no reconstruction in 2.6.14.2?

no reconstruction in 2.6.14.2?

[Carlos Carvalho]

I've just installed a new server with a 5-disk raid5 and kernel
2.6.14.2. To check something I did a hard reset without shutdown and
on reboot the machine didn't do any automatic resync; all arrays are
shown clean.

Is this some automatic intent-logging or another 2.6 feature (the
machine had been really idle for minutes before the reset)??? This is
my first 2.6 raid5 and with 2.4 resync does happen...

Re: no reconstruction in 2.6.14.2?

[Mike Hardy]

I meant to send this to the list as well, and sent it to just Carlos
instead. Obviously chime in if I'm off base.

-Mike

-------- Original Message --------
Subject: Re: no reconstruction in 2.6.14.2?
Date: Sat, 19 Nov 2005 13:13:11 -0800
From: Mike Hardy <mhardy@h3c.com>
To: Carlos Carvalho <carlos@fisica.ufpr.br>
References: <17279.33939.476421.616066@fisica.ufpr.br>


One of the things you get with newer kernels is that the md driver more
aggressively marks the array clean, typically very quickly after the
last write was issued.

The benefit is that more often than not even if you shut down
unexpectedly, the arrays are clean and you don't have to resync on startup.

So I think the behavior is expected.

If you were to run bonnie++ against the array, and shut down abruptly,
you would most likely be unclean and get the resync.

-Mike

Carlos Carvalho wrote:
> I've just installed a new server with a 5-disk raid5 and kernel
> 2.6.14.2. To check something I did a hard reset without shutdown and
> on reboot the machine didn't do any automatic resync; all arrays are
> shown clean.
> 
> Is this some automatic intent-logging or another 2.6 feature (the
> machine had been really idle for minutes before the reset)??? This is
> my first 2.6 raid5 and with 2.4 resync does happen...
> -
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Re: no reconstruction in 2.6.14.2?

[Neil Brown]

On Saturday November 19, carlos@fisica.ufpr.br wrote:
> I've just installed a new server with a 5-disk raid5 and kernel
> 2.6.14.2. To check something I did a hard reset without shutdown and
> on reboot the machine didn't do any automatic resync; all arrays are
> shown clean.
> 
> Is this some automatic intent-logging or another 2.6 feature (the
> machine had been really idle for minutes before the reset)??? This is
> my first 2.6 raid5 and with 2.4 resync does happen...

You could put it this way:
  2.6 has a default 'intend log' of 1 bit that covers the whole
  array.  Before writing, it sets the bit.  20msecs after there has
  been no write activity, it clears the bit.

This bit is always known as the 'dirty' or 'active' bit.

(When you have full bitmap intent logging, each bit covers much less
space, and the timeout for clearing bits is about 10seconds (by
default) rather than 20msecs)

NeilBrown