On Thu, Apr 8, 2010 at 9:21 PM, Olivier Le Cam
<address@hidden> wrote:
Hi again,
Thanks for information.
Raghavendra G wrote:
Metadata caching is done in stat-prefetch. But that is not suitable to your use-case. in stat-prefetch stats of dentries are stored from the period of readdir to close on directory fd.
Although this translators is not easy to understand for a new comer in glusterfs devel, I'll carefully read its code.
Did you try with a very high cache-timeout value (say 5 minutes or more)? Also a bigger cache-size might help.
Yes, I did. In my situation, the problem is indeed the latency caused by tons of stats. My guess is that it should be possible to reach better improvement with metadata caching than with the io-cache (files are small and can usually be fetched with only a single frame).
Well, now I have a crutial question regarding the way to wrote a translator. I used the rot-13 code as a starting point.
I have a "lookup" xlator fop and its associated callback function. Here is a part my (very simple) speudo code:
int32_t
mdc_lookup_cbk (call_frame_t *frame, void *cookie, xlator_t *this,
int32_t op_ret, int32_t op_errno, inode_t *inode,
struct stat *stbuf, dict_t *dict,
struct stat *postparent)
{
/* TODO: cache metadata */
STACK_UNWIND_STRICT (lookup, frame, op_ret, op_errno, inode,
stbuf, dict, postparent);
}
int32_t
mdc_lookup (call_frame_t *frame, xlator_t *this, loc_t *loc,
dict_t *xattr_req)
{
uint8_t cached = 0;
if (xattr_req == NULL)
goto out;
/* TODO: check if we have cached metadata for "loc->inode"
If so, prepare data and set cache to 1 */
if (cached) {
/* How to "re-inject" metadata in the engine? */
STACK_UNWIND_STRICT (lookup, frame, -1, 0, NULL, NULL,
NULL, NULL);
return 0;
}
out :
STACK_WIND (frame, mdc_lookup_cbk, FIRST_CHILD (this),
FIRST_CHILD (this)->fops->lookup, loc, xattr_req);
return 0;
}
In mdc_lookup, I think I can stop going through the lower-level xlators with the help of STACK_UNWIND_STRICT. But how can I deliver cached metadata back to the engine at this step?
you can do that by sending the cached stats (here stat of file, stat of parent directory) through STACK_UNWIND.
STACK_UNWIND_STRICT (lookup, frame, 0, 0, loc->inode, cached_stat, NULL, parent_stat);
you can look into libglusterfs/src/defaults.c for default definitions of each of fop (file operations) and their call backs.
Thanks,