new Data Recovery on deferred errors [94-067r2]

Gerry Houlder Gerry_Houlder at
Fri Aug 19 12:59:37 PDT 1994

Jerry Witalka made this comment about the data recovery proposal:
>I assume that once the ACA condition is cleared, the Target will make
>another attempt to write the data to the disk and mark all affected areas on
>the disk as corrupted if the error persists.  Our Host software will
>probably not attempt to retrieve the data from the cache.  The probability
>of occurrence for this error should be much lower that a failure on the disk
>itself after the Write.
No, Jerry, do not make that assumption. The target is reporting an
unrecoverable write error with this mechanism because auto-reallocation and all
other retries have already failed. When this error is posted, the target is
sure that recovery is impossible without host intervention. You are correct
that this is a rare event, but a recevery method is needed even for rare
events. The expectation is that, under cover of the ACA condition, host
software could recover the unwritten data and write it to a different target.
Note that this type of recovery is only needed in systems that have discarded
the local copy of the write data. If that write data hasn't been discarded yet,
the host doesn't need to recover it from the disk; it is still present in local
This host recovery method should only be needed by a host that enables Write
Caching (so GOOD status is returned before data is written on the disk) and
discards the write data before the drive later reports the unrecoverable write
error. The lack of recovery mechanism for this type of failure keeps many hosts
|from enabling write caching, even though they would rather enable it to get a
performance benefit.
Gerry Houlder -- Gerry_Houlder at
Seagate Technology   -   920 Disc Drive   -   Scotts Valley, CA 95066 USA
Main Phone 408-438-6550   -   Email Problems postmaster at
Technical Support: BBS 408-438-8771  Fax 408-438-8137  Voice 408-438-8222  

### OGATE Version 8 message trace and attachment information:
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### Org Date:    08-19-94 12:38:04 PM
### From:        Gerry Houlder at SEAGATE
### To:          scsi @ WichitaKS.NCR.COM @ internet
### Subject:     RE: new Data Recovery on deferred errors [94-067r2]
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