features/shard: Introducing ".shard/.remove_me" for atomic shard deletion (part 1)

PROBLEM:
Shards are deleted synchronously when a sharded file is unlinked or
when a sharded file participating as the dst in a rename() is going to
be replaced. The problem with this approach is it makes the operation
really slow, sometimes causing the application to time out, especially
with large files.

SOLUTION:
To make this operation atomic, we introduce a ".remove_me" directory.
Now renames and unlinks will simply involve two steps:
1. creating an empty file under .remove_me named after the gfid of the file
participating in unlink/rename
2. carrying out the actual rename/unlink
A synctask is created (more on that in part 2) to scan this directory
after every unlink/rename operation (or upon a volume mount) and clean
up all shards associated with it. All of this happens in the background.
The task takes care to delete the shards associated with the gfid in
.remove_me only if this gfid doesn't exist in backend, ensuring that the
file was successfully renamed/unlinked and its shards can be discarded now
safely.

Change-Id: Ia1d238b721a3e99f951a73abbe199e4245f51a3a
updates: bz#1568521
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/bugs/shard/bug-1245547.t b/tests/bugs/shard/bug-1245547.t
index c19b2a6a042..3c46785d10f 100644
--- a/tests/bugs/shard/bug-1245547.t
+++ b/tests/bugs/shard/bug-1245547.t
@@ -25,11 +25,11 @@ TEST touch $M0/bar
 TEST truncate -s 10G $M0/bar
 #Unlink on such a file should succeed.
 TEST unlink $M0/bar
-#
+
 #Create a file 'baz' with holes.
 TEST touch $M0/baz
 TEST truncate -s 10G $M0/baz
 #Rename with a sharded existing dest that has holes must succeed.
 TEST mv -f $M0/foo $M0/baz
 
-cleanup;
+cleanup