Recover data from accidentally created raid5 over raid1

Recover data from accidentally created raid5 over raid1

[Moritz Rosin]

Hey there,

unfortunately I have to admit, that I learned my lesson the hard way 
dealing with software raids.

I had a raid1 running reliable over month using two 4TB HDDs.
Since I ran short on free space I tried to convert the raid1 to a raid5 
in-place (with the plan to add the 3rd HDD after converting).
That's where my incredibly stupid mistake kicked in.

I followed an internet tutorial that told me to do:
mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/sdX1 /dev/sdY1

I learned that I re-created a raid5 array instead of converting the 
raid1 :-(

Is there any chance to un-do the conversion or restore the data?
Has the process of creation really overwritten data or is there anythins 
left on the disk itself that can be rescued?

Any help would be appreciated. Thanks!!!

Moritz

Recover data from accidentally created raid5 over raid1

[Moritz Rosin]

Hey there,

unfortunately I have to admit, that I learned my lesson the hard way 
dealing with software raids.

I had a raid1 running reliable over month using two 4TB HDDs.
Since I ran short on free space I tried to convert the raid1 to a raid5 
in-place (with the plan to add the 3rd HDD after converting).
That's where my incredibly stupid mistake kicked in.

I followed an internet tutorial that told me to do:
mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/sdX1 /dev/sdY1

I learned that I re-created a raid5 array instead of converting the 
raid1 :-(

Is there any chance to un-do the conversion or restore the data?
Has the process of creation really overwritten data or is there anythins 
left on the disk itself that can be rescued?

Any help would be appreciated. Thanks!!!

Moritz

Re: Recover data from accidentally created raid5 over raid1

[John Stoffel]

>>>>> "Moritz" == Moritz Rosin <moritz.rosin@itrosinen.de> writes:

> Hey there,
> unfortunately I have to admit, that I learned my lesson the hard way 
> dealing with software raids.

> I had a raid1 running reliable over month using two 4TB HDDs.
> Since I ran short on free space I tried to convert the raid1 to a raid5 
> in-place (with the plan to add the 3rd HDD after converting).
> That's where my incredibly stupid mistake kicked in.

> I followed an internet tutorial that told me to do:
> mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/sdX1 /dev/sdY1

Please share the link to the tutorial so we can maybe shame that
person into fixing it.  Or removing it.  

> I learned that I re-created a raid5 array instead of converting the 
> raid1 :-(

Yeah, I think you're out of luck here.  What kind of filesystem did
you have on your setup?  Were you using MD -> LVM -> filesystem stack?
Or just a raw filesystem on top of the /dev/md?  device?

> Is there any chance to un-do the conversion or restore the data?
> Has the process of creation really overwritten data or is there
> anythins left on the disk itself that can be rescued?

If you have any information on your setup before you did this, then
you might be ok, but honestly, I think you're toast.

John

Re: Recover data from accidentally created raid5 over raid1

[Wol]

On 11/04/2023 20:47, John Stoffel wrote:
>>>>>> "Moritz" == Moritz Rosin <moritz.rosin@itrosinen.de> writes:
> 
>> Hey there,
>> unfortunately I have to admit, that I learned my lesson the hard way
>> dealing with software raids.
> 
>> I had a raid1 running reliable over month using two 4TB HDDs.
>> Since I ran short on free space I tried to convert the raid1 to a raid5
>> in-place (with the plan to add the 3rd HDD after converting).
>> That's where my incredibly stupid mistake kicked in.
> 
>> I followed an internet tutorial that told me to do:
>> mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/sdX1 /dev/sdY1
> 
> Please share the link to the tutorial so we can maybe shame that
> person into fixing it.  Or removing it.

See below. There's no reason why it shouldn't work, PROVIDED nothing has 
happened to the mirror since you created it.
> 
>> I learned that I re-created a raid5 array instead of converting the
>> raid1 :-(
> 
> Yeah, I think you're out of luck here.  What kind of filesystem did
> you have on your setup?  Were you using MD -> LVM -> filesystem stack?
> Or just a raw filesystem on top of the /dev/md?  device?

I dunno. A two-disk raid-5 is the same as a 2-disk mirror. That raid-5 
MAY just start and run and you'll be okay. You can try mounting it 
read-only and see what happens ...
> 
>> Is there any chance to un-do the conversion or restore the data?
>> Has the process of creation really overwritten data or is there
>> anythins left on the disk itself that can be rescued?
> 
If the conversion has overwritten the data, it will merely have 
overwritten one copy of the data with the other.

> If you have any information on your setup before you did this, then
> you might be ok, but honestly, I think you're toast.
> 
It might be a bit of a forensic job, but no I don't think so. Do you 
have that third 4TB HDD? If so, MAKE A BACKUP of one of the drives. That 
way, you'll have three copies to play with to try and recover the data.

As John says, please give us all the information you can. If you've just 
put a file system on top of the array, you should now have three copies 
of the filesystem to try and recover. I can't help any further here. but 
all you have to do is track down the start of said filesystem, work out 
where you tell linux to start a partition so it correctly contains the 
filesystem, and then mount said partition. Your data should all be there.

Actually, you might be better off not copying onto drive 3. If you can 
work out where your filesystem partition should start, create a 
partition on drive 3 and copy the filesystem contents into said partition.

I've cc'd a couple of people I hope can help, but basically, you need to 
find out where in the raid array your data has been put, and then work 
out how to access it. Your data SHOULD be recoverable, but you've got 
some detective work ahead of you.

Cheers,
Wol

Re: Recover data from accidentally created raid5 over raid1

[Phil Turmel]

Hi Moritz, et al,

On 4/11/23 20:18, Wol wrote:
> On 11/04/2023 20:47, John Stoffel wrote:
>>>>>>> "Moritz" == Moritz Rosin <moritz.rosin@itrosinen.de> writes:
>>
>>> Hey there,
>>> unfortunately I have to admit, that I learned my lesson the hard way
>>> dealing with software raids.
>>
>>> I had a raid1 running reliable over month using two 4TB HDDs.
>>> Since I ran short on free space I tried to convert the raid1 to a raid5
>>> in-place (with the plan to add the 3rd HDD after converting).
>>> That's where my incredibly stupid mistake kicked in.
>>
>>> I followed an internet tutorial that told me to do:
>>> mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/sdX1 /dev/sdY1

Ewww.

>> Please share the link to the tutorial so we can maybe shame that
>> person into fixing it.  Or removing it.
> 
> See below. There's no reason why it shouldn't work, PROVIDED nothing has 
> happened to the mirror since you created it.
>>
>>> I learned that I re-created a raid5 array instead of converting the
>>> raid1 :-(

Indeed.  It would have sync'd every other chunk in opposite directions 
to place "parity" in the right rotation, but otherwise equivalent to a 
mirror.

>> Yeah, I think you're out of luck here.  What kind of filesystem did
>> you have on your setup?  Were you using MD -> LVM -> filesystem stack?
>> Or just a raw filesystem on top of the /dev/md?  device?
> 
> I dunno. A two-disk raid-5 is the same as a 2-disk mirror. That raid-5 
> MAY just start and run and you'll be okay. You can try mounting it 
> read-only and see what happens ...

The odds of matching offsets depends entirely on how old the original 
raid1 was.

>>> Is there any chance to un-do the conversion or restore the data?
>>> Has the process of creation really overwritten data or is there
>>> anythins left on the disk itself that can be rescued?
>>
> If the conversion has overwritten the data, it will merely have 
> overwritten one copy of the data with the other.

Concur.

>> If you have any information on your setup before you did this, then
>> you might be ok, but honestly, I think you're toast.
>>
> It might be a bit of a forensic job, but no I don't think so. Do you 
> have that third 4TB HDD? If so, MAKE A BACKUP of one of the drives. That 
> way, you'll have three copies to play with to try and recover the data.

This.

> As John says, please give us all the information you can. If you've just 
> put a file system on top of the array, you should now have three copies 
> of the filesystem to try and recover. I can't help any further here. but 
> all you have to do is track down the start of said filesystem, work out 
> where you tell linux to start a partition so it correctly contains the 
> filesystem, and then mount said partition. Your data should all be there.

The trick will be to determine the offset.  Please share as much 
information as possible as to the layering of the original setup, 
preferable with the fstab contents if available.

> Actually, you might be better off not copying onto drive 3. If you can 
> work out where your filesystem partition should start, create a 
> partition on drive 3 and copy the filesystem contents into said partition.

Or overlays with dmsetup.

> I've cc'd a couple of people I hope can help, but basically, you need to 
> find out where in the raid array your data has been put, and then work 
> out how to access it. Your data SHOULD be recoverable, but you've got 
> some detective work ahead of you.

Your odds are decent.  Again, share all the info you can.

> 
> Cheers,
> Wol

Phil

Re: Recover data from accidentally created raid5 over raid1

[Moritz Rosin]

Hi Phil, et.al.,

first of all, thank y'all so much for your thoughts and answers.
I am going to add as much information as possible.

Am 12.04.2023 um 02:26 schrieb Phil Turmel:
> Hi Moritz, et al,
>
> On 4/11/23 20:18, Wol wrote:
>> On 11/04/2023 20:47, John Stoffel wrote:
>>>>>>>> "Moritz" == Moritz Rosin <moritz.rosin@itrosinen.de> writes:
>>>
>>>> Hey there,
>>>> unfortunately I have to admit, that I learned my lesson the hard way
>>>> dealing with software raids.
>>>
>>>> I had a raid1 running reliable over month using two 4TB HDDs.
>>>> Since I ran short on free space I tried to convert the raid1 to a 
>>>> raid5
>>>> in-place (with the plan to add the 3rd HDD after converting).
>>>> That's where my incredibly stupid mistake kicked in.
>>>
>>>> I followed an internet tutorial that told me to do:
>>>> mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/sdX1 /dev/sdY1
>
> Ewww.
>
>>> Please share the link to the tutorial so we can maybe shame that
>>> person into fixing it.  Or removing it.
I followed this tutorial, but found similar suggesting to use "mdadm -- 
create" -> 
https://dev.to/csgeek/converting-raid-1-to-raid-5-on-linux-file-systems-k73
>>>
>>
>> See below. There's no reason why it shouldn't work, PROVIDED nothing 
>> has happened to the mirror since you created it.
>>>
>>>> I learned that I re-created a raid5 array instead of converting the
>>>> raid1 :-(
>
> Indeed.  It would have sync'd every other chunk in opposite directions 
> to place "parity" in the right rotation, but otherwise equivalent to a 
> mirror.
>
>>> Yeah, I think you're out of luck here. What kind of filesystem did
>>> you have on your setup?  Were you using MD -> LVM -> filesystem stack?
>>> Or just a raw filesystem on top of the /dev/md?  device?
>>
>> I dunno. A two-disk raid-5 is the same as a 2-disk mirror. That 
>> raid-5 MAY just start and run and you'll be okay. You can try 
>> mounting it read-only and see what happens ...
>
> The odds of matching offsets depends entirely on how old the original 
> raid1 was.
The original raid1 was about 2 years old.
I was using ext4 directly ontop of the array.
>
>
>>>> Is there any chance to un-do the conversion or restore the data?
>>>> Has the process of creation really overwritten data or is there
>>>> anythins left on the disk itself that can be rescued?
>>>
>> If the conversion has overwritten the data, it will merely have 
>> overwritten one copy of the data with the other.
>
> Concur.
>
>>> If you have any information on your setup before you did this, then
>>> you might be ok, but honestly, I think you're toast.
>>>
>> It might be a bit of a forensic job, but no I don't think so. Do you 
>> have that third 4TB HDD? If so, MAKE A BACKUP of one of the drives. 
>> That way, you'll have three copies to play with to try and recover 
>> the data.
>
> This.
>
>> As John says, please give us all the information you can. If you've 
>> just put a file system on top of the array, you should now have three 
>> copies of the filesystem to try and recover. I can't help any further 
>> here. but all you have to do is track down the start of said 
>> filesystem, work out where you tell linux to start a partition so it 
>> correctly contains the filesystem, and then mount said partition. 
>> Your data should all be there.
>
> The trick will be to determine the offset.  Please share as much 
> information as possible as to the layering of the original setup, 
> preferable with the fstab contents if available.
Unfortunately I have no output (e.g. of fdisk -l) _bevore_ converting 
the array. What I found in the syslogs is pasted here: 
https://pastebin.com/iktUtYyt
Output of lsblk: https://pastebin.com/LNyUizGq
Output of fdisk -l (after conversion): https://pastebin.com/LH6ngUjc -- 
as you can see there is no partition table exitent for "md0"
Output of "mdadm --detail": https://pastebin.com/vYhjphKY
>
>> Actually, you might be better off not copying onto drive 3. If you 
>> can work out where your filesystem partition should start, create a 
>> partition on drive 3 and copy the filesystem contents into said 
>> partition.
>
> Or overlays with dmsetup.
>
>> I've cc'd a couple of people I hope can help, but basically, you need 
>> to find out where in the raid array your data has been put, and then 
>> work out how to access it. Your data SHOULD be recoverable, but 
>> you've got some detective work ahead of you.
>
> Your odds are decent.  Again, share all the info you can.
Is there anything else I can provide?
What is a good point to start the detective work?
>
>
>>
>> Cheers,
>> Wol
>
> Phil
Thanks
Moritz

Re: Recover data from accidentally created raid5 over raid1

[Reindl Harald]

Am 12.04.23 um 02:18 schrieb Wol:
> A two-disk raid-5 is the same as a 2-disk mirror

this is completly nonsense like it was nonsense that a RAID10 of 4 
drives after remove two drives is the same as a RAID1 and just a 
metadata update should be enough

why are you doing that always?

the problem is there are people out there which believe what you are 
pretending here and spread that nonsense over the web as there isn't 
enough wrong information already out there

Re: Recover data from accidentally created raid5 over raid1

[Phil Turmel]

Reindl,

On 4/12/23 06:22, Reindl Harald wrote:
> 
> 
> Am 12.04.23 um 02:18 schrieb Wol:
>> A two-disk raid-5 is the same as a 2-disk mirror
> 
> this is completly nonsense like it was nonsense that a RAID10 of 4 
> drives after remove two drives is the same as a RAID1 and just a 
> metadata update should be enough
> 
> why are you doing that always?
> 
> the problem is there are people out there which believe what you are 
> pretending here and spread that nonsense over the web as there isn't 
> enough wrong information already out there
> 

What is nonsense?  A two-disk raid5 does have exactly the same content 
on both disks, just like a mirror.  The parity for any given single byte 
is that same byte.

Please, just once, limit yourself to *positive* contributions.

Phil

Re: Recover data from accidentally created raid5 over raid1

[Mark Wagner]

On Wed, Apr 12, 2023 at 5:45 AM Phil Turmel <philip@turmel.org> wrote:
>
> What is nonsense?  A two-disk raid5 does have exactly the same content
> on both disks, just like a mirror.  The parity for any given single byte
> is that same byte.

Only if it's using even parity.  If it's using odd parity, the second
disk is the inverse of the first.

(I think Linux RAID uses even parity, but if it's documented anywhere,
I haven't found it.)

-- 
Mark

Re: Recover data from accidentally created raid5 over raid1

[Moritz Rosin]

Good Morning everyone.

Thank god I have very good news, since I managed to get access to 
(hopefully most of) the data.
I want to share the path I took with you for further reference.

First thing I tried was to use "foremost" directly on the raid array 
(/dev/md0) that had no partition table.
That was just partially successful since it ran *verry* slowly and 
produced mixed results. Many broken files, no dir structure, no 
filenames.
But there were files that came out correctly (content wise) - so I had 
hope.

Following your advice I then start off on this page: 
https://raid.wiki.kernel.org/index.php/Recovering_a_failed_software_RAID 
(chapter "Making the harddisks read-only using an overlay file").
With what you assumed, that the data should be there on the raw disks, I 
created overlays as explained.
One note here: In step 3 the page used "blockdev --getsize ...." but in 
my version (2.36.1) that parameter is marked as "deprecated" an didn't 
work. I had to use "blockdev --getsz" instead.

Having the overlays I fiddled around and ended up using "testdisk", let 
it analyze the overlay device "/dev/mapper/sdX1". After selecting "EFI 
GPT" partition type, it found a partition table I was able to use.
 From here on the process was pretty straight forward. testdisk showed 
the disk's old file structure and I was able to copy the "lost" files to 
a backup hdd.
The process is still running but spot tests were pretty promising that 
most of the data can be recovered.

When backup has finished. I will completely re-create the array and 
start with a clean setup :-)

Thanks again for your support, input and thoughts.

Best
Moritz

Am 2023-04-12 12:15, schrieb Moritz Rosin:
> Hi Phil, et.al.,
> 
> first of all, thank y'all so much for your thoughts and answers.
> I am going to add as much information as possible.
> 
> Am 12.04.2023 um 02:26 schrieb Phil Turmel:
>> Hi Moritz, et al,
>> 
>> On 4/11/23 20:18, Wol wrote:
>>> On 11/04/2023 20:47, John Stoffel wrote:
>>>>>>>>> "Moritz" == Moritz Rosin <moritz.rosin@itrosinen.de> writes:
>>>> 
>>>>> Hey there,
>>>>> unfortunately I have to admit, that I learned my lesson the hard 
>>>>> way
>>>>> dealing with software raids.
>>>> 
>>>>> I had a raid1 running reliable over month using two 4TB HDDs.
>>>>> Since I ran short on free space I tried to convert the raid1 to a 
>>>>> raid5
>>>>> in-place (with the plan to add the 3rd HDD after converting).
>>>>> That's where my incredibly stupid mistake kicked in.
>>>> 
>>>>> I followed an internet tutorial that told me to do:
>>>>> mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/sdX1 
>>>>> /dev/sdY1
>> 
>> Ewww.
>> 
>>>> Please share the link to the tutorial so we can maybe shame that
>>>> person into fixing it.  Or removing it.
> I followed this tutorial, but found similar suggesting to use "mdadm -- 
> create" -> 
> https://dev.to/csgeek/converting-raid-1-to-raid-5-on-linux-file-systems-k73
>>>> 
>>> 
>>> See below. There's no reason why it shouldn't work, PROVIDED nothing 
>>> has happened to the mirror since you created it.
>>>> 
>>>>> I learned that I re-created a raid5 array instead of converting the
>>>>> raid1 :-(
>> 
>> Indeed.  It would have sync'd every other chunk in opposite directions 
>> to place "parity" in the right rotation, but otherwise equivalent to a 
>> mirror.
>> 
>>>> Yeah, I think you're out of luck here. What kind of filesystem did
>>>> you have on your setup?  Were you using MD -> LVM -> filesystem 
>>>> stack?
>>>> Or just a raw filesystem on top of the /dev/md?  device?
>>> 
>>> I dunno. A two-disk raid-5 is the same as a 2-disk mirror. That 
>>> raid-5 MAY just start and run and you'll be okay. You can try 
>>> mounting it read-only and see what happens ...
>> 
>> The odds of matching offsets depends entirely on how old the original 
>> raid1 was.
> The original raid1 was about 2 years old.
> I was using ext4 directly ontop of the array.
>> 
>> 
>>>>> Is there any chance to un-do the conversion or restore the data?
>>>>> Has the process of creation really overwritten data or is there
>>>>> anythins left on the disk itself that can be rescued?
>>>> 
>>> If the conversion has overwritten the data, it will merely have 
>>> overwritten one copy of the data with the other.
>> 
>> Concur.
>> 
>>>> If you have any information on your setup before you did this, then
>>>> you might be ok, but honestly, I think you're toast.
>>>> 
>>> It might be a bit of a forensic job, but no I don't think so. Do you 
>>> have that third 4TB HDD? If so, MAKE A BACKUP of one of the drives. 
>>> That way, you'll have three copies to play with to try and recover 
>>> the data.
>> 
>> This.
>> 
>>> As John says, please give us all the information you can. If you've 
>>> just put a file system on top of the array, you should now have three 
>>> copies of the filesystem to try and recover. I can't help any further 
>>> here. but all you have to do is track down the start of said 
>>> filesystem, work out where you tell linux to start a partition so it 
>>> correctly contains the filesystem, and then mount said partition. 
>>> Your data should all be there.
>> 
>> The trick will be to determine the offset.  Please share as much 
>> information as possible as to the layering of the original setup, 
>> preferable with the fstab contents if available.
> Unfortunately I have no output (e.g. of fdisk -l) _bevore_ converting 
> the array. What I found in the syslogs is pasted here: 
> https://pastebin.com/iktUtYyt
> Output of lsblk: https://pastebin.com/LNyUizGq
> Output of fdisk -l (after conversion): https://pastebin.com/LH6ngUjc -- 
> as you can see there is no partition table exitent for "md0"
> Output of "mdadm --detail": https://pastebin.com/vYhjphKY
>> 
>>> Actually, you might be better off not copying onto drive 3. If you 
>>> can work out where your filesystem partition should start, create a 
>>> partition on drive 3 and copy the filesystem contents into said 
>>> partition.
>> 
>> Or overlays with dmsetup.
>> 
>>> I've cc'd a couple of people I hope can help, but basically, you need 
>>> to find out where in the raid array your data has been put, and then 
>>> work out how to access it. Your data SHOULD be recoverable, but 
>>> you've got some detective work ahead of you.
>> 
>> Your odds are decent.  Again, share all the info you can.
> Is there anything else I can provide?
> What is a good point to start the detective work?
>> 
>> 
>>> 
>>> Cheers,
>>> Wol
>> 
>> Phil
> Thanks
> Moritz

Re: Recover data from accidentally created raid5 over raid1

[Wols Lists]

On 12/04/2023 21:22, Mark Wagner wrote:
> On Wed, Apr 12, 2023 at 5:45 AM Phil Turmel <philip@turmel.org> wrote:
>>
>> What is nonsense?  A two-disk raid5 does have exactly the same content
>> on both disks, just like a mirror.  The parity for any given single byte
>> is that same byte.
> 
> Only if it's using even parity.  If it's using odd parity, the second
> disk is the inverse of the first.
> 
It's documented on the raid wiki (because I think it was documented 
there before I started editing it), that the way raid switches between 
different raid types is through the fact (accidental or on purpose I 
don't know) that most of the two-disk raid types just happen to have an 
"identical in practice" disk layout.

So converting between raid-1 and raid-5 is as simple as changing the 
config of a raid-1 to a degraded raid-5 and vice versa.

Cheers,
Wol

Re: Recover data from accidentally created raid5 over raid1

[Phil Turmel]

On 4/13/23 02:10, Moritz Rosin wrote:
> Good Morning everyone.
> 
> Thank god I have very good news, since I managed to get access to 
> (hopefully most of) the data.
> I want to share the path I took with you for further reference.
> 
> First thing I tried was to use "foremost" directly on the raid array 
> (/dev/md0) that had no partition table.
> That was just partially successful since it ran *verry* slowly and 
> produced mixed results. Many broken files, no dir structure, no filenames.
> But there were files that came out correctly (content wise) - so I had 
> hope.
> 
> Following your advice I then start off on this page: 
> https://raid.wiki.kernel.org/index.php/Recovering_a_failed_software_RAID 
> (chapter "Making the harddisks read-only using an overlay file").
> With what you assumed, that the data should be there on the raw disks, I 
> created overlays as explained.
> One note here: In step 3 the page used "blockdev --getsize ...." but in 
> my version (2.36.1) that parameter is marked as "deprecated" an didn't 
> work. I had to use "blockdev --getsz" instead.
> 
> Having the overlays I fiddled around and ended up using "testdisk", let 
> it analyze the overlay device "/dev/mapper/sdX1". After selecting "EFI 
> GPT" partition type, it found a partition table I was able to use.
>  From here on the process was pretty straight forward. testdisk showed 
> the disk's old file structure and I was able to copy the "lost" files to 
> a backup hdd.
> The process is still running but spot tests were pretty promising that 
> most of the data can be recovered.
> 
> When backup has finished. I will completely re-create the array and 
> start with a clean setup :-)
> 
> Thanks again for your support, input and thoughts.
> 
> Best
> Moritz

Good morning Moritz,

That's great news.  Thanks for reporting back.

Phil

Re: Recover data from accidentally created raid5 over raid1

[Phil Turmel]

On 4/13/23 04:05, Wols Lists wrote:
> On 12/04/2023 21:22, Mark Wagner wrote:
>> On Wed, Apr 12, 2023 at 5:45 AM Phil Turmel <philip@turmel.org> wrote:
>>>
>>> What is nonsense?  A two-disk raid5 does have exactly the same content
>>> on both disks, just like a mirror.  The parity for any given single byte
>>> is that same byte.
>>
>> Only if it's using even parity.  If it's using odd parity, the second
>> disk is the inverse of the first.
>>
> It's documented on the raid wiki (because I think it was documented 
> there before I started editing it), that the way raid switches between 
> different raid types is through the fact (accidental or on purpose I 
> don't know) that most of the two-disk raid types just happen to have an 
> "identical in practice" disk layout.
> 
> So converting between raid-1 and raid-5 is as simple as changing the 
> config of a raid-1 to a degraded raid-5 and vice versa.

Be careful!  This is only true for a *two* disk raid.

> Cheers,
> Wol
> 

Phil

Re: Recover data from accidentally created raid5 over raid1

[Wols Lists]

On 13/04/2023 13:12, Phil Turmel wrote:
> On 4/13/23 04:05, Wols Lists wrote:
>> On 12/04/2023 21:22, Mark Wagner wrote:
>>> On Wed, Apr 12, 2023 at 5:45 AM Phil Turmel <philip@turmel.org> wrote:
>>>>
>>>> What is nonsense?  A two-disk raid5 does have exactly the same content
>>>> on both disks, just like a mirror.  The parity for any given single 
>>>> byte
>>>> is that same byte.
>>>
>>> Only if it's using even parity.  If it's using odd parity, the second
>>> disk is the inverse of the first.
>>>
>> It's documented on the raid wiki (because I think it was documented 
>> there before I started editing it), that the way raid switches between 
>> different raid types is through the fact (accidental or on purpose I 
>> don't know) that most of the two-disk raid types just happen to have 
>> an "identical in practice" disk layout.
>>
>> So converting between raid-1 and raid-5 is as simple as changing the 
>> config of a raid-1 to a degraded raid-5 and vice versa.
> 
> Be careful!  This is only true for a *two* disk raid.
> 
Which is why (and I've tried) mdadm won't let you convert a 3-disk 
raid-1 :-)

But yes, two-disk raids only ...

Cheers,
Wol