I *think* this is correct. I'm a user, not a coder. If nothing else it should help you search the archives for clarification :)
In general I think the answer lies around md's superblocks
Can Sar wrote:
Hi,
I am working with a research group that is currently building a tool to automatically find bugs in file systems, and related questions. We are trying to check whether file systems really guarantee the consistencies they promise, and one aspect we are looking at is running them on top of Raid devices. In order to do this we have to understand a few things about the Linux Raid driver/tools and I haven't been able to figure this out from the documention/source code, so maybe you can help me.
I asked this same question a few days ago, but I think I didn't really state it clearly, so let me try to rephrase it.
For Raid 4-6 and for say 5 disks say we write a block that is striped across all the disks, and after 4 of the disks write their part of the block to disk the machine crashes without the 5th disk being able to complete the write. Because of this, the checksum for this stripe should be incorrect, right?
If I understand correctly, the superblocks are updated after each device sync - in this case superblock on disk 5 is different to 1-4. This means that disk 5 is kicked on restart and the array re-syncs using 1-4 to verify or write (not sure) disk 5.
The raid array is a Linux soft raid array set up using mdadm, and none of the disks actually crashed or wrote had any errors during this operation (the machine crashed for some other reason) We then reboot the machine and recreate the array,
this should be 'automatic'
It's not so much 'recreate' (special recovery related meaning in md terminology) as 'start the md device'
then remount it and then try to read the sector that was previously written (that has an incorrect checksum). At what point will the raid driver discover that something is wrong?
as it starts it checks the superblock sequence number, notices that one disk is wrong and not use it.
Will it ever (I feel that it should discover this during the read at latest). Will it try to perform any kind of recovery or simply fail?
so, since the superblock is wrong it starts in 'degraded' mode and resyncs.
How would this change if only 3 of the 5 disk writes made it to disk? Fixing the error would be impossible of course (at least with Raid 4 and 5, i know little about 6), but detection should still work. Will the driver complain?
I don't know what happens if the superblock fails to update on say, 3 out of 6 disks in an array.
The driver _will_ complain.
Newer kernels have an experimental fail facility that you may be interested in:
CONFIG_MD_FAULTY:
The "faulty" module allows for a block device that occasionally returns
read or write errors. It is useful for testing.
HTH
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