RE: Scrub?

As long as the relocation contains the original information; which is
only the case if the relocation is done on a write to the disk.

A RAID-5 generated relocation allows the data to be reconstructed on a
media failure on erad, and the relocation (triggered by a re-write)
would contain the correct information preventing corruption.

Sincerely -- Mark Salyzyn

-----Original Message-----
From: dean gaudet [mailto:dean-list-linux-raid@arctic.org] 
Sent: Saturday, August 07, 2004 4:13 PM
To: Salyzyn, Mark
Cc: Kanoa Withington; Derek Listmail Acct; linux-raid@vger.kernel.org
Subject: RE: Scrub?

no, that's not how it works.  i'm referring to the hard disk itself
relocating a sector -- it's transparent to the host/raid.  the only
thing
the raid software might see is that the disk will be less snappy while
it's running the SMART long test.  (mind you i do this on live busy
systems and i don't really tend to notice it -- although on particularly
busy weeks, some disks can take several days to complete their self test
in the few spare cycles they find.)

-dean

On Sat, 7 Aug 2004, Salyzyn, Mark wrote:

Problem with running the relocation is that the RAID-5 will now be
corrupt. The RAID-5 algorithm needs to be in-touch with disk block
relocation so that it can correct the parity and the data.

Sincerely -- Mark Salyzyn

-----Original Message-----
From: dean gaudet [mailto:dean-list-linux-raid@arctic.org]
Sent: Friday, August 06, 2004 5:59 PM
To: Kanoa Withington
Cc: Salyzyn, Mark; Derek Listmail Acct; linux-raid@vger.kernel.org
Subject: RE: Scrub?

On Fri, 6 Aug 2004, Kanoa Withington wrote:

quoted

On Fri, 6 Aug 2004, Salyzyn, Mark wrote:

quoted

Just reading the entire array should correct the bad blocks, so

reverse

quoted

quoted

the sense of the dd:

	dd if=/dev/md0 of=/dev/null bs=200b

to find and replace the bad blocks (making the assumption that md

works

quoted

quoted

like the H/W RAID cards).

In this case software RAID does not work like the H/W cards. Finding
an unreadable block that way in a software array would cause it to

go

quoted

into a degraded state.

if the disks support SMART (i.e. they're less than a few years old)

then

try running the smart long selftest... it can be done online and on

many

disks it will force sector reallocation (and produce a SMART log event
so
you know it happenned).

get smartmontools and run "smartctl -a" to see info on your drive, and
"smartctl -t long" to launch the long test.  man page has more

examples.

i run smart long tests on each my disks once a week (staggerred over
many
nights)... see /etc/smartd.conf.

-dean