always the case.
So people experiencing corruption: would you also do this test and report? It's easy and takes little time...
Thank you