[OpenAFS-devel] good and bad performance of memory cache

[Nickolai Zeldovich]

Edward Moy wrote:

> So I ran the full suite of iozone tests (13), but at a single file size  
> (128M) and one record size (64K).  I set the AFS cache size to 80000K  
> for both memcache and diskcache.

Note that memcache size and diskcache size are different things.
In the case of memcache, a fixed number of chunks are allocated
in memory, such that numChunks * chunkSize = memCacheSize.  In
the case of disk cache, there are a lot more chunks, because the
disk cache assumes not every chunk will be filled (the underlying
filesystem handles disk block allocation for us).  Thus, when you
have small file segments, they use up an entire chunk worth of
cache in the memcache case, but only their size worth of cache
in the diskcache cache.

-- kolya