Avoiding resync of RAID1 during creation

Avoiding resync of RAID1 during creation

[Bryan Wann]

With FC2, when installing a fresh new system we would create a RAID10 
array by creating several RAID1s, then adding all of those to a RAID0 
array.  To make the RAID1 devices, we'd use the command:
   /sbin/mkraid --really-force --dangerous-no-resync /dev/mdX

Then we'd set up the RAID0 and mke2fs our filesystems on top of it. 
This worked well for us, never had any problems later.  As soon as the 
kickstart was finished, the system was ready to go.

Now with FC4, raidtools is gone and I'm left with mdadm tools.  As far 
as I can tell, mdadm has nothing resembling --dangerous-no-resync.  I've 
updated my kickstart to use mdadm instead of mkraid using:

   /sbin/mdadm --create /dev/md4 --force --run --level=1 --chunk=256 \
   --raid-disks=2 --spare-devices=0 /dev/sda5 /dev/sde5

This causes all of the newly created RAID1 devices to start syncing.  On 
a system with many large disks and RAID1 arrays, syncing takes a 
considerably long time.

Is there any way to avoid the sync after creation when using mdadm like 
I could with mkraid?

The compelling argument I've read in the archives indicates this would 
run counter to ensuring both partitions were completely clean at a block 
level.  I would think creation of the filesystem on top of the array 
would ensure they're clean, at least on that level for all intents and 
purposes.


--bryan

Re: Avoiding resync of RAID1 during creation

[Tuomas Leikola]

On 2/20/06, Bryan Wann <bryan@datafoundry.com> wrote:
> Now with FC4, raidtools is gone and I'm left with mdadm tools.  As far
> as I can tell, mdadm has nothing resembling --dangerous-no-resync.  I've
> updated my kickstart to use mdadm instead of mkraid using:
>

mdadm --assume-clean

from the man page:

It can also be used when creating a RAID1 or RAID10 if you want to
avoid the initial resync, however this  practice - while normally safe
- is not recommended.

/ tuomas

Re: Avoiding resync of RAID1 during creation

[Bryan Wann]

Tuomas Leikola wrote:
> mdadm --assume-clean
> 
> from the man page:
> 
> It can also be used when creating a RAID1 or RAID10 if you want to
> avoid the initial resync, however this  practice - while normally safe
> - is not recommended.

What version of mdadm was that from?  From mdadm(8) in 
mdadm-1.11.0-4.fc4 on my systems:

   --assume-clean
     Tell  mdadm that the array pre-existed and is known to be clean.
     This is only really useful for Building RAID1 array.   Only  use
     this  if  you really know what you are doing.  This is currently
     only supported for --build.

I tried with --assume-clean, it still wanted to sync.  From what my man 
page was telling me, it only works with --build.  If I use --build it'll 
go ahead without syncing, but I need per-device superblocks.  Why mdadm 
didn't error when I used --assume-clean with --create, I don't know.

--bryan

Re: Avoiding resync of RAID1 during creation

[Tuomas Leikola]

On 2/20/06, Bryan Wann <bryan@datafoundry.com> wrote:
> > mdadm --assume-clean
> >
> What version of mdadm was that from?  From mdadm(8) in
> mdadm-1.11.0-4.fc4 on my systems:
>
<cut>
> I tried with --assume-clean, it still wanted to sync.

The man page i quoted was from 2.3.1 (6 feb) - relatively new.

I tested this with 2 boxes: 1.9.0 starts the resync and 2.3.1 doesn't.
Used kernel 2.6.14 - altough I don't expect that to make much of a
difference.

-tuomas