Hi all,
I'm trying to understand if, and how, mdadm can be used with network
attached devices (iSCSI, in this case). I have a very simple setup with
two 1 GB drives, the first being a local disk (a logical volume, really)
and the second a remote iSCSI disk.
First question: even if in my preliminary tests this seems to work
reasonably well, do you feel that such solution can be used for
production workloads? Or something with a more specific focus, as DRBD,
remains the preferred solution?
I'm using two CentOS 7.3 x86-64 boxes, with kernel version
3.10.0-514.16.1.el7.x86_64 and mdadm v3.4 - 28th January 2016. Here you
can find my current RAID1 setup, where /dev/sdb is the iSCSI disk:
[root@gdanti-laptop g.danti]# cat /proc/mdstat
Personalities : [raid1]
md200 : active raid1 sdb[1] dm-3[0]
1047552 blocks super 1.2 [2/2] [UU]
bitmap: 0/1 pages [0KB], 65536KB chunk
unused devices: <none>
[root@gdanti-laptop g.danti]# mdadm -D /dev/md200
/dev/md200:
Version : 1.2
Creation Time : Wed May 10 08:53:12 2017
Raid Level : raid1
Array Size : 1047552 (1023.00 MiB 1072.69 MB)
Used Dev Size : 1047552 (1023.00 MiB 1072.69 MB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Intent Bitmap : Internal
Update Time : Wed May 10 10:27:35 2017
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Name : gdanti-laptop.assyoma.it:200 (local to host
gdanti-laptop.assyoma.it)
UUID : 9d6fb056:c1d49780:149f9391:68fc267f
Events : 62
Number Major Minor RaidDevice State
0 253 3 0 active sync /dev/dm-3
1 8 16 1 active sync /dev/sdb
So far, so good: the array seems to work well, with good read/write
speed. Now, suppose the remote disk become unavailable:
[root@gdanti-laptop g.danti]# iscsiadm -m node --targetname
iqn.2008-09.com.example:server.target1 --portal 172.31.255.11 --logout
Logging out of session [sid: 6, target:
iqn.2008-09.com.example:server.target1, portal: 172.31.255.11,3260]
Logout of [sid: 6, target: iqn.2008-09.com.example:server.target1,
portal: 172.31.255.11,3260] successful.
[root@gdanti-laptop g.danti]# cat /proc/mdstat
Personalities : [raid1]
md200 : active (auto-read-only) raid1 dm-3[0]
1047552 blocks super 1.2 [2/1] [U_]
bitmap: 0/1 pages [0KB], 65536KB chunk
unused devices: <none>
The device is correctly kicked-off the array.
So, second question: how to enable auto re-add for the remote device
when it become available again? For example:
[root@gdanti-laptop g.danti]# iscsiadm -m node --targetname
iqn.2008-09.com.example:server.target1 --portal 172.31.255.11 --login
Logging in to [iface: default, target:
iqn.2008-09.com.example:server.target1, portal: 172.31.255.11,3260]
(multiple)
Login to [iface: default, target:
iqn.2008-09.com.example:server.target1, portal: 172.31.255.11,3260]
successful.
[root@gdanti-laptop g.danti]# cat /proc/mdstat
Personalities : [raid1]
md200 : active (auto-read-only) raid1 dm-3[0]
1047552 blocks super 1.2 [2/1] [U_]
bitmap: 0/1 pages [0KB], 65536KB chunk
unused devices: <none>
Even if /dev/sdb is now visible, it is not auto re-added to the array.
If I run mdadm /dev/sdb --incremental --run I see the device added as a
spare:
[root@gdanti-laptop g.danti]# cat /proc/mdstat
Personalities : [raid1]
md200 : active (auto-read-only) raid1 sdb[1](S) dm-3[0]
1047552 blocks super 1.2 [2/1] [U_]
bitmap: 0/1 pages [0KB], 65536KB chunk
unused devices: <none>
Third question: with --incremental adds device as a spare, rather than
active?
I've looked at the POLICY directive in mdadm.conf, but I am unable to
make it work by auto re-adding iSCSI devices when they become up again.
Sorry for the long post, I am really trying to learn something!
Thanks.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx
GPG public key ID: FF5F32A8
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