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apurkrt
Tux's lil' helper
Joined: 26 Feb 2011
Posts: 116
Location: Czechia, Europe
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 Posted: Sat May 30, 2020 6:44 am Post subject: Is it possible to change the order of disks in raid1? |
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Hi!
I have done this experiment (in virtual environment) with raid1:
| Code: |
vbox ~ # cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md126 : active raid1 sdc[0] sdd[1]
1048512 blocks [2/2] [UU]
...
vbox ~ # shasum /dev/md126
35b4e07c30f11e6ad81fc45705376bc8d661415d /dev/md126
vbox ~ # dd if=/dev/urandom of=/dev/sdd bs=1M
dd: error writing '/dev/sdd': No space left on device
1025+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 6.33977 s, 169 MB/s
vbox ~ # shasum /dev/md126
35b4e07c30f11e6ad81fc45705376bc8d661415d /dev/md126
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i.e. I've overwritten one of the disks constituting the raid - the disk denoted as [1] - with random data
Surprisingly, the outcome of reading /dev/md126 was still the same (same shasum).
So my conclusion is: raid1 writes to both drives, but when it comes to reading, it reads only from one of the drives - namely the one which is seen as [0] in /proc/mdstat
This is in line with the observation of a real hw test machine, where I have raid1 made from an old platter HDD and SSD; when reading from the raid, I can hear the platter disk seek, also it is rather slow - the conclusion being the data are being read from the HDD (the raid partition on the platter HDD being denoted as [0])
My question would be: Is it possible to somehow change this order? I.e. change which disk (partition) is [0] and which is [1]?
I've tried to search "man mdadm" for this, to no avail. Maybe I missed something? |
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Bones McCracker
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Joined: 14 Mar 2006
Posts: 1611
Location: U.S.A.
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| Posted: Sat May 30, 2020 11:34 pm Post subject: |
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I don't know, but think you would have to disassemble the array then assemble it again in reverse order.
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| patrix_neo wrote: | The human thought: I cannot win.
The ratbrain in me : I can only go forward and that's it. |
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apurkrt
Tux's lil' helper
Joined: 26 Feb 2011
Posts: 116
Location: Czechia, Europe
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| Posted: Sun May 31, 2020 11:52 am Post subject: |
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| Bones McCracker wrote: | | I don't know, but think you would have to disassemble the array then assemble it again in reverse order. |
Reassembling in reverse order does not change anything:
| Code: | vbox ~ # cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md126 : active raid1 sdc[0] sdd[1]
1048512 blocks [2/2] [UU]
unused devices: <none>
vbox ~ # mdadm --stop /dev/md126
mdadm: stopped /dev/md126
vbox ~ # mdadm -A /dev/md126 /dev/sdd /dev/sdc
mdadm: /dev/md126 has been started with 2 drives.
vbox ~ # cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md126 : active raid1 sdc[0] sdd[1]
1048512 blocks [2/2] [UU]
unused devices: <none>
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even starting the array with sdd alone (and only later adding sdc) does not force sdd to be marked as [0]
| Code: | vbox ~ # cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md126 : active raid1 sdc[0] sdd[1]
1048512 blocks [2/2] [UU]
unused devices: <none>
vbox ~ # mdadm --stop /dev/md126
mdadm: stopped /dev/md126
vbox ~ # mdadm -A /dev/md126 --run /dev/sdd
mdadm: /dev/md126 has been started with 1 drive (out of 2).
vbox ~ # cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md126 : active raid1 sdd[1]
1048512 blocks [2/1] [_U]
vbox ~ # mdadm /dev/md126 -a /dev/sdc
mdadm: hot added /dev/sdc
vbox ~ # cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md126 : active raid1 sdc[2] sdd[1]
1048512 blocks [2/1] [_U]
[===============>.....] recovery = 76.9% (806976/1048512) finish=0.0min speed=201744K/sec
(and later)
vbox ~ # cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md126 : active raid1 sdc[0] sdd[1]
1048512 blocks [2/2] [UU]
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apurkrt
Tux's lil' helper
Joined: 26 Feb 2011
Posts: 116
Location: Czechia, Europe
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| Posted: Sun May 31, 2020 12:01 pm Post subject: |
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the number in square brackets [ ] seems to correspond to "RaidDevice" number printed in the metadata;
| Code: | vbox ~ # mdadm -E /dev/sdc
/dev/sdc:
Magic : a92b4efc
Version : 0.90.00
UUID : ece21326:d9c89e34:eff5119f:08dda25b (local to host vbox)
Creation Time : Thu May 28 09:37:13 2020
Raid Level : raid1
Used Dev Size : 1048512 (1023.94 MiB 1073.68 MB)
Array Size : 1048512 (1023.94 MiB 1073.68 MB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 126
Update Time : Sun May 31 13:50:26 2020
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Checksum : 265bbadb - correct
Events : 143
Number Major Minor RaidDevice State
this 0 8 32 0 active sync /dev/sdc
0 0 8 32 0 active sync /dev/sdc
1 1 8 48 1 active sync /dev/sdd
vbox ~ # mdadm -E /dev/sdd
/dev/sdd:
Magic : a92b4efc
Version : 0.90.00
UUID : ece21326:d9c89e34:eff5119f:08dda25b (local to host vbox)
Creation Time : Thu May 28 09:37:13 2020
Raid Level : raid1
Used Dev Size : 1048512 (1023.94 MiB 1073.68 MB)
Array Size : 1048512 (1023.94 MiB 1073.68 MB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 126
Update Time : Sun May 31 13:50:26 2020
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Checksum : 265bbaed - correct
Events : 143
Number Major Minor RaidDevice State
this 1 8 48 1 active sync /dev/sdd
0 0 8 32 0 active sync /dev/sdc
1 1 8 48 1 active sync /dev/sdd
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apurkrt
Tux's lil' helper
Joined: 26 Feb 2011
Posts: 116
Location: Czechia, Europe
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Bones McCracker
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Joined: 14 Mar 2006
Posts: 1611
Location: U.S.A.
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| Posted: Sun May 31, 2020 2:35 pm Post subject: |
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When I said "disassemble", I meant completely destroy the raid array and re-create it.
_________________
| patrix_neo wrote: | The human thought: I cannot win.
The ratbrain in me : I can only go forward and that's it. |
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