Neil Brown wrote:
On Monday April 2, gmccullagh@xxxxxxxxx wrote:
Neil's post here suggests either this is all normal or I'm seriously up the
creek.
http://www.mail-archive.com/linux-raid@xxxxxxxxxxxxxxx/msg07349.html
My questions:
1. Should I be worried or is this normal? If so can you explain why the
number is non-zero?
Probably not too worried.
Is it normal? I'm not really sure what 'normal' is. I'm beginning to
think that it is 'normal' to get strange errors from disk drives, by
maybe I have a jaded perspective.
If you have a swap-partition or a swap-file on the device then you
should consider it normal. If not, then it is much less likely but
still possible.
2. Should I repair, fsck, replace a disk, something else?
'repair' is probably a good idea.
'fsck' certainly wouldn't hurt and might show something, though I
suspect it will find the filesystem to be structurally sound.
I wouldn't replace the disk on the basis on a single difference report
from mismatch_cnt. I don't know what the SMART message means so I
don't know if that suggests that the drive needs to be replaced.
3. Can someone explain how this quote can be true:
"Though it is less likely, a regular filesystem could still (I think)
genuinely write different data to difference devices in a raid1/10."
when I thought the point of RAID1 was that the data should be the same on
both disks.
Suppose I memory-map a file and often modify the mapped memory.
The system will at some point decide to write that block of the file
to the device. It will send a request to raid1, which will send one
request each to two different devices. They will each DMA the data
out of that memory to the controller at different times so they could
quite possibly get different data (if I changed the mapped memory
between those two DMA request). So the data on the two drives in a
mirror can easily be different. If a 'check' happens at exactly this
time it will notice.
Normally that block will be written out again (as it is still 'dirty')
and again and again if necessary as long as I keep writing to the
memory. Once I stop writing to the memory (e.g. close the file,
unmount the filesystem) a final write will be made with the same data
going to both devices. During this time we will never read that block
from the filesystem, so the filesystem will never be able to see any
difference between the two devices in a raid1.
So: if you are actively writing to a file while 'check' is running on
a raid1, it could show up as a difference in mismatch_cnt. But you
have to get the timing just right (or wrong).
I think it is possible in the above scenario to truncate the file
while a write is underway but with new data in memory. If you do
this, the system might not write out that last 'new' data, so the last
write to the particular block on storage may have written different
data to the two different drives, and this difference will not be
corrected by the filesystem e.g on unmount. Note that the inconsistent
data will never be read by the filesystem (the file has been
truncated, remember) so there is no risk of data corruption.
In this case the difference could remain for some time until later
when a 'check' or 'repair' notices it.
Some time ago I suggested that marking a block in memory copy on write
(COW) would allow preserving a coherent block to write. You noted that
it was harder than it sounds, and I never thought it sounded easy, due
to issues with multiple processes or threads modifying the data.
But I do have another thought, which might be more useful, if not easier
to implement. In the case of a repair, you really don't want to guess
wrong which copy is the most recent. When a mismatch is detected, would
it be feasible to either scan for a dirty block which is waiting to be
written to that location, or just sync and check again? The performance
hit might be considerable, but (a) running check on a busy system is
already a serious hit, and (b) it would only happen when a problem was
detected.
Does any of that sound useful?
Does that help explain the above quote?
It is still the case that:
filesystem corruption won't happen in normal operation
a small mismatch_cnt does not necessarily imply a problem.
--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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