Add support for opening the disk images with O_DIRECT. This allows
bypassing the host's file system cache, which is useful to avoid
polluting its cache and for better data integrity.
This mode of operation can be enabled by adding the "direct=<bool>"
parameter to the "backend" argument:
./target/debug/vhost_user_blk --backend image=test.raw,sock=/tmp/vhostblk,direct=true
The "direct" parameter defaults to "false", to preserve the original
behavior.
Signed-off-by: Sergio Lopez <slp@redhat.com>
Use RawFile as backend instead of File. This allows us to abstract
the access to the actual image with a specialized layer, so we have a
place where we can deal with the low-level peculiarities.
Signed-off-by: Sergio Lopez <slp@redhat.com>
Doing I/O on an image opened with O_DIRECT requires to adhere to
certain restrictions, requiring the following elements to be aligned:
- Address of the source/destination memory buffer.
- File offset.
- Length of the data to be read/written.
The actual alignment value depends on various elements, and according
to open(2) "(...) there is currently no filesystem-independent
interface for an application to discover these restrictions (...)".
To discover such value, we iterate through a list of alignments
(currently, 512 and 4096) calling pread() with each one and checking
if the operation succeeded.
We also extend RawFile so it can be used as a backend for QcowFile,
so the later can be easily adapted to support O_DIRECT too.
Signed-off-by: Sergio Lopez <slp@redhat.com>
The current backend only support rw, and we also need
add readonly support.
The new command:
vhost_user_blk \
--backend "image=/home/test.img, \
sock=/home/path/vhost.socket, \
readonly=true"
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Create a vhost-user-blk backend using vhost-user-backend and following
the conventions established by the existing vhost-user-net
implementation.
This backend is based on https://github.com/slp/vhost-user-backend,
but a bit simplified, making it closer to the original implementation
in Firecracker. The main features missing are EVENT_IDX, support for
asynchronous I/O and multiqueue, but it's still fully functional and
provides a good starting point for evolving it into a more complete
implementation.
Signed-off-by: Sergio Lopez <slp@redhat.com>
