Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer patches

# gpg: Signature made Mon 05 Mar 2018 17:45:51 GMT
# gpg:                using RSA key 7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream: (38 commits)
  block: Fix NULL dereference on empty drive error
  qcow2: Replace align_offset() with ROUND_UP()
  block/ssh: Add basic .bdrv_truncate()
  block/ssh: Make ssh_grow_file() blocking
  block/ssh: Pull ssh_grow_file() from ssh_create()
  qemu-img: Make resize error message more general
  qcow2: make qcow2_co_create2() a coroutine_fn
  block: rename .bdrv_create() to .bdrv_co_create_opts()
  Revert "IDE: Do not flush empty CDROM drives"
  block: test blk_aio_flush() with blk->root == NULL
  block: add BlockBackend->in_flight counter
  block: extract AIO_WAIT_WHILE() from BlockDriverState
  aio: rename aio_context_in_iothread() to in_aio_context_home_thread()
  docs: document how to use the l2-cache-entry-size parameter
  specs/qcow2: Fix documentation of the compressed cluster descriptor
  iotest 033: add misaligned write-zeroes test via truncate
  block: fix write with zero flag set and iovector provided
  block: Drop unused .bdrv_co_get_block_status()
  vvfat: Switch to .bdrv_co_block_status()
  vpc: Switch to .bdrv_co_block_status()
  ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

# Conflicts:
#	include/block/block.h

2 files changed, 56 insertions, 6 deletions

diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index d7fdb1fee3..feb711fb6a 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -426,10 +426,20 @@ Standard Cluster Descriptor:
 
 Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
 
-    Bit  0 -  x:    Host cluster offset. This is usually _not_ aligned to a
-                    cluster boundary!
+    Bit  0 - x-1:   Host cluster offset. This is usually _not_ aligned to a
+                    cluster or sector boundary!
 
-       x+1 - 61:    Compressed size of the images in sectors of 512 bytes
+         x - 61:    Number of additional 512-byte sectors used for the
+                    compressed data, beyond the sector containing the offset
+                    in the previous field. Some of these sectors may reside
+                    in the next contiguous host cluster.
+
+                    Note that the compressed data does not necessarily occupy
+                    all of the bytes in the final sector; rather, decompression
+                    stops when it has produced a cluster of data.
+
+                    Another compressed cluster may map to the tail of the final
+                    sector used by this compressed cluster.
 
 If a cluster is unallocated, read requests shall read the data from the backing
 file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index b0571de4b8..170191a242 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -1,6 +1,6 @@
 qcow2 L2/refcount cache configuration
 =====================================
-Copyright (C) 2015 Igalia, S.L.
+Copyright (C) 2015, 2018 Igalia, S.L.
 Author: Alberto Garcia <berto@igalia.com>
 
 This work is licensed under the terms of the GNU GPL, version 2 or
@@ -118,8 +118,8 @@ There are three options available, and all of them take bytes:
 
 There are two things that need to be taken into account:
 
- - Both caches must have a size that is a multiple of the cluster
-   size.
+ - Both caches must have a size that is a multiple of the cluster size
+   (or the cache entry size: see "Using smaller cache sizes" below).
 
  - If you only set one of the options above, QEMU will automatically
    adjust the others so that the L2 cache is 4 times bigger than the
@@ -143,6 +143,46 @@ much less often than the L2 cache, so it's perfectly reasonable to
 keep it small.
 
 
+Using smaller cache entries
+---------------------------
+The qcow2 L2 cache stores complete tables by default. This means that
+if QEMU needs an entry from an L2 table then the whole table is read
+from disk and is kept in the cache. If the cache is full then a
+complete table needs to be evicted first.
+
+This can be inefficient with large cluster sizes since it results in
+more disk I/O and wastes more cache memory.
+
+Since QEMU 2.12 you can change the size of the L2 cache entry and make
+it smaller than the cluster size. This can be configured using the
+"l2-cache-entry-size" parameter:
+
+   -drive file=hd.qcow2,l2-cache-size=2097152,l2-cache-entry-size=4096
+
+Some things to take into account:
+
+ - The L2 cache entry size has the same restrictions as the cluster
+   size (power of two, at least 512 bytes).
+
+ - Smaller entry sizes generally improve the cache efficiency and make
+   disk I/O faster. This is particularly true with solid state drives
+   so it's a good idea to reduce the entry size in those cases. With
+   rotating hard drives the situation is a bit more complicated so you
+   should test it first and stay with the default size if unsure.
+
+ - Try different entry sizes to see which one gives faster performance
+   in your case. The block size of the host filesystem is generally a
+   good default (usually 4096 bytes in the case of ext4).
+
+ - Only the L2 cache can be configured this way. The refcount cache
+   always uses the cluster size as the entry size.
+
+ - If the L2 cache is big enough to hold all of the image's L2 tables
+   (as explained in the "Choosing the right cache sizes" section
+   earlier in this document) then none of this is necessary and you
+   can omit the "l2-cache-entry-size" parameter altogether.
+
+
 Reducing the memory usage
 -------------------------
 It is possible to clean unused cache entries in order to reduce the