7a6b2b6429
On vnode backed md(4) devices over a certain, currently undetermined size relative to the buffer cache our "lemming-syncer" can provoke a buffer starvation which puts the md thread to sleep on wdrain. This generally tends to grind the entire system to a stop because the event that is supposed to wake up the thread will not happen until a fair bit of the piled up I/O requests in the system finish, and since a lot of those are on a md(4) vnode backed device which is currently waiting on wdrain until a fair amount of the piled up ... you get the picture. The cure is to issue all VOP_WRITES on the vnode backing the device with IO_SYNC. In addition to more closely emulating a real disk device with a non-lying write-cache, this makes the writes exempt from rate-limited (there to avoid starving the buffer cache) and consequently prevents the deadlock. Unfortunately performance takes a hit. Add "async" option to give people who know what they are doing the old behaviour. |
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| md.c | ||