[Gluster-devel] lookup caching

[Raghavendra G]

Comments are inlined.

On Thu, Apr 8, 2010 at 9:21 PM, Olivier Le Cam <
Olivier.LeCam at crdp.ac-versailles.fr> 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
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