Date: Thu, 22 Feb 2001 18:42:30 -0800 From: Peter Wemm <peter@netplex.com.au> To: Terry Lambert <tlambert@primenet.com> Cc: patl@phoenix.volant.org (Pat Lashley), freebsd-fs@FreeBSD.ORG Subject: Re: Status of nullfs Message-ID: <200102230242.f1N2gUf43386@mobile.wemm.org> In-Reply-To: <200102222215.PAA26750@usr05.primenet.com>
next in thread | previous in thread | raw e-mail | index | archive | help
Terry Lambert wrote: > > The man page in -STABLE for mount_null contains dire warnings about > > its lack of support and potential to cause problems. But how > > solid/shakey is it really? Is it, in practice, safe to use? Is > > there any particular usage that is more likely to cause problems, > > or are the warnings just there because nobody is actively keeping > > it up to date? > > > > In other words, if I'm willing to take the risk of using it, just > > how big of a risk am I actually taking? > > Any mmap'ed writes will fail due to cache coherency problems. If > the system isn't quiescent long enough to flush everything out > before an unmount, you can panic. If you have exposed the NULL > mount , e.g. by mounting an FS on /usr, and then nullfs mounting > it to /null_usr, then modifications to the /usr FS will not > result in coherency notification, and files will become corrupt, > if accessed both ways. Paging from executables can cause problems > for multiple copies of executables, if statically initialized data > is modified (copy on write) due to the way backing page handling > is limited. > > All of these derive from the fact that the vnode in the upper > layer (nullfs) and the vnode in the lower layer (ffs or whatever) > have seperate vm_object_t's, even though the data contents of the > file are supposed to be treated as identical for both. Terry, please check your facts. This has been recently fixed. See the VOP_GETVOBJECT() stuff. The trick though is to get it MFC'ed into 4.x... > There are a number of patches to manually enforce coherency; as > far as I know, they don't deal with the pager path, and, since > the coherency is explicit, the /usr vs. /null_usr problem is > not resolved. Check the list archives to obtain the patches. nullfs is fully functional in -current. Backporting the changes should be easy. Cheers, -Peter -- Peter Wemm - peter@FreeBSD.org; peter@yahoo-inc.com; peter@netplex.com.au "All of this is for nothing if we don't go to the stars" - JMS/B5 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200102230242.f1N2gUf43386>