Re: lookup caching

[Raghavendra G <raghavendra@xxxxxxxxxxx>]

Comments are inlined.

On Thu, Apr 8, 2010 at 9:21 PM, Olivier Le Cam <Olivier.LeCam@xxxxxxxxxxxxxxxxxxxxx> 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,

--
Olivier


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regards,
-- 
Raghavendra G