Re: end to end error recovery musings

Douglas Gilbert
2007-02-26T06:01:50Z

H. Peter Anvin wrote:
> Ric Wheeler wrote:
>>
>> We still have the following challenges:
>>
>> (1) read-ahead often means that we will retry every bad sector at
>> least twice from the file system level. The first time, the fs read
>> ahead request triggers a speculative read that includes the bad sector
>> (triggering the error handling mechanisms) right before the real
>> application triggers a read does the same thing. Not sure what the
>> answer is here since read-ahead is obviously a huge win in the normal
>> case.
>>
>
> Probably the only sane thing to do is to remember the bad sectors and
> avoid attempting reading them; that would mean marking "automatic"
> versus "explicitly requested" requests to determine whether or not to
> filter them against a list of discovered bad blocks.

Some disks are doing their own "read-ahead" in the form
of a background media scan. Scans are done on request or
periodically (e.g. once per day or once per week) and we
have tools that can fetch the scan results from a disk
(e.g. a list of unreadable sectors). What we don't have
is any way to feed such information to a file system
that may be impacted.

Doug Gilbert