Re: Improvements to fsck performance in -current ...?

Don Lewis wrote:
> On  2 Oct, Terry Lambert wrote:

[...]

>>Actually, write caching is not so much the problem, as the disk
>>reporting that the write has completed before the contents of
>>the transaction saved in the write cache have actually been
>>committed to stable storage.
>>
>>Unfortunately, IDE disks do not permit disconnected writes, due
>>to a bug in the original IDE implementation, which has been
>>carried forward for [insert no good reason here].
>>
>>Therefore IDE disks almost universally lie to the driver any
>>time write caching is enabled on an IDE drive.
>>
>>In most cases, if you use SCSI, the problem will go away.
> 
> 
> Nope, they "lie" as well unless you turn of the WCE bit.  Fortunately
> with tagged command queuing there is very little performance penalty for
> doing this in most cases.  The main exception to this is when you run
> newfs which talks to the raw partition and only has one command
> outstanding at a time.
> 
> Back in the days when our SCSI implementation would spam the console
> whenever it reduced the number of tagged openings because the drive
> indicated that its queue was full, I'd see the number of tagged openings
> stay at 63 if write caching was disabled, but the number would drop
> significantly under load (50%?) if write caching was enabled.  I always
> suspected that the drive's cache was full of data for write commands
> that it had indicated to the host as being complete even though the data
> hadn't been written to stable storage.
> 
> Unfortunately SCSI drives all seem to ship with the WCE bit set,
> probably for "benchmarking" reasons, so I always have to remember to
> turn this bit off whenever I install a new drive.

A message from this morning ('file system (UFS2) consistancy
after -current crash?') to this list describes exactly the
situation on my fileserver a few month ago, except my machine
runs with FreeBSD 4-STABLE and has an ICP-Vortex 6528RD controller.

I think, disk's or controllers (short hardware) write cache
is a problem. Maybe it shouldn't be in theory, but it is in
real world :-)

Best regards,
Jens