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| -rw-r--r-- | doc/README.notifications | 37 |
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diff --git a/doc/README.notifications b/doc/README.notifications new file mode 100644 index 0000000..2ca6204 --- /dev/null +++ b/doc/README.notifications @@ -0,0 +1,37 @@ +During the life-cycle of a user-space filesystem the usual flow is: + + 1. User-space application does a filesystem-related syscall + 2. Kernel VFS calls into the FUSE kernel driver + 3. FUSE kernel redirects request to the user-space filesystem + 4. User-space server replies to request + 5. FUSE returns reply to VFS + 6. User-space application gets reply from the kernel + +However, there are occasions where the filesystem needs to send notifications to +the kernel that are not in reply to any particular request. If, for example, +when a READ request of 4096 bytes results in the filesystem having more data +available for the specific inode, it may be useful to provide this extra data to +the kernel cache so that future read operations will be faster. + +FUSE provides mechanisms for a user-space server to send the kernel certain +types of asynchronous notifications. Currently, these are the available +notifications: + +|-------------+----------------------------------| +| Operation | libfuse function | +|-------------+----------------------------------| +| POLL | fuse_lowlevel_notify_poll | +| INVAL_INODE | fuse_lowlevel_notify_inval_inode | +| ENTRY | fuse_lowlevel_notify_inval_entry | +| STORE | fuse_lowlevel_notify_store | +| RETRIEVE | fuse_lowlevel_notify_retrieve | +| DELETE | fuse_lowlevel_notify_delete | +| RESEND | - | +|-------------+----------------------------------| + +One important restriction is that these asynchronous operations SHALL NOT be +performed while executing other FUSE requests. Doing so will likely result in +deadlocking the user-space filesystem server. In the example above, if the +server is replying to a READ request and has extra data to add to the kernel +cache, it needs to reply to the READ request first, and, e.g., signal a +different thread to do the STORE. |