resync (what is it doing when making a new array?)

resync (what is it doing when making a new array?)

[Eli Cantu]

Hello All,

I've gone through the faq, archives, newsgroups, google etc.

I cannot find an answer to my question: After creating a new array, why is the
time consuming resync required?


I've built a few hardware RAID 5 arrays in my time using Mylex, Adaptec, AMI
etc. I don't recall this hours long, sometimes days long initial resync using
hardware raid.

What's written to the disks using linux raid, and why isn't it used in hardware
raids?

Thanks,
~e

(watching a raid 5 array, composed of five 250gb disks, resyncing...to be
completed 16 hours from now...)

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Re: resync (what is it doing when making a new array?)

[Daniel Pittman]

On 8 Aug 2004, Eli Cantu wrote:
> I cannot find an answer to my question: After creating a new array, why is the
> time consuming resync required?

Because the disks need to contain the correct data, or they will cause
faults when you try to use them after a disk failure. :)

> I've built a few hardware RAID 5 arrays in my time using Mylex, Adaptec, AMI
> etc. I don't recall this hours long, sometimes days long initial resync using
> hardware raid.

Usually, this is handled by either doing the sync in the background
quietly, or by keeping an NVRAM bitmap of "unclean" blocks that starts
with all the bits set.

Regards,
        Daniel
-- 
I have always detested the belief that sex is the chief bond
between man and woman. Friendship is far more human.
        -- Agnes Smedley

Re: resync (what is it doing when making a new array?)

[Eli Cantu]

> Usually, this is handled by either doing the sync in the background
> quietly, or by keeping an NVRAM bitmap of "unclean" blocks that starts
> with all the bits set.
>
> Regards,
>         Daniel
> --

Thanks Daniel & Gordon,

I suspected as much. Well, it's a new day and my array finished resyncing. Yea!

~e


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Re: resync (what is it doing when making a new array?)

[Gordon Henderson]

On Sat, 7 Aug 2004, Eli Cantu wrote:

> (watching a raid 5 array, composed of five 250gb disks, resyncing...to
> be completed 16 hours from now...)

One thing to check (if it's IDE drives) is that they are working in DMA
mode - use hdparm /dev/hda, etc. to check and force DMA if necessary
(although you might need to re-compile your kernel to include the correct
drivers for your IDE hardware)

It'll still take a long time with 250GB drives, though.

Other things you can fiddle with are

  /proc/sys/dev/raid/speed_limit_max
  /proc/sys/dev/raid/speed_limit_min

which will affect resync speed versus array accessibility during the
recync.

Gordon

Re: resync (what is it doing when making a new array?)

[Jon Lewis]

On Sun, 8 Aug 2004, Gordon Henderson wrote:

> On Sat, 7 Aug 2004, Eli Cantu wrote:
>
> > (watching a raid 5 array, composed of five 250gb disks, resyncing...to
> > be completed 16 hours from now...)
>
> One thing to check (if it's IDE drives) is that they are working in DMA
> mode - use hdparm /dev/hda, etc. to check and force DMA if necessary
> (although you might need to re-compile your kernel to include the correct
> drivers for your IDE hardware)
>
> It'll still take a long time with 250GB drives, though.

16 hours seems kind of long.  I've recently been dealing with a 6 200gb
drive (SATA) sw RAID5, and resyncs took around 3 hours on an otherwise
totally idle P4 2.4ghz.

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