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
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