2.4.x swim3 performance problem on PM4400 identified

[Paul Mackerras]

Boris Bezlaj writes:

> | kista:/tmp# dd if=swimmer of=/dev/fd0 bs=512 count=2880
> | swim3: timeout reading sector 2647

Hmmm, wonder why it's reading that sector?

The timeout probably indicates that the sector header is corrupted or
unreadable.  The driver should probably retry after verifying that it
is on the right track.

> | end_request: I/O error, dev 02:00 (floppy), sector 2647   <--bad sector
> | dd: writing `/dev/fd0': Input/output error
> | 2647+0 records in
> | 2646+0 records out
> | 1354752 bytes transferred in 134.614549 seconds (10064 bytes/sec)
>
> after the error at sector 2647, swim3 seems to move the heads to sector 0
> and write? another 2647 sectors. Any particular reason for that ?

I don't think it's the swim3 driver itself that is doing that.  I
would start by stracing the dd to see what writes it is doing.  It may
be the block I/O layer doing something funny.

> Most importantly, when changing the floppy, it still works (not the
> case with the unpatched driver).

Great!

Paul.

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