mdadm: Bitmap size

mdadm: Bitmap size

[Ste]

Hi all, i have this problem:

I am going to do raid 1 over two virtual devices. These virtual devices 
are two hd situated on other two servers.

The problem is:
A network failure could happen often, and if it happen, i don't want to 
recover all the disks. I want to recover just last changed things.

As i can see the bitmap do exactly this, but the default bitmap is too 
small!
There is no problem to have a bitmap file of 1 Gb, but i can't find the 
way to tell to mdadm the size that he has to use for the bitmap file.

Thanks, Stefano.

mdadm: bitmap size

[Ste]

Hi all, i have this problem:

I am going to do raid 1 over two virtual devices. These virtual devices 
are two hard disks situated on other two servers.

The problem is:
A network failure could happen often, and if it happen, i don't want to 
recover all the disks. I want to recover just last changed things.

As i can see the bitmap do exactly this, but the default bitmap is too 
small!
There is no problem to have a bitmap file of 1 Gb, but i can't find the 
way to tell to mdadm the size to use for the bitmap file.

Thanks,
Stefano.

Re: mdadm: bitmap size

[Neil Brown]

On Friday May 19, ste@phx6a.ath.cx wrote:
> Hi all, i have this problem:
> 
> I am going to do raid 1 over two virtual devices. These virtual devices 
> are two hard disks situated on other two servers.
> 
> The problem is:
> A network failure could happen often, and if it happen, i don't want to 
> recover all the disks. I want to recover just last changed things.
> 
> As i can see the bitmap do exactly this, but the default bitmap is too 
> small!

Why do you say that?
Are you using an internal bitmap, or a bitmap in a separate file?

> There is no problem to have a bitmap file of 1 Gb, but i can't find the 
> way to tell to mdadm the size to use for the bitmap file.

The only way to control the size of the bitmap is the change the
bitmap chunk size.
Warning: if you have more than 1 million bits in the bitmap, the
kernel may fail in memory allocation and may not be able to assemble
your array.

NeilBrown

Re: mdadm: bitmap size

[Neil Brown]

(Please don't reply off-list.  If the conversation starts on the list,
please leave it there unless there is a VERY GOOD reason).

On Monday May 22, stefano.russo@gmail.com wrote:
> On 5/19/06, Neil Brown <neilb@suse.de> wrote:
> >
> > On Friday May 19, ste@phx6a.ath.cx wrote:
> > > As i can see the bitmap do exactly this, but the default bitmap is too
> > > small!
> >
> > Why do you say that?
> > Are you using an internal bitmap, or a bitmap in a separate file?
> 
> 
> I was using bitmap in a separate file.
> Why i said that tha bitmap is too small? I try to explain:
> 
> the raid device is a raid1, created on /dev/md0 trought mdadm, and the
> bitmap use a 4 kb chunk-size on external file (in root directory)
> 
> 
> setfaulty /dev/md0 /dev/nda
> raidhotremove /dev/md0 /dev/nda
> cd /mnt/md0
> wget http://...   (240 kb file...)
> raidhotadd /dev/md0 /dev/nda
> 
> And now dmesg said that the bitmap was obsolete (0<1 or something like that)
> and that the md driver will force a total recovery.

raidhotadd doesn't know anything about bitmaps.
If you use 'mdadm /dev/mda --add /dev/nda' you should find that it
works better.

I recommend getting rid of setfaulty / raidhotadd /raidhotremove etc
and just using mdadm.

NeilBrown


> 
> A recovery of 40 gb for a 240 kb file is a little bit expensive.. :-)
> 
> Unfortunately i cannot give you the exact output because the server is down
> now.:-|
> 
> 
> The only way to control the size of the bitmap is the change the
> > bitmap chunk size.
> 
> 
> Okay thanks.
> 
> Warning: if you have more than 1 million bits in the bitmap, the
> > kernel may fail in memory allocation and may not be able to assemble
> > your array.
> 
> 
> Thankyou for your help.
> 
> Stefano.

Re: mdadm: bitmap size

[Ste]

Neil Brown wrote:
> (Please don't reply off-list.  If the conversation starts on the list,
> please leave it there unless there is a VERY GOOD reason).
>   
Sorry this wasn't my intent.
> raidhotadd doesn't know anything about bitmaps.
> If you use 'mdadm /dev/mda --add /dev/nda' you should find that it
> works better.
>
> I recommend getting rid of setfaulty / raidhotadd /raidhotremove etc
> and just using mdadm.
>   
Okay, thankyou. :-)

Stefano.

Re: mdadm: bitmap size

[Ste]

Ste wrote:
>> raidhotadd doesn't know anything about bitmaps.
>> If you use 'mdadm /dev/mda --add /dev/nda' you should find that it
>> works better.
>>
>> I recommend getting rid of setfaulty / raidhotadd /raidhotremove etc
>> and just using mdadm.
>>
Okay now it works really fine. You could write somewere to not mix mdadm 
and standard raidtools.

Thanks a lot!
Bye! :-)