(Illustration by Gaich Muramatsu)
.. To: codalist_at_TELEMANN.coda.cs.cmu.edu .. From: soma_at_cs.unm.edu (Anil B. Somayaji) Date: 19 Jan 1999 13:44:35 -0700 Message-ID: <ut24spnq9fw.fsf_at_lydia.adaptive.net> .. * Does a client's cache have to be big enough to hold any single file? Or does it have to be big enough to hold a whole directory? We definitely had problems when we tried copying a 23M file into coda when the client only was set up with a 20M cache. .. Anil and Fellow Codaphiles, At the core of this question is an important design(?) limitation of Coda. By insisting that all open files be cached (and so read and written) locally, the total size of all open files must be less than the size of the local cache. In practice this is not too critical a limitation. At least a part of Coda's target is the realm of mobile, disconnected computing. For mobile users this often means a single-user laptop which I would expect would open only a small number of small files at once. But in the (nearly) orthogonal realm of large-scale reliable computing (redundancy, higher performance) this limitation is more of a problem. In fact, the current Coda implementation makes things worse than they need be. A cached file can grow to consume all free space on the local cache. At this point the kernel could request that Venus (i.e. the cache manager) free up cache space if possible, presumably by removing cached but unopened (and unhoarded?) files. At present the kernel code and kernel<->Venus upcall that would be required to do this do not exist. Venus also must be ready to perform cache `garbage collection' when it tries to install a file into the cache during that file's first open. I am not sure whether Venus does this at present (the Venus source code was less than scrutible on this point :-)). Peter [Braam], at present I expect that in our (ongoing, nefarious, ab)use of the kernel<->Venus protocol we will move towards a hybrid Coda/Potemkin/NFS system which will give us finer grained (i.e. block-level) caching as well as neatly side-step the above local caching limits. Nevertheless, I would be interested to hear what, if any, plans the Coda development team has with regard to these issues. Regards, bruce.Received on 1999-01-20 02:26:27