RAID 1 metadata - keep separate from mirror disks ?

RAID 1 metadata - keep separate from mirror disks ?

[Suresh Babu Kandukuru]

Hi There, 

On the RAID 1  metadata: is there  any way to keep the metadata separate from the mirror disks?  Could you  guide us on this ?, please. In general, we need to keep all metadata off the device itself, leaving all the device available for user data. This is particularly important in the migration case, where we want to take an existing LUN and add a second leg to it to create the mirror device without changing any of the data or metadata on the LUN. 

Thanks
/Suresh

Re: RAID 1 metadata - keep separate from mirror disks ?

[Phil Turmel]

Good morning Suresh,

On 02/11/2015 07:14 AM, Suresh Babu Kandukuru wrote:
> Hi There,
> 
> On the RAID 1  metadata: is there  any way to keep the metadata
> separate from the mirror disks?  Could you  guide us on this ?,
> please. In general, we need to keep all metadata off the device
> itself, leaving all the device available for user data. This is
> particularly important in the migration case, where we want to take
> an existing LUN and add a second leg to it to create the mirror
> device without changing any of the data or metadata on the LUN.

If you look at "man 4 md" you'll see some options.  If a legacy array
type meets your needs, you can operate without metadata at all.  Use
"mdadm --build" to assemble your raid at each boot.

Or, if your storage server can insert a leg ahead of you current LUN,
you can then create the array with an explicit data offset matching the
size of the inserted leg.  Create it degraded with the existing LUN,
then add (a) LUN(s) to start mirroring.  This process will leave you the
option to resize with more legs later.

Or you can add a leg to the end and create your array with version 1.0
metadata, which is placed at the end of the device.

Finally, you could write your own metadata container service for use
with mdmon.  (That's a bit beyond my ability, sorry.)

Phil

RE: RAID 1 metadata - keep separate from mirror disks ?

[Suresh Babu Kandukuru]

Thanks Phil . This helps.

/Suresh


-----Original Message-----
From: Phil Turmel [mailto:philip@turmel.org] 
Sent: Wednesday, February 11, 2015 8:19 PM
To: Suresh Babu Kandukuru; linux-raid@vger.kernel.org
Subject: Re: RAID 1 metadata - keep separate from mirror disks ?

Good morning Suresh,

On 02/11/2015 07:14 AM, Suresh Babu Kandukuru wrote:
> Hi There,
> 
> On the RAID 1  metadata: is there  any way to keep the metadata 
> separate from the mirror disks?  Could you  guide us on this ?, 
> please. In general, we need to keep all metadata off the device 
> itself, leaving all the device available for user data. This is 
> particularly important in the migration case, where we want to take an 
> existing LUN and add a second leg to it to create the mirror device 
> without changing any of the data or metadata on the LUN.

If you look at "man 4 md" you'll see some options.  If a legacy array type meets your needs, you can operate without metadata at all.  Use "mdadm --build" to assemble your raid at each boot.

Or, if your storage server can insert a leg ahead of you current LUN, you can then create the array with an explicit data offset matching the size of the inserted leg.  Create it degraded with the existing LUN, then add (a) LUN(s) to start mirroring.  This process will leave you the option to resize with more legs later.

Or you can add a leg to the end and create your array with version 1.0 metadata, which is placed at the end of the device.

Finally, you could write your own metadata container service for use with mdmon.  (That's a bit beyond my ability, sorry.)

Phil
--
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RE: RAID 1 metadata - keep separate from mirror disks ?

[Suresh Babu Kandukuru]

Yeh . Thank Phil . it is quite useful .  Now we see there are two options . 1) without metadata 2) externally managed metadata . But we would like to keep the metadata external to mirror leg ( like on host local drive )  for the reasons mentioned below , not the externally managed metadata , just by keeping metadata on mirror legs
 . Do we have any option with md driver ?

/Suresh
Principal developer | Hyderabad Team Lead
Phone: +914067246370 | Mobile: +919701451727 
Oracle Maxrep development

ORACLE India Hyderabad 
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-----Original Message-----
From: Phil Turmel [mailto:philip@turmel.org] 
Sent: Wednesday, February 11, 2015 8:19 PM
To: Suresh Babu Kandukuru; linux-raid@vger.kernel.org
Subject: Re: RAID 1 metadata - keep separate from mirror disks ?

Good morning Suresh,

On 02/11/2015 07:14 AM, Suresh Babu Kandukuru wrote:
> Hi There,
> 
> On the RAID 1  metadata: is there  any way to keep the metadata 
> separate from the mirror disks?  Could you  guide us on this ?, 
> please. In general, we need to keep all metadata off the device 
> itself, leaving all the device available for user data. This is 
> particularly important in the migration case, where we want to take an 
> existing LUN and add a second leg to it to create the mirror device 
> without changing any of the data or metadata on the LUN.

If you look at "man 4 md" you'll see some options.  If a legacy array type meets your needs, you can operate without metadata at all.  Use "mdadm --build" to assemble your raid at each boot.

Or, if your storage server can insert a leg ahead of you current LUN, you can then create the array with an explicit data offset matching the size of the inserted leg.  Create it degraded with the existing LUN, then add (a) LUN(s) to start mirroring.  This process will leave you the option to resize with more legs later.

Or you can add a leg to the end and create your array with version 1.0 metadata, which is placed at the end of the device.

Finally, you could write your own metadata container service for use with mdmon.  (That's a bit beyond my ability, sorry.)

Phil
--
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: RAID 1 metadata - keep separate from mirror disks ?

[Phil Turmel]

Hi Suresh,

On 02/16/2015 06:37 AM, Suresh Babu Kandukuru wrote:
> Yeh . Thank Phil . it is quite useful .  Now we see there are two
> options . 1) without metadata 2) externally managed metadata . But we
> would like to keep the metadata external to mirror leg ( like on host
> local drive )  for the reasons mentioned below , not the externally
> managed metadata , just by keeping metadata on mirror legs . Do we
> have any option with md driver ?

I'm not sure I understand your follow-up question.  You've restated my
answer, and re-iterated your preference to not have metadata on the
member devices.

So you need to write a service to handle metadata the way you want.  Or
write scripts that will do --build operations at appropriate times
without metadata.

I'm not entirely clear why the on-member metadata is unacceptable, but
as such, either remaining option needs some code of your own.

Phil