I stand corrected. I guess it is true what they say about memory as one ages. Near-term is the first to go; long term stays around a little longer. I haven't used T-disks for a long, long time. If I read the doc correctly, if you are using FBA disks, only the first 8 blocks are cleared if you have disabled Clear_Tdisk. The easy way no longer exists.
Regards, Richard Schuh ________________________________ From: The IBM z/VM Operating System [mailto:ib...@listserv.uark.edu] On Behalf Of Kris Buelens Sent: Thursday, September 17, 2009 1:55 PM To: IBMVM@LISTSERV.UARK.EDU Subject: Re: TDISK and SYSTEM CONFIG question. As far as I know, without Clear_Tdisk, CP still formats the first cylinder of a T-disk, so ACCESS will always fail even if you'd get a T-disk at the same place as the previous user. But, with DDR or alike one could still find raw data on other cylinders. With Clear-Tdisk enabled, CP will format all cylinders of a detached T-disk before handing it out again. Using V-disks: less I/O involved: no CP format, and CMS format doesn't need to write nK data blocks all over the disk. But, V-disk pages are considered as shared storage by CP. So, V-disk pages may stay longer in real storage than other user pages. We once used large V-disks as sort work area, performance of the system was degraded. This was in the VM/ESA R2(?) era, with less real storage available than in a modern system. 2009/9/17 Schuh, Richard <rsc...@visa.com<mailto:rsc...@visa.com>> The simplest mode of recovering data from a T-disk in CMS requires only that the starting cylinder be the same as it was for the previous user. If you are in a shop that does not have Clear_Tdisk enabled, simply ACCESS the disk when you define it. If the ACCESS works, you are in, you have access to the previous user's data. If your disk is at least as large as the former disk, you have full access; else, only partial. Requiring the person who defines the T-disk to format it is a nice rule that can easily be broken, either inadvertently or on purpose. If you use a shared space to store sensitive data, you should take the responsibility for insuring that it cannot be recovered by the next person who uses that space. Having CP do it is a secondary line of defense; there is always the possibility, maybe very low probability, that conditions may change in a way that thwarts CP's protection. Do not assume that you are safe. On the other hand, protection when using V-disk is absolute. The first time you reference a page, you are given one that has been cleared to zeros. There is no possibility of retrieving someone else's data from it. Regards, Richard Schuh > -----Original Message----- > From: The IBM z/VM Operating System > [mailto:IBMVM@LISTSERV.UARK.EDU<mailto:IBMVM@LISTSERV.UARK.EDU>] On Behalf Of > Tom Rae > Sent: Thursday, September 17, 2009 12:05 PM > To: IBMVM@LISTSERV.UARK.EDU<mailto:IBMVM@LISTSERV.UARK.EDU> > Subject: Re: TDISK and SYSTEM CONFIG question. > > Without CLEAR_Tdisk enabled, whatever data the last user > stored in T-DISK space is still present when it is allocated > to the next user. To successfully use the space with CMS you > will most likely have to format it, unless the boundaries of > your TDISK allocation exactly overlay the previous user's > allocation, but there is nothing forcing you to use CMS to > access the space. A utility such as DDR would be quite happy > to copy the contents of T-DISK to a permanent location, from > which you could use other utilities to recover the data, > track by track, without relying on CMS. > > Gentry, Stephen wrote: > > I guess I should have read Richard's response closer. So, > I'll echo Dennis's question as well. > > What security problem? > > We do not have CLEAR_TDisk enabled. Every time we define a > t-disk, it HAS to be formatted, no exceptions. > > Steve > -- Kris Buelens, IBM Belgium, VM customer support
