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Re: [Gluster-devel] lookup caching

From: Olivier Le Cam
Subject: Re: [Gluster-devel] lookup caching
Date: Thu, 08 Apr 2010 19:21:59 +0200
User-agent: Thunderbird (Windows/20100228)

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:

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);

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?


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