mdadm 2.2 new option

mdadm 2.2 new option

[Robert Heinzmann]

Hello,

i just downloaded the mdadm 2.2 version. It seems that there are some 
new options which are not explained in details. of special interest are 
2 options.

-GROW
The new grow mode is very very helpfull for us. It would allow us to do 
resize operations with md devices at last - without the need for LVM.
The manpage states that the GROW mode requires kernel support to work. 
There is no note that states, which kernel supports the options. Also 
with various distributions with patched kernels its not possible to say 
version X.Y.Z. of the kernel contains the required patches.

Is there a way to find out via mdadm, if the current kernel supports the 
GROW mode, prior to performing the GROW ?

-bitmap

As far as I understand this bitmap options, in incorperates write intend 
logging.
Is this based on paper by James Bottomley ? Also is there a way to find 
out via mdadm if the running kernel supports intend logging via mdadm ?

-There is also an option --write-mostly since 2.0.

I guess this especially usefull for MD devices with large storage arrays 
as backend. They have clever caches and read ahead paramaters, that can 
be non-effective if we change the disk for each read. Does this option 
require a kernel modification too ?

One last question. Besindes the source code and manpage is there oterh 
documentation available on how the md works internally ? (update of 
superblock, handling of device renaming inside the superblock etc.)

Thanks,
Robert Heinzmann

Re: mdadm 2.2 new option

[Neil Brown]

On Thursday December 15, Robert.Heinzmann@gmx.net wrote:
> Hello,
> 
> i just downloaded the mdadm 2.2 version. It seems that there are some 
> new options which are not explained in details. of special interest are 
> 2 options.
> 
> -GROW
> The new grow mode is very very helpfull for us. It would allow us to do 
> resize operations with md devices at last - without the need for LVM.
> The manpage states that the GROW mode requires kernel support to work. 
> There is no note that states, which kernel supports the options. Also 
> with various distributions with patched kernels its not possible to say 
> version X.Y.Z. of the kernel contains the required patches.
> 
> Is there a way to find out via mdadm, if the current kernel supports the 
> GROW mode, prior to performing the GROW ?

No, there isn't.  However if your kernel doesn't support the
operation, it will fail cleanly so there is no risk.

> 
> -bitmap
> 
> As far as I understand this bitmap options, in incorperates write intend 
> logging.
> Is this based on paper by James Bottomley ? Also is there a way to find 
> out via mdadm if the running kernel supports intend logging via
> mdadm ?

Yes, it is comes (indirectly) from James.
No, you cannot query the kernel.

> 
> -There is also an option --write-mostly since 2.0.
> 
> I guess this especially usefull for MD devices with large storage arrays 
> as backend. They have clever caches and read ahead paramaters, that can 
> be non-effective if we change the disk for each read. Does this option 
> require a kernel modification too ?

Yes, this also requires a recent kernel - 2.6.14 I think.
It is intended for mirrored situations where one mirror is over a slow
link (e.g. over the Internet). .

> 
> One last question. Besindes the source code and manpage is there oterh 
> documentation available on how the md works internally ? (update of 
> superblock, handling of device renaming inside the superblock etc.)

No there isn't, though I am seeing more and more requests for this and
am starting to wonder if I should commit some time to writing
something.

NeilBrown