From: Jim Brankin [jbrankin@ntlworld.com] Sent: Tuesday, February 04, 2003 11:54 AM To: Info-VAX@Mvb.Saic.Com Subject: Re: Very large disks on VMS "Jim Geier" wrote in message news:<000301c2cbb0$caebaa40$6b3e11ac@jcgt23>... > With the evolution towards larger and larger disks, and now with the EVA > moreso than its predecessors, it is possible to have VERY large disks on > a VMs system. Are there any problems or issues regarding how well VMS > will handle disks that are 300, 400, 500 GB or even larger? For example, > how does VMS handle things like the free block list or cache tables? > Will these things get unmanageable with extremely large disks? > > --jim geier-- The file system records free blocks as a bitmap in [00000]BITMAP.SYS. This is cached and the cache size is defined by the ACP_MAPCACHE sysgen parameter. If BITMAP.SYS becomes very big then it seems reasonable to increase the size of the cache. The cache should be able to hold ALL of the biggest bitmap.sys on your system and a bit more besides, IMHO. That way when a disk is almost full and the file system is scratching around looking for the last few blocks it will have the bitmap in memory. Since the change which allows smaller cluster sizes I have increased the ACP_EXTCACHE parameter to 2000. I don't have any scientific reason for doing this but it seems to me that if there are more clusters on the disk it is likely there will be more extents and therefore a bigger cache seems like a good idea. Increasing the size of the extent cache makes some disks less prone to fragmentation and some more so. I cannot figure out what is going on there. - Jim