mdadm and disk failures

I was looking around the wiki and I could not find the answer.

I wanted to know if the raid system will recover bad blocks lost on the
one device when the copies of those blocks are available on another.
From what I understand when a hard drive loses a block it replaces it
with another block but it can not recover the data. At this point I do
not know what the raid system does. At one point in the past my
understanding is, the raid system did nothing. If it read from the disk
that had the block it gets garbage for data. Conceptually the raid
system could read the data from one disk and write it to the copy on the
other and restore the data.

Is the raid system capable of recovering and restoring bad blocks?

Mike Power
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html

Re: mdadm and disk failures

On 16/06/11 11:37, Mike Power wrote:
> Is the raid system capable of recovering and restoring bad blocks?
>
Yes, and it does so admirably. You will see a message in your system log to the effect that it had
recovered the (or those) bad sector(s).

Brad
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html

Re: mdadm and disk failures

--Sig_/17GAR60NTo/eAtBwc8_uqqA
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: quoted-printable

On Wed, 15 Jun 2011 20:37:31 -0700
Mike Power wrote:

> From what I understand when a hard drive loses a block it replaces it=20
> with another block but it can not recover the data.
> If it read from the disk that had the block it gets garbage for data.

This is not true, an HDD will never return garbage data just because it nee=
ded
to remap a bad sector. And that's the reason why bad sectors will return I/O
error (unreadable) instead, and are not remapped anywhere until you try to
WRITE into these sectors. At that moment, when the hard disk has new user d=
ata
for that sector, it can finally do the remap and put it somewhere else on a
remapped good sector.

--=20
With respect,
Roman

--Sig_/17GAR60NTo/eAtBwc8_uqqA
Content-Type: application/pgp-signature; name=signature.asc
Content-Disposition: attachment; filename=signature.asc

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.11 (GNU/Linux)

iEYEARECAAYFAk35jZwACgkQTLKSvz+PZwghnACgjF8sSbdRQWXbYLF+Twrv NKV6
N74AoI6W7M0HflSHwu0X6dRhmzdAeAfH
=7ROB
-----END PGP SIGNATURE-----

--Sig_/17GAR60NTo/eAtBwc8_uqqA--
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html

Re: mdadm and disk failures

On Thu, 16 Jun 2011, Roman Mamedov wrote:

> At that moment, when the hard disk has new user data for that sector, it
> can finally do the remap and put it somewhere else on a remapped good
> sector.

Just to be clear, it won't necessarily remap the sector. It might succeed
in writing the new data to it and determine the sector isn't really bad,
and not remap it.

--
Mikael Abrahamsson email: swmike@swm.pp.se
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html

Re: mdadm and disk failures

Ahh I understand more clearly now. Because the failing drive will
report an IO error the raid system can respond reactively by getting the
data else where and writing it to the failing disk. Earlier I had
thought that the raid system might periodically poll the disks to see
what blocks were lost and correct them.

This makes me much more interested in setting up a raid system above and
beyond the performance benefit.
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html