From 6c23479357ed3a810e3b56b1076fc6d9a2fee6e8 Mon Sep 17 00:00:00 2001 From: James Bottomley Date: Wed, 24 Apr 2013 14:02:53 -0700 Subject: [PATCH] sd: fix array cache flushing bug causing performance problems commit 39c60a0948cc06139e2fbfe084f83cb7e7deae3b upstream. Some arrays synchronize their full non volatile cache when the sd driver sends a SYNCHRONIZE CACHE command. Unfortunately, they can have Terrabytes of this and we send a SYNCHRONIZE CACHE for every barrier if an array reports it has a writeback cache. This leads to massive slowdowns on journalled filesystems. The fix is to allow userspace to turn off the writeback cache setting as a temporary measure (i.e. without doing the MODE SELECT to write it back to the device), so even though the device reported it has a writeback cache, the user, knowing that the cache is non volatile and all they care about is filesystem correctness, can turn that bit off in the kernel and avoid the performance ruinous (and safety irrelevant) SYNCHRONIZE CACHE commands. The way you do this is add a 'temporary' prefix when performing the usual cache setting operations, so echo temporary write through > /sys/class/scsi_disk//cache_type Reported-by: Ric Wheeler Signed-off-by: James Bottomley Signed-off-by: Ben Hutchings --- Reading git-format-patch failed