Just a thought - linux raid wiki - raid 5 grows getting stuck at 0%

Just a thought - linux raid wiki - raid 5 grows getting stuck at 0%

[Wols Lists]

I've just been doing a bit of work on the wiki when I put 2 and 2
together, and hope I haven't made 5 ...

Bitmaps interfere with grow operations, I believe ...

And mdadm has recently been modified so that bitmaps are switched on by
default, I also believe ...

Could this be silently blocking the grow, so you don't get any error and
it just sits there doing nothing?

But that brings up two points. Should mdadm now explicitly look for a
bitmap when growing an array, and warn that the bitmap needs to be
disabled? Apparently it currently comes up with some errors that hint at
the cause rather than explicitly say so.

And secondly, I'm sure someone sent an email to the list that explained
when a bitmap was created. I can't find it in my archives. The man page
says it's created if the array is over 100G, but I'm sure the email gave
rather more detail than that and that the figure actually varied.

But I'm sure you can see from this that I'm updating this bit of the
wiki. Is there anything else I ought to know about bitmaps, so I can
document it? :-)

Cheers,
Wol

Re: Just a thought - linux raid wiki - raid 5 grows getting stuck at 0%

[NeilBrown]

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On Thu, Nov 17 2016, Wols Lists wrote:

> I've just been doing a bit of work on the wiki when I put 2 and 2
> together, and hope I haven't made 5 ...
>
> Bitmaps interfere with grow operations, I believe ...

Used to, yes.  Don't any more.

>
> And mdadm has recently been modified so that bitmaps are switched on by
> default, I also believe ...
>
> Could this be silently blocking the grow, so you don't get any error and
> it just sits there doing nothing?

Nope.  If it was a problem it would explicitly fail.

>
> But that brings up two points. Should mdadm now explicitly look for a
> bitmap when growing an array, and warn that the bitmap needs to be
> disabled? Apparently it currently comes up with some errors that hint at
> the cause rather than explicitly say so.

If you try to reshape an array which has a bitmap, on a kernel that
doesn't support reshaping arrays with bitmaps, mdadm hits this line of
code:

			if (err == EBUSY &&
			    (array.state & (1<<MD_SB_BITMAP_PRESENT)))
				cont_err("Bitmap must be removed before size can be changed\n");

or maybe this one

		pr_err("Cannot set array shape for %s\n",
		       devname);
		if (err == EBUSY &&
		    (info->array.state & (1<<MD_SB_BITMAP_PRESENT)))
			cont_err("       Bitmap must be removed before shape can be changed\n");

or

			if (err == EBUSY &&
			    (array.state & (1<<MD_SB_BITMAP_PRESENT)))
				cont_err("Bitmap must be removed before level can be changed\n");



>
> And secondly, I'm sure someone sent an email to the list that explained
> when a bitmap was created. I can't find it in my archives. The man page
> says it's created if the array is over 100G, but I'm sure the email gave
> rather more detail than that and that the figure actually varied.

	if (!s->bitmap_file &&
	    s->level >= 1 &&
	    st->ss->add_internal_bitmap &&
	    (s->write_behind || s->size > 100*1024*1024ULL)) {
		if (c->verbose > 0)
			pr_err("automatically enabling write-intent bitmap on large array\n");
		s->bitmap_file = "internal";
	}

So it looks like "over 100G" is the only test.

>
> But I'm sure you can see from this that I'm updating this bit of the
> wiki. Is there anything else I ought to know about bitmaps, so I can
> document it? :-)

If only I could run "diff" between my brain and the wiki....

Thanks for doing this!

NeilBrown

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Re: Just a thought - linux raid wiki - raid 5 grows getting stuck at 0%

[Wols Lists]

On 17/11/16 06:18, NeilBrown wrote:
>> > But I'm sure you can see from this that I'm updating this bit of the
>> > wiki. Is there anything else I ought to know about bitmaps, so I can
>> > document it? :-)

> If only I could run "diff" between my brain and the wiki....

:-)
> 
> Thanks for doing this!
> 
> NeilBrown

I'll update the wiki in the next day or so.

I'm a bit worried I'll spread myself too thin, but I've been following
the threads on the recent patch sets, and I've also picked up on the new
Sphinx kernel documentation stuff.

So my aim now is, on top of generally updating the wiki, to link it in
to the kernel doc, and update a lot of that doc using re-structured text
in the md source files. That's not going to be a problem, I hope?

I'm going to dig and find out more, but if anybody can point me to some
kerneldoc code in the md source, and the corresponding links to the doc
on the kernel.org website, that'll be wonderful. It'll just give me a
leg up on the hardest part - actually getting started.

And if there isn't any at present, it'll just tell me it's needed even
more :-)

Cheers,
Wol

Re: Just a thought - linux raid wiki - raid 5 grows getting stuck at 0%

[NeilBrown]

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On Thu, Nov 17 2016, Wols Lists wrote:

> On 17/11/16 06:18, NeilBrown wrote:
>>> > But I'm sure you can see from this that I'm updating this bit of the
>>> > wiki. Is there anything else I ought to know about bitmaps, so I can
>>> > document it? :-)
>
>> If only I could run "diff" between my brain and the wiki....
>
> :-)
>> 
>> Thanks for doing this!
>> 
>> NeilBrown
>
> I'll update the wiki in the next day or so.
>
> I'm a bit worried I'll spread myself too thin, but I've been following
> the threads on the recent patch sets, and I've also picked up on the new
> Sphinx kernel documentation stuff.
>
> So my aim now is, on top of generally updating the wiki, to link it in
> to the kernel doc, and update a lot of that doc using re-structured text
> in the md source files. That's not going to be a problem, I hope?
>
> I'm going to dig and find out more, but if anybody can point me to some
> kerneldoc code in the md source, and the corresponding links to the doc
> on the kernel.org website, that'll be wonderful. It'll just give me a
> leg up on the hardest part - actually getting started.

There is no kerneldoc documentation in md - just a few ad-hoc large
comments.
Even Documentation/md.txt isn't in very good shape.

I would be nice to have more documentation, but documentation can so
quickly become stale....

Thanks,
NeilBrown


>
> And if there isn't any at present, it'll just tell me it's needed even
> more :-)
>
> Cheers,
> Wol

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