Date: Tue, 24 Feb 1998 10:58:42 -0600 From: Karl Denninger <karl@mcs.net> To: hackers@FreeBSD.ORG Subject: SCSI Sense ASC 11, ASCQ 0x0c - Unrecovered read errors Message-ID: <19980224105842.07731@mcs.net>
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Hi folks,
I have a question...
Right now, as the driver stands, if you get a sense return on a disk of
0x11,0x0c ("Unrecovered read error - recommend rewrite the block"), the
driver does not attempt to do anything about it.
Why?
You're screwed in this case - the data is gone. But, some RAID controllers
(notably the CMD adapters) will *FIX* such an error if you write back to the
block.
Here's the scenario:
1) You have a failure on a data drive. It gets reported back with
sense ASC 0x11, 0x0c.
2) The driver does not attempt to do anything other than report the
error.
This sounds like bogus behavior to me. Here's why:
You've ALREADY lost the data. There is no harm in trying to
"fix it". Thus, why not do the following:
a) Attempt a forced reassign of the block.
b) If that FAILS, write zeros into the block.
Why do these things you ask? Simple:
1) The error, if repeated (or even singly) may cause a panic. If its
in a swap area, for example, you're screwed - you're probably
reading back a page of an executable from the paging space, and if
its corrupted you're going down.
2) If its a data file you MIGHT die. There's no way to know.
3) IF YOU DON'T "FIX" IT, YOU WILL GET KILLED EVENTUALLY.
With a regular disk, (a) above will succeed. You may still crash, but at
least you should come back up. If the data was a file, its gone anyway -
likewise for a directory. There is no harm in trying to prevent FUTURE
errors at that point.
If you have a RAID adapter, you got the error because BOTH the parity and
primary data were unreadable. (a) will probably fail; most RAID controllers
refuse reassignment. Writing zeros will be buffered in the controller
though - now the error is "gone". A rebuild (which should already be in
process) now fixes the error *permanently*.
What am I missing here, and why isn't this done?
--
--
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