Linux RAID migration

Linux RAID migration

[saeed bishara]

Hi,
 I'm looking for a method for doing RAID migration while keeping the
data available.
the migrations I'm interested with are:
1. Single drive ->RAID1/RAID5
2. RAID1 -> RAID5.

the mdadm and kernel doesn't seem to provide built-in and crash-proof
solution. but I found that RAID5 on 2 drives in degraded mode looks
like a RAID1 (mirroring), and since linux supports RAID5 reshape, then
I can use it to do the migration from RAID1 to  RAID5 with 3 drives or
more.
- can I really assume that RAID 5 on 2 hdds (degraded mode) will
function as raid 5?
- how to build raid while keeping the contents of an existing drive available?

saeed

Re: Linux RAID migration

[Tuomas Leikola]

On 8/7/07, saeed bishara <saeed.bishara@gmail.com> wrote:
> Hi,
>  I'm looking for a method for doing RAID migration while keeping the
> data available.
> the migrations I'm interested with are:
> 1. Single drive ->RAID1/RAID5
> 2. RAID1 -> RAID5.
>

1. is a bit complicated, as a raid device on a disk is slightly
smaller than the original device. you might need to copy data
manually.

2. should be simple as (offline) re-creating the raid as raid5.

> - can I really assume that RAID 5 on 2 hdds (degraded mode) will
> function as raid 5?

You should test by using loopback devices and files. But why degraded?
raid5 of two disks should look like raid1.

> - how to build raid while keeping the contents of an existing drive available?

Normally the array is available when building/rebuilding.

- tuomas

Re: Linux RAID migration

[saeed bishara]

On 8/7/07, Tuomas Leikola <tuomas.leikola@gmail.com> wrote:
> On 8/7/07, saeed bishara <saeed.bishara@gmail.com> wrote:
> > Hi,
> >  I'm looking for a method for doing RAID migration while keeping the
> > data available.
> > the migrations I'm interested with are:
> > 1. Single drive ->RAID1/RAID5
> > 2. RAID1 -> RAID5.
> >
>
> 1. is a bit complicated, as a raid device on a disk is slightly
> smaller than the original device. you might need to copy data
> manually.
   I think I can overcome this problem by creating linear raid on the
initial disk.
>
> 2. should be simple as (offline) re-creating the raid as raid5.
this didn't work, I created raid1 array on sda[12], then stopped it,
and created new raid5 on sda[123], the data on the raid1 was not
preserved.
>
> > - can I really assume that RAID 5 on 2 hdds (degraded mode) will
> > function as raid 5?
>
> You should test by using loopback devices and files. But why degraded?
> raid5 of two disks should look like raid1.
I meant to ask whether raid 5 on 2 disks will function as raid1 :(
>
> > - how to build raid while keeping the contents of an existing drive available?
>
> Normally the array is available when building/rebuilding.
>
> - tuomas
>

Re: Linux RAID migration

[Neil Brown]

On Tuesday August 7, saeed.bishara@gmail.com wrote:
> > 2. should be simple as (offline) re-creating the raid as raid5.
> this didn't work, I created raid1 array on sda[12], then stopped it,
> and created new raid5 on sda[123], the data on the raid1 was not
> preserved.

Create the new raid5 on sda[12] (same 2 devices as the raid1) and it
will work.  Then grow to make it raid5 on 3 drives if you want.

> >
> > > - can I really assume that RAID 5 on 2 hdds (degraded mode) will
> > > function as raid 5?
> >
> > You should test by using loopback devices and files. But why degraded?
> > raid5 of two disks should look like raid1.
> I meant to ask whether raid 5 on 2 disks will function as raid1 :(

A 2-disk raid5 and a 2-disk raid1 put the data in exactly the same
place - is that what you wanted to know?

NeilBrown

Re: Linux RAID migration

[saeed bishara]

>
> A 2-disk raid5 and a 2-disk raid1 put the data in exactly the same
> place - is that what you wanted to know?
>
yes, exactly.
> NeilBrown
>

Re: Linux RAID migration

[Tomasz Chmielewski]

Neil Brown schrieb:

(...)

>>>> - can I really assume that RAID 5 on 2 hdds (degraded mode) will
>>>> function as raid 5?
>>> You should test by using loopback devices and files. But why degraded?
>>> raid5 of two disks should look like raid1.
>> I meant to ask whether raid 5 on 2 disks will function as raid1 :(
> 
> A 2-disk raid5 and a 2-disk raid1 put the data in exactly the same
> place - is that what you wanted to know?

Is the same valid for RAID-10?

Let's say, you degraded a 4-disk RAID-10 to just 2 (proper) disks.

Then, you want to migrate it to 2-disk, 3, 4, and eventually, 5-disk 
RAID-5 array.


Would the RAID-10 -> RAID-5 migration be as easy as in RAID-1 -> RAID-5 
case?



-- 
Tomasz Chmielewski
http://wpkg.org

Re: Linux RAID migration

[saeed bishara]

how can I create raid 5 array that preserves the contents of a given
disk (part of the array)?
I can do that in two steps, raid 5 on two disks then reshape it to be
real raid5, but is there a direct way to do it?

saeed

Re: Linux RAID migration

[Neil Brown]

On Tuesday August 7, saeed.bishara@gmail.com wrote:
> how can I create raid 5 array that preserves the contents of a given
> disk (part of the array)?
> I can do that in two steps, raid 5 on two disks then reshape it to be
> real raid5, but is there a direct way to do it?

What do you mean by "real raid 5".  Two disks are just as much an
array as 10.
What do you mean by "direct".  If I understand your requirements
correctly, there are two obvious steps.
  1/ make the data appear in a raid5
  2/ change the number of devices in the raid5.

Are two steps too many?

I guess I don't understand your question.

NeilBrown

Re: Linux RAID migration

[saeed bishara]

On 8/7/07, Neil Brown <neilb@suse.de> wrote:
> On Tuesday August 7, saeed.bishara@gmail.com wrote:
> > how can I create raid 5 array that preserves the contents of a given
> > disk (part of the array)?
> > I can do that in two steps, raid 5 on two disks then reshape it to be
> > real raid5, but is there a direct way to do it?
>
> What do you mean by "real raid 5".  Two disks are just as much an
> array as 10.
I meant by real raid 5 to raid 5 over 3 drives or above.
> What do you mean by "direct".  If I understand your requirements
in one mdadm command
> correctly, there are two obvious steps.
>   1/ make the data appear in a raid5
>   2/ change the number of devices in the raid5.
>
> Are two steps too many?
in this case I two options:
1. keep the filesystem unmounted tell step 1 in done -> the fs will
not be available for long time
2. force umount before step 2.


>
> I guess I don't understand your question.
>
> NeilBrown
>

Re: Linux RAID migration

[Neil Brown]

On Tuesday August 7, saeed.bishara@gmail.com wrote:
> On 8/7/07, Neil Brown <neilb@suse.de> wrote:
> > On Tuesday August 7, saeed.bishara@gmail.com wrote:
> > > how can I create raid 5 array that preserves the contents of a given
> > > disk (part of the array)?
> > > I can do that in two steps, raid 5 on two disks then reshape it to be
> > > real raid5, but is there a direct way to do it?
> >
> > What do you mean by "real raid 5".  Two disks are just as much an
> > array as 10.
> I meant by real raid 5 to raid 5 over 3 drives or above.
> > What do you mean by "direct".  If I understand your requirements
> in one mdadm command
> > correctly, there are two obvious steps.
> >   1/ make the data appear in a raid5
> >   2/ change the number of devices in the raid5.
> >
> > Are two steps too many?
> in this case I two options:
> 1. keep the filesystem unmounted tell step 1 in done -> the fs will
> not be available for long time

No, it shouldn't take a long time....

What exactly do you have, and what exactly do you want to achieve?

If think you started of by saying you wanted to convert a raid1 to a
raid5.  Yes, that requires the filesystem to be unmounted, but only
takes seconds to achieve:

 mdadm -S /dev/md0
 mdadm -C /dev/md0 -l5 -n2 /dev/sda1 /dev/sdb1
 mount /dev/md0

If you have a single-disk, you first might need to shrink the
filesystem to make sure there is 128K free at the end of the device:

  umount /filesystem
  resize2fs /dev/sda1 whatever-number
  mdadm -C /dev/md0 -l5 -n1 /dev/sda1 missing
  mount /dev/md0 /filesystem
  # now add the drives and grow
  mdadm /dev/md0 --add /dev/sdb1 /dev/sdc1
  # wait for recovery to finish
  mdadm --grow /dev/md0 --raid-disks=3

something like that..

NeilBrown

Re: Linux RAID migration

[saeed bishara]

On 8/7/07, Neil Brown <neilb@suse.de> wrote:
> On Tuesday August 7, saeed.bishara@gmail.com wrote:
> > On 8/7/07, Neil Brown <neilb@suse.de> wrote:
> > > On Tuesday August 7, saeed.bishara@gmail.com wrote:
> > > > how can I create raid 5 array that preserves the contents of a given
> > > > disk (part of the array)?
> > > > I can do that in two steps, raid 5 on two disks then reshape it to be
> > > > real raid5, but is there a direct way to do it?
> > >
> > > What do you mean by "real raid 5".  Two disks are just as much an
> > > array as 10.
> > I meant by real raid 5 to raid 5 over 3 drives or above.
> > > What do you mean by "direct".  If I understand your requirements
> > in one mdadm command
> > > correctly, there are two obvious steps.
> > >   1/ make the data appear in a raid5
> > >   2/ change the number of devices in the raid5.
> > >
> > > Are two steps too many?
> > in this case I two options:
> > 1. keep the filesystem unmounted tell step 1 in done -> the fs will
> > not be available for long time
>
> No, it shouldn't take a long time....
>
> What exactly do you have, and what exactly do you want to achieve?
>
> If think you started of by saying you wanted to convert a raid1 to a
> raid5.  Yes, that requires the filesystem to be unmounted, but only
> takes seconds to achieve:
>
>  mdadm -S /dev/md0
>  mdadm -C /dev/md0 -l5 -n2 /dev/sda1 /dev/sdb1
>  mount /dev/md0
>
> If you have a single-disk, you first might need to shrink the
> filesystem to make sure there is 128K free at the end of the device:
>
>   umount /filesystem
>   resize2fs /dev/sda1 whatever-number
>   mdadm -C /dev/md0 -l5 -n1 /dev/sda1 missing
-n1 didn't work, but -n2 succeeded.
>   mount /dev/md0 /filesystem
>   # now add the drives and grow
>   mdadm /dev/md0 --add /dev/sdb1 /dev/sdc1
>   # wait for recovery to finish
>   mdadm --grow /dev/md0 --raid-disks=3
>
> something like that..
great, this actually does the migration from single drive to raid5 as
I was looking for.
thanks.
>
> NeilBrown
>

Re: Linux RAID migration

[saeed bishara]

> >   mdadm --grow /dev/md0 --raid-disks=3
the mdadm --grow supports saving backup file that can be used to
re-assemble the array in case of crash during the "critical section".
Any idea what is the size of file? or is there any upper limit in the
case of array of 5 drives?

saeed

Re: Linux RAID migration

[Neil Brown]

On Tuesday August 7, saeed.bishara@gmail.com wrote:
> > >   mdadm --grow /dev/md0 --raid-disks=3
> the mdadm --grow supports saving backup file that can be used to
> re-assemble the array in case of crash during the "critical section".
> Any idea what is the size of file? or is there any upper limit in the
> case of array of 5 drives?

The file size depends on the chunk size and the number of disks.

I think for 5 disks it would be less than 30 times the chunk size, so
for a 128K chunk, it would be under 3 Meg.

NeilBrown

SWAP file on a RAID-10 array possible?

[Tomas France]

Hi everyone,

I apologize for asking such a fundamental question on the Linux-RAID list 
but the answers I found elsewhere have been contradicting one another.

So, is it possible to have a swap file on a RAID-10 array?

Thanks!

Tomas 

Re: SWAP file on a RAID-10 array possible?

[David Greaves]

Tomas France wrote:
> Hi everyone,
> 
> I apologize for asking such a fundamental question on the Linux-RAID 
> list but the answers I found elsewhere have been contradicting one another.
> 
> So, is it possible to have a swap file on a RAID-10 array?
yes.

mkswap /dev/md<X>
swapon /dev/md<X>

Should you use RAID-10 for swap? That's philosophy :)

David

Re: SWAP file on a RAID-10 array possible?

[Tomas France]

Thanks for the answer, David!

I kind of think RAID-10 is a very good choice for a swap file. For now I 
will need to setup the swap file on a simple RAID-1 array anyway, I just 
need to be prepared when it's time to add more disks and transform the whole 
thing into RAID-10... which will be big fun anyway, for sure ;)

By the way, does anyone know if there is a comprehensive how-to on software 
RAID with mdadm available somewhere? I mean a website where I could get 
answers to questions like "How to convert your system from no RAID to 
RAID-1, from RAID-1 to RAID-5/10, how to setup LILO/GRUB to boot from a 
RAID-1 array" etc. Don't take me wrong, I have "done my homework" and found 
a lot of info on the topic but a lot of it is several years old and many 
things have changed since then. And it's quite scattered too..

Tomas


----- Original Message ----- 
From: "David Greaves" <david@dgreaves.com>
To: "Tomas France" <tomfra@centrum.cz>
Cc: <linux-raid@vger.kernel.org>
Sent: Wednesday, August 15, 2007 12:10 PM
Subject: Re: SWAP file on a RAID-10 array possible?


> Tomas France wrote:
>> Hi everyone,
>>
>> I apologize for asking such a fundamental question on the Linux-RAID list 
>> but the answers I found elsewhere have been contradicting one another.
>>
>> So, is it possible to have a swap file on a RAID-10 array?
> yes.
>
> mkswap /dev/md<X>
> swapon /dev/md<X>
>
> Should you use RAID-10 for swap? That's philosophy :)
>
> David
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 

Re: SWAP file on a RAID-10 array possible?

[David Greaves]

Tomas France wrote:
> Thanks for the answer, David!
you're welome

> By the way, does anyone know if there is a comprehensive how-to on 
> software RAID with mdadm available somewhere? I mean a website where I 
> could get answers to questions like "How to convert your system from no 
> RAID to RAID-1, from RAID-1 to RAID-5/10, how to setup LILO/GRUB to boot 
> from a RAID-1 array" etc. Don't take me wrong, I have "done my homework" 
> and found a lot of info on the topic but a lot of it is several years 
> old and many things have changed since then. And it's quite scattered too..

Yes, I got to thinking that. About a year ago I got a copy of the old raid FAQ 
from the authors and have started to modify it when I have time and it bubbles 
up my list.

Try
http://linux-raid.osdl.org/

It's a community wiki - welcome to the community :) Please feel free to edit,,,
Also feel free to post questions/suggestions here if you're not sure.

David

RE: SWAP file on a RAID-10 array possible?

[Daniel Korstad]

I used this site to bring my existing Linux install to a RAID 1.   It worked great for me.
 
http://wiki.clug.org.za/wiki/RAID-1_in_a_hurry_with_grub_and_mdadm
 
 
 
----- Original Message -----
From: linux-raid-owner@vger.kernel.org on behalf of Tomas France 
Sent: Wed, 8/15/2007 5:28am
To: linux-raid@vger.kernel.org
Subject: Re: SWAP file on a RAID-10 array possible? 
 
 
Thanks for the answer, David!

I kind of think RAID-10 is a very good choice for a swap file. For now I 
will need to setup the swap file on a simple RAID-1 array anyway, I just 
need to be prepared when it's time to add more disks and transform the whole 
thing into RAID-10... which will be big fun anyway, for sure ;)

By the way, does anyone know if there is a comprehensive how-to on software 
RAID with mdadm available somewhere? I mean a website where I could get 
answers to questions like "How to convert your system from no RAID to 
RAID-1, from RAID-1 to RAID-5/10, how to setup LILO/GRUB to boot from a 
RAID-1 array" etc. Don't take me wrong, I have "done my homework" and found 
a lot of info on the topic but a lot of it is several years old and many 
things have changed since then. And it's quite scattered too..

Tomas


----- Original Message ----- 
From: "David Greaves" <david@dgreaves.com>
To: "Tomas France" <tomfra@centrum.cz>
Cc: <linux-raid@vger.kernel.org>
Sent: Wednesday, August 15, 2007 12:10 PM
Subject: Re: SWAP file on a RAID-10 array possible?


> Tomas France wrote:
>> Hi everyone,
>>
>> I apologize for asking such a fundamental question on the Linux-RAID list 
>> but the answers I found elsewhere have been contradicting one another.
>>
>> So, is it possible to have a swap file on a RAID-10 array?
> yes.
>
> mkswap /dev/md<X>
> swapon /dev/md<X>
>
> Should you use RAID-10 for swap? That's philosophy :)
>
> David
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 

-
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: SWAP file on a RAID-10 array possible?

[Michael Tokarev]

Tomas France wrote:
> Thanks for the answer, David!
> 
> I kind of think RAID-10 is a very good choice for a swap file. For now I
> will need to setup the swap file on a simple RAID-1 array anyway, I just
> need to be prepared when it's time to add more disks and transform the
> whole thing into RAID-10... which will be big fun anyway, for sure ;)

By the way, you don't really need raid10 for swap.  Built-in linux
swap code can utilize multiple swap areas just fine - mkswap + swapon
on multiple devices/files.  This is essentially a raid0.  For raid10,
one thing needed is the mirroring, with is provided by raid1.  So
when you've two drives, use single partition on both to form a raid1
array for swap space.  If you've 4 drives, create 2 raid1 arrays and
specify them both as swap space, giving them appropriate priority
(prio=xxx in swap line in fstab).  With 6 drives, have 3 raid1 arrays
and so on...  This way, the whole thing is much simpler and more
manageable.

/mjt

Re: SWAP file on a RAID-10 array possible?

[Bill Davidsen]

Tomas France wrote:
> Hi everyone,
>
> I apologize for asking such a fundamental question on the Linux-RAID 
> list but the answers I found elsewhere have been contradicting one 
> another.
>
> So, is it possible to have a swap file on a RAID-10 array?

Yes, and very fast as well. Do note that if you (a) need to boot from a 
recovery CD and (b) have a low memory machine which really needs swap 
early, most of the ones I've tried don't use such a swap, you have to 
start it by hand. Also, I have not tried suspend to disk and restore 
using RAID-10, it's on my to-do list, but *way* down.

But in general you will find it very fast and reliable.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

Re: SWAP file on a RAID-10 array possible?

[Tomas France]

This wouldn't be probably the best solution in my situation. The computer we
are talking about will be quad-core web server with 8GB RAM and initially
2x500GB SATA HDDs setup in a RAID-1 array. When it begins running low on
space or more HDDs performance is needed, I plan to convert the RAID-1 to
RAID-10 by adding 2-4 more hard disks (I've found some info on how to do
this so hopefully it will work).

If I should follow the commonly accepted strategy saying that the swap space
should be 2X+ the amount of RAM, that means 16GB. If I add 4 more HDDs in
RAID-1 pairs later, then I would end up with 3 swap partitions on RAID-1
taking 96GB (6*16GB) of space on the harddrives which would be a
considerable waste of space.

Sure, when adding the more hard disks I could probably create a smaller swap
partition on each of them but that would be yet another complication. Using
a swap file initally on the RAID-1 array and then on the RAID-10 array
sounds like a much simpler solution to me as it  will allow me to change the
size of the swap space more flexibly.

Tomas



----- Original Message ----- 
From: "Michael Tokarev" <mjt@tls.msk.ru>
To: "Tomas France" <tomfra@centrum.cz>
Cc: <linux-raid@vger.kernel.org>
Sent: Wednesday, August 15, 2007 11:42 PM
Subject: Re: SWAP file on a RAID-10 array possible?


> Tomas France wrote:
>> Thanks for the answer, David!
>>
>> I kind of think RAID-10 is a very good choice for a swap file. For now I
>> will need to setup the swap file on a simple RAID-1 array anyway, I just
>> need to be prepared when it's time to add more disks and transform the
>> whole thing into RAID-10... which will be big fun anyway, for sure ;)
>
> By the way, you don't really need raid10 for swap.  Built-in linux
> swap code can utilize multiple swap areas just fine - mkswap + swapon
> on multiple devices/files.  This is essentially a raid0.  For raid10,
> one thing needed is the mirroring, with is provided by raid1.  So
> when you've two drives, use single partition on both to form a raid1
> array for swap space.  If you've 4 drives, create 2 raid1 arrays and
> specify them both as swap space, giving them appropriate priority
> (prio=xxx in swap line in fstab).  With 6 drives, have 3 raid1 arrays
> and so on...  This way, the whole thing is much simpler and more
> manageable.
>
> /mjt
>