On Thu, 25 Jun 2015 10:19:59 +0500 Roman Mamedov [off-list ref] wrote:

On Thu, 25 Jun 2015 11:33:35 +1000
NeilBrown [off-list ref] wrote:

quoted

On Sun, 14 Jun 2015 20:13:16 +0300 tlknv [off-list ref] wrote:

quoted

Hello,

quoted

I have raid 1 which mirrors a root/boot partition on 1SSD and 2HDD
(write-mostly). mismatch_cnt goes up even when there are very few
writes to the partition as /var is mounted separatly. After I update
several packages I typically see mismatch_cnt somewhere between
500,000 and 2,000,000. I have read a number of threads in this DL
but could not find an explanation of what could cause mismatch_cnt
to grow that much. I checked md5 sums using
/var/lib/dpkg/info/*.md5sums, and didn't see many errors, even
though there are few, mostly in text files which look ok to me. I
guess when I check, all reads go to SSD (as both HDDs in this raid
are write-mostly), and thus md5sum only shows no problem on
SSD. Note, this partition is used as both boot and root and just in
case here is some more info about my system:

This does surprise me.

I had another look at the code and there could be a bug that would let
'check' see the difference between when the first write completes and
when the write-behind writes complete, but you would need to run the
check while the install was happening for that to be noticed, and even
then you would need to be unlucky.

Couldn't this be simply the normal observed effect of using TRIM on SSD?

Yes, of course it could.  I try not to think about TRIM to much - makes me ill :-)

Thanks,
NeilBrown

After deleting some files, the filesystem issues a discard request, it
does nothing to the HDDs, but the content of the discared areas on SSD is no
longer deterministic (or mostly zeroed, as mentioned in the original report).
So there is now a mismatch between the content of HDDs and SSD, but since it
is in the area of deleted files, it doesn't affect the system in any way.