X-Git-Url: https://git.whamcloud.com/?p=fs%2Flustre-release.git;a=blobdiff_plain;f=lustre%2Fkernel_patches%2Fpatches%2Fdev_read_only-2.6.27-vanilla.patch;h=8e465e21150c702e72dd1523f92624958999925d;hp=de7804d9d729a97c9f26c7a3856cc4a5929a93c6;hb=c65b502fe9feda61c7ad7137f5022c654c6ba501;hpb=d21876c689b7339132064a3fcc9defdb96dd8501

diff --git a/lustre/kernel_patches/patches/dev_read_only-2.6.27-vanilla.patch b/lustre/kernel_patches/patches/dev_read_only-2.6.27-vanilla.patch
index de7804d..8e465e2 100644
--- a/lustre/kernel_patches/patches/dev_read_only-2.6.27-vanilla.patch
+++ b/lustre/kernel_patches/patches/dev_read_only-2.6.27-vanilla.patch
@@ -1,3 +1,24 @@
+This patch is no longer needed for Lustre.  It is only included
+for testing and ease of using the same kernel with older Lustre
+versions.  This testing functionality was replaced in Linux 3.0
+by the dm-flakey driver.
+
+This functionality is mainly used during testing, in order to
+simulate a server crash for ldiskfs by discarding all of the
+writes to the filesystem.  For recovery testing we could simulate
+this by using a special loopback or DM device that also discards
+writes to the device.
+
+This functionality is also used by target "failback" in order
+to speed up service shutdown and takeover by the other node
+during controlled operation.  However, it would also be possible
+to do this by simply allowing all of the in-flight requests to
+complete and then waiting for the service to stop.  This will
+also be needed by the DMU-OSD, because discarding of writes on
+a DMU-based target is not safe as it could trigger a storage
+failure if the data is ever read from disk again and the
+checksum does not match that expected by the block pointer.
+
 Index: linux-2.6.27.21-0.1/block/blk-core.c
 ===================================================================
 --- linux-2.6.27.21-0.1.orig/block/blk-core.c	2009-04-23 02:12:51.000000000 -0600