What uncached directories mean?

asked 2014-07-01 07:23:55 -0500

birben gravatar image

updated 2014-07-02 12:22:11 -0500

smaffulli gravatar image

Hi all,

I'm reading the http://docs.openstack.org/admin-guide-cloud/content/section_object-replication.html (Object Replication) and I couldn't understand the following phrase.

The number of uncached directories that object replication must traverse, usually as a result of invalidated suffix directory hashes, impedes performance. To provide acceptable replication speeds, object replication is designed to invalidate around 2 percent of the hash space on a normal node each day.

Is there anyone that can give me a little bit detail explanation about this phrase and what uncached directory means?

Thanks

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answered 2014-07-05 07:43:32 -0500

dachary gravatar image

The directory cache is a copy of file system directories that are kept in memory by the linux kernel filesystem. Since swift stores objects in the file system, accessing them means walking directories, either using the copy from the cache or, if there is no copy of the directory content in the cache, loading the content from the disk to the cache. For frequently used directory entries one can expect that they are quickly accessed because they are in the cache most of the time.

The comment you quoted explains that the daily maintenance operation implies accessing 2% of the hash space, most of which are probably not accessed frequently. Since they are not accessed frequently, the directories in which they reside are not cached and one should expect delay because they are loaded from disk.

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Asked: 2014-07-01 07:23:55 -0500

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Last updated: Jul 05 '14