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Lock-ahead translator
---------------------
The objective of the lock-ahead translator is to speculatively
hold locks (inodelk and entrylk) on the universal set (0 - infinity
in case of inodelk and all basenames in case of entrylk) even
when a lock is requested only on a subset, in anticipation that
further locks will be requested within the same universal set.
So, for example, when cluster/replicate locks a region before
writing to it, lock-ahead would instead lock the entire file.
On further writes, lock-ahead can immediately return success for
the lock requests, since the entire file has been previously locked.
To avoid starvation of other clients/mountpoints, we employ a
notify mechanism, described below.
typedef struct {
struct list_head subset_locks;
} la_universal_lock_t;
Universal lock structure is stored in the inode context.
typedef struct {
enum {LOCK_AHEAD_ENTRYLK, LOCK_AHEAD_FENTRYLK,
LOCK_AHEAD_INODELK, LOCK_AHEAD_FINODELK};
union {
fd_t *fd;
loc_t loc;
};
off_t l_start;
off_t l_len;
const char *basename;
struct list_head universal_lock;
} la_subset_lock_t;
fops implemented:
* inodelk/finodelk/entrylk/fentrylk:
lock:
if universal lock held:
add subset to it (save loc_t or fd) and return success
else:
send lock-notify fop
hold universal lock and return
(set inode context, add subset to it, save loc_t or fd)
if this fails:
forward the lock request
unlock:
if subset exists in universal lock:
delete subset lock from list
else:
forward it
* release:
hold subset locks (each subset lock using the saved loc_t or fd)
and release universal lock
* lock-notify (on unwind) (new fop)
hold subset locks and release universal lock
lock-notify in locks translator:
if a subset lock in entrylk/inodelk cannot be satisfied
because of a universal lock held by someone else:
unwind the lock-notify fop
==============================================
$ Last updated: Tue Feb 17 11:31:18 IST 2009 $
$ Author: Vikas Gorur <vikas@gluster.com> $
==============================================
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