Am 06.03.2017 um 12:56 schrieb Pasi Kärkkäinen:
On Sun, Mar 05, 2017 at 09:15:39AM +0100, Mikael Abrahamsson wrote:
On Fri, 3 Mar 2017, Gandalf Corvotempesta wrote:
This is what ZFS does. This is what Gluster does. This is what BRTFS does.
Adding this in mdadm could be an interesting feature.
This has been discussed several times. Yes, it would be interesting.
It's not easy to do because mdadm maps 4k blocks to 4k blocks. Only
way to "easily" add this I imagine, would be to have an additional
"checksum" block, so that raid6 would require 3 extra drives instead
of 2.
The answer historically has been "patches welcome".
There was/is an early prototype implementation of checksums for Linux MD RAID:
http://pages.cs.wisc.edu/~bpkroth/cs736/md-checksums/
http://pages.cs.wisc.edu/~bpkroth/cs736/md-checksums/md-checksums-presentation.pdf
http://pages.cs.wisc.edu/~bpkroth/cs736/md-checksums/md-checksums-paper.pdf
http://pages.cs.wisc.edu/~bpkroth/cs736/md-checksums/md-checksums-code.tar.bz2
well, it would help when the raid-check just verify that in a RAID10
both mirrors of the stripe have identical data and not just wait for a
read-error of the drives
given that when you mix HDD/SSD and after the first fstrim which only
affects the SSD sha1sum of the first MB no longer matches while it does
on a 4 drive HD array while both machines are clones that is not the case
________________________________________
machine 1 running since 2011
for disk in sda2 sdb2 sdd2 sdc2
do
echo -n "$disk = ";
dd if=/dev/$disk bs=1M skip=10 count=10 2>/dev/null | sha1sum
done
sda2 = 61efc1017cac02b1be7a95618215485b70a0d18d -
sdb2 = ac4ec9b1a96c9c6bbd9ba196fcb7d6cd2dbb0faa -
sdd2 = ac4ec9b1a96c9c6bbd9ba196fcb7d6cd2dbb0faa -
sdc2 = 61efc1017cac02b1be7a95618215485b70a0d18d -
________________________________________
the same on a cloned machine (just take two of the drives to the other
machine and resync both with 2 new drives)
sda2 = 766fde5907aebc4dca39e31475b295035c95e3b4 -
sdb2 = 4f4b7f3b8f8893b2fb2f0f8b86944aa88f2cf2b6 -
sdd2 = 940ecae52580759abb33328dc464a937a66339ba -
sdc2 = 9f79a56f0f09bb422a8d40787ca28cb719866e8e -
________________________________________
* remove the 2 HDD on the mixed one
* overwrite it with zeros
* add them and wait rebuild
* sha1sum is identical
* fstrim -a
* sha1sum mismatch
* no alter from "raid-check"
you can repeat that as often as you want with the same results, the
simple reason is that when on that first MB a free ext4 blocks on top
after fstrim the SSD returns zeros while the HDD don't
________________________________________
and yes i am aware that someone or automatism needs to make a decision
which of the 2 halfs is the truth but at least it should alert by default
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
