UFS2 fsck Question (semantics of -p)

[Can Sar]

We ran our experiment on top of a very simple RAM disk which does not  
have any caches or anything of that sort.

The dmesg log is at http://keeda.stanford.edu/dmesg

The resultant images are at:
http://keeda.stanford.edu/ufs-umount-image
http://keeda.stanford.edu/ufs-mount-sync-image


If you run fsck -p on them, fsck will not be able to recover, while  
fsck without the -p option will be able to.

Can


On Aug 29, 2006, at 9:55 AM, Chuck Swiger wrote:

> Can Sar wrote:
> [ ... ]
>> Would you consider it an error if the -p option does not fix  
>> inconsistencies caused by a simple power failure, without any  
>> hardware or software corruption?
>
> You're asking an interesting question, but the issue of data  
> integrity depends not only on the software which comprises the OS,  
> but also on the hardware being used.
>
> In particular, the system depends upon the hard drives to reliably  
> report when data being written actually has been; SCSI drives,  
> using tagged command queuing, especially in conjunction with a  
> battery-backup which ensures the drive stays up long enough to  
> flush it's write cache even if system power is removed, will tend  
> to fare pretty well.
>
> IDE drives, by contrast, have a bad habit of lying about whether  
> data has actually been written to the disk itself rather than  
> simply making it to the write cache on the drive.  (Such drives  
> ignore the ATA "FLUSH CACHE" command, specificly.)
>
> In other words, showing that a filesystem can become inconsistent  
> in a fashion that "fsck -p" cannot correct is interesting and a  
> concern regardless of the circumstances, but showing it in cases  
> where you are using battery-backed drives and/or SCSI rather than  
> IDE is a lot more meaningful.  If you are using IDE devices, your  
> testing will be more meaningful if you disable the IDE write-cache  
> entirely.  Also, you should put your results somewhere, perhaps on  
> a webpage with links to the filesystem images and a complete dmesg  
> so that the OS version and hardware being used is well-documented.
>
> -- 
> -Chuck
>