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author | phk <phk@FreeBSD.org> | 2004-03-10 20:41:09 +0000 |
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committer | phk <phk@FreeBSD.org> | 2004-03-10 20:41:09 +0000 |
commit | 0f56e66e2f53df9e66c87c4c703a093c7926dc1c (patch) | |
tree | 59ebe2a7ed44e7865042f27309af6372e0e6c948 /sys/netinet/tcp_seq.h | |
parent | 1ea153590eb705528aa8cdf33a034b97c6cd57ba (diff) | |
download | FreeBSD-src-0f56e66e2f53df9e66c87c4c703a093c7926dc1c.zip FreeBSD-src-0f56e66e2f53df9e66c87c4c703a093c7926dc1c.tar.gz |
Fix a long-standing deadlock issue with vnode backed md(4) devices:
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.
Diffstat (limited to 'sys/netinet/tcp_seq.h')
0 files changed, 0 insertions, 0 deletions