bitmap questions

Hi,

I have a question on how bitmap works and what's shown in /proc/mdstat,=
for=20
example:

md2 : active raid10 sdc3[0] sdd3[3] sdb3[2] sda3[1]
60002560 blocks 64K chunks 2 near-copies [4/4] [UUUU]
bitmap: 1/1 pages [4KB], 65536KB chunk

- 1 bitmap for 4096 chunks

- if it shows 0/1 pages [0KB] what does it actually mean? Does that mea=
n that=20
in case of resync it will resync _all_ 4096 chunks in that page or only=
chunks=20
that require that (for example 7 chunks) ?

- if it will resync all 4096 chunks what's the point of even allowing 1=
page=20
setup (via mdadm)?

Thanks!
--=20
Arkadiusz Miśkiewicz PLD/Linux Team
arekm / maven.pl http://ftp.pld-linux.org/

--=20
Arkadiusz Miśkiewicz PLD/Linux Team
arekm / maven.pl http://ftp.pld-linux.org/
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Re: bitmap questions

On Wed, Oct 13, 2010 at 12:24 PM, Arkadiusz Miskiewicz
wrote:

> I have a question on how bitmap works and what's shown in /proc/mdsta=
t, for
> example:
>
> md2 : active raid10 sdc3[0] sdd3[3] sdb3[2] sda3[1]
> =A0 =A0 =A060002560 blocks 64K chunks 2 near-copies [4/4] [UUUU]
> =A0 =A0 =A0bitmap: 1/1 pages [4KB], 65536KB chunk
>
> - 1 bitmap for 4096 chunks
>
> - if it shows 0/1 pages [0KB] what does it actually mean?

If it's 0 pages then no resync.
I assume you mean 1/1 ?

> Does that mean that
> in case of resync it will resync _all_ 4096 chunks in that page or o=
nly chunks
> that require that (for example 7 chunks) ?

It only resyncs what is dirty. A page dirty in the in-memory bitmap
does not mean ALL bits are dirty, just that the page is allocated.

--
Paul
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Re: bitmap questions

On Thursday 14 of October 2010, Paul Clements wrote:
> On Wed, Oct 13, 2010 at 12:24 PM, Arkadiusz Miskiewicz
>=20
> wrote:
> > I have a question on how bitmap works and what's shown in /proc/mds=
tat,
> > for example:
> >=20
> > md2 : active raid10 sdc3[0] sdd3[3] sdb3[2] sda3[1]
> > 60002560 blocks 64K chunks 2 near-copies [4/4] [UUUU]
> > bitmap: 1/1 pages [4KB], 65536KB chunk
> >=20
> > - 1 bitmap for 4096 chunks
> >=20
> > - if it shows 0/1 pages [0KB] what does it actually mean?
>=20
> If it's 0 pages then no resync.
> I assume you mean 1/1 ?

Ok, I thought differently since it's bitmap->pages - bitmap->missing_pa=
ges=20
(thought that missing_pages == dirty one).

> > Does that mean that
> > in case of resync it will resync _all_ 4096 chunks in that page or=
only
> > chunks that require that (for example 7 chunks) ?
>=20
> It only resyncs what is dirty. A page dirty in the in-memory bitmap
> does not mean ALL bits are dirty, just that the page is allocated.

So there is no way to know in advance how much is going to be resynced =
when=20
sudden reset would happen in that moment?

Anyway good to know that 1 page setup also makes sense :)

> --
> Paul

--=20
Arkadiusz Miśkiewicz PLD/Linux Team
arekm / maven.pl http://ftp.pld-linux.org/
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Re: bitmap questions

On 14/10/2010 07:00, Arkadiusz Miskiewicz wrote:
[...]
> So there is no way to know in advance how much is going to be resynced when
> sudden reset would happen in that moment?

Only in the general sense that there's a limit to how many chunks could
have been dirtied in whatever period your kernel will hang on to stuff
before insisting on flushing it. I think that defaults to 5 seconds, so
you'd have to have a pathological workload to dirty so many chunks the
resync might take more than a minute.

Cheers,

John.

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Re: bitmap questions

Arkadiusz Miskiewicz wrote:
> So there is no way to know in advance how much is going to be resynced when
> sudden reset would happen in that moment?

mdadm -X /dev/component-device


regards
Mario
--
To err is human. To really foul things up requires a computer.

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