mirror of
https://github.com/ceph/ceph
synced 2024-12-25 12:54:16 +00:00
77 lines
3.1 KiB
ReStructuredText
77 lines
3.1 KiB
ReStructuredText
======
|
|
LazyIO
|
|
======
|
|
|
|
LazyIO relaxes POSIX semantics. Buffered reads/writes are allowed even when a
|
|
file is opened by multiple applications on multiple clients. Applications are
|
|
responsible for managing cache coherency themselves.
|
|
|
|
Libcephfs supports LazyIO since nautilus release.
|
|
|
|
Enable LazyIO
|
|
=============
|
|
|
|
LazyIO can be enabled by following ways.
|
|
|
|
- ``client_force_lazyio`` option enables LAZY_IO globally for libcephfs and
|
|
ceph-fuse mount.
|
|
|
|
- ``ceph_lazyio(...)`` and ``ceph_ll_lazyio(...)`` enable LAZY_IO for file handle
|
|
in libcephfs.
|
|
|
|
Using LazyIO
|
|
============
|
|
|
|
LazyIO includes two methods ``lazyio_propagate()`` and ``lazyio_synchronize()``.
|
|
With LazyIO enabled, writes may not be visble to other clients until
|
|
``lazyio_propagate()`` is called. Reads may come from local cache (irrespective of
|
|
changes to the file by other clients) until ``lazyio_synchronize()`` is called.
|
|
|
|
- ``lazyio_propagate(int fd, loff_t offset, size_t count)`` - Ensures that any
|
|
buffered writes of the client, in the specific region (offset to offset+count),
|
|
has been propagated to the shared file. If offset and count are both 0, the
|
|
operation is performed on the entire file. Currently only this is supported.
|
|
|
|
- ``lazyio_synchronize(int fd, loff_t offset, size_t count)`` - Ensures that the
|
|
client is, in a subsequent read call, able to read the updated file with all
|
|
the propagated writes of the other clients. In CephFS this is facilitated by
|
|
invalidating the file caches pertaining to the inode and hence forces the
|
|
client to refetch/recache the data from the updated file. Also if the write cache
|
|
of the calling client is dirty (not propagated), lazyio_synchronize() flushes it as well.
|
|
|
|
An example usage (utilizing libcephfs) is given below. This is a sample I/O loop for a
|
|
particular client/file descriptor in a parallel application:
|
|
|
|
::
|
|
|
|
/* Client a (ca) opens the shared file file.txt */
|
|
int fda = ceph_open(ca, "shared_file.txt", O_CREAT|O_RDWR, 0644);
|
|
|
|
/* Enable LazyIO for fda */
|
|
ceph_lazyio(ca, fda, 1));
|
|
|
|
for(i = 0; i < num_iters; i++) {
|
|
char out_buf[] = "fooooooooo";
|
|
|
|
ceph_write(ca, fda, out_buf, sizeof(out_buf), i);
|
|
/* Propagate the writes associated with fda to the backing storage*/
|
|
ceph_propagate(ca, fda, 0, 0);
|
|
|
|
/* The barrier makes sure changes associated with all file descriptors
|
|
are propagated so that there is certainty that the backing file
|
|
is upto date */
|
|
application_specific_barrier();
|
|
|
|
char in_buf[40];
|
|
/* Calling ceph_lazyio_synchronize here will ascertain that ca will
|
|
read the updated file with the propagated changes and not read
|
|
stale cached data */
|
|
ceph_lazyio_synchronize(ca, fda, 0, 0);
|
|
ceph_read(ca, fda, in_buf, sizeof(in_buf), 0);
|
|
|
|
/* A barrier is required here before returning to the next write
|
|
phase so as to avoid overwriting the portion of the shared file still
|
|
being read by another file descriptor */
|
|
application_specific_barrier();
|
|
}
|