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Tue, 22 Apr 2008

Cache coherency in POHMELFS. Continue.

While moving home I thought a lot about cache coherency issues. While we belive that NFS has coherent cache, since it is somewhat write-through, its cache actually is not synchronous, since between object creation and moment when other clients see new object really lot of time can run, for example when client, which create an object, has slow link... So, object creation and removal should not be synced to other clients during writeback on one of them, instead clients which are interested in object perform a lookup, which may or may not return object, this is not a race or cache non-coherency, this is usual multithreaded environment without client's synchronization.

What we really care about, is data consistency on the server. When we have multipage write, which overlaps with another write from different client, we should not read data back from the middle of the transactions. Locking the whole file is not an issue, instead proper byte-range (page-range actually) locking has to be implemented. I already have a prototype, but have to check it in real life.

So, other competing projects may or may not follow my way and drop creation/removal/stat coherency from the TODO list (afacs, no one implemented that yet :) based on my analysis and concentrate on server read/write locking.

And I will start some bits of VM hacking: plan is to implement generic enough (well, working on x86 for start :) mechanism to copy data from different (i.e. not that one which started a syscall) thread to userspace, while original one sleeps in syscall, via copy_to_user(). Likely it will be somewhat similar to what I did for zero-copy userspace sniffer and how get_user_pages() work.
Result, which has to be as fast as usual copy_to_user(), otherwise it is not interesting solution, will be used in POHMELFS client and its async reading.

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