Hi, Below is a message I received from the DataPerfect discussion group.
Regards, Marcos -------------------------------------- Marcos Fávero Florence de Barros Campinas, Brazil Date: Tue, 19 Jun 2012 13:54:57 +1000 From: "Brian Hancock" <brian.hanc...@brileigh.com> To: <datap...@dataperfect.nl> Subject: Re: [Dataperf] Networking DP Hi Marcos, MSDOS, and presumably FreeDOS, does not natively provide a framework for safe filesharing and file locks, this needs to come from the server software working in combination with requests from the application via the client networking. Normally when a simple DOS application opens a file it opens it in a specific mode, eg read or read/write, and when a file is opened in write mode it is opened exclusively, so any process that tried to open the file is given an access denied. When task switching application, multitasking or networking came about a DOS application share.exe (later share.com and vshare.exe, vshare.vsd) wedged into the operating system (a TSR) and allowed 5 access modes: deny none, deny all, deny read, deny write and another mode for programs that did not like how share.exe worked. Share.exe maintains two tables in memory, one for each file that is opened, and another for which portions of the file were locked, and what mode the portion was locked in File server operating systems such as Novell Netware and Unix, Windows Server etc, have their own layers which provides this file locking mechanism. Early Windows used a variant of share.exe for its locking mechanism. So when a client application, which can be running on the same machine or a different network machines requests a file to be opened it communicates with the networking client to make the request, so that the entire file is not locked when someone opens it, and so only a specific portion of the file is loaded into memory and locked. For advanced server application the server can cache the reading and the writing of files. Caching can be dangerous because a process that has opened a part of a file and has cached it in memory may not be aware that another process has also tried to modify the same segment of data, however in advanced fileserver operating system this conflict is resolved within the fileserver operating system and manages this contention and gives the best performance. A bigger problem however is where the client application or rather the client networking software decides it can increase the performance by caching file reads and write, so instead of immediately requesting the server to write the data to disk it waits a moment until it has more time, when the cpu's utilisation is a little lower. In many application this can speed up the application, however in database application it is almost a death wish. I have run DP over many many different server and networking client combinations, with up to about 40 simultaneous connections and not ever had a problem unless I was running share.exe on the client or a setting on the client networking software was to enable network caching. For a short while one of the Novell Netware clients for Windows 3.x had client caching of network reads and writes turned on by default, and this was the only time that I have ever had DP data corruption. The major corruption over that period was with the STR file which keeps track of the consecutive numbering of records, and I kept getting duplicates, but I also got other random corruption problems with TXX and the index file. The other application that I know of which suffered badly with this setting was Lotus Notes. Once the culprit was found and caching turned off everything worked fine. Although this might not be the problem it might help you in isolating where some of the problems might lie. Regards Brian ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Freedos-user mailing list Freedos-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freedos-user
