Path: news.mitre.org!blanket.mitre.org!philabs!newsjunkie.ans.net!newsfeeds.ans.net!news-was.dfn.de!news-spur1.maxwell.syr.edu!news.maxwell.syr.edu!cpk-news-hub1.bbnplanet.com!su-news-hub1.bbnplanet.com!news.bbnplanet.com!news1.digital.com!pa.dec.com!usenet From: Andy Goldstein Newsgroups: comp.os.vms Subject: Re: verification errors Date: Tue, 18 Nov 1997 15:14:17 -0400 Organization: OpenVMS Engineering Lines: 30 Message-ID: <3471B0C9.7B336CCF@star.zko.dec.youknowwhere> References: <0033000000600773000002L032*@MHS> <01bcf146$684fdb80$8ac868ce@reis.chipsupply.com> <346FFB14.1EBC@iolnospam.ie> NNTP-Posting-Host: davros.zko.dec.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 3.0b8Gold (X11; I; OpenVMS X6AB-FT2 AlphaStation 400 4/233) Ian Shiel wrote: > > I had a similar problem years ago with a non DEC SCSI disk installed > in a Vaxstation (model 30 I think) running VMS 5.5+ > > The problem had something to do with "high water marking" on the > disk. When enabled it gave verification errors so we were told > to turn off high water marking (set volume/nohigh dkannn:) Ouch!! Yes, unfortunately highwater marking can cause BACKUP verification errors in some circumstances. This will occur on files with other than sequential organization. For sequential files, BACKUP reads only up to the RMS end of file mark, making the issue moot. However, for non-sequential organizations, BACKUP reads the entire allocated file space to cover some obscure cases in which the EOF has not been correctly set. This means BACKUP may attempt to read file blocks that were never written. The highwater marking logic acts to prevent this. Problem is there's a shortcut in the logic that leaves the previous buffer contents intact for the affected blocks, rather than zeroing the buffer, and signalling a successful completion for the entire read. Thus BACKUP saves indeterminate data at the tail of the file. Files are processed in a different order in the verify pass than in the save pass, so the buffer residue will be different. Turning off highwater marking allows BACKUP to read the actual file contents all the way out, so the results are consistent.