[OpenAFS] Re: Linux tmpfs

Jason Edgecombe jason@rampaginggeek.com
Fri, 14 Nov 2008 09:15:43 -0500

Derek Atkins wrote:
> "Chas Williams (CONTRACTOR)" <chas@cmf.nrl.navy.mil> writes:
>> In message <5FB03AFA-8CA1-4B39-A743-188E5DBF4666@ece.cmu.edu>,"Brandon S. Allbe
>> ry KF8NH" writes:
>>> It used to be said (back when warlord was maintaining linux-afs and  
>>> Transarc 3.4a was the main release) that the memcache was much less  
>>> efficient than the disk cache and that it was better to use disk cache  
>>> in a ramdisk.  Both have been pretty thoroughly overhauled since then,  
>>> though.
>> "efficient" isnt meaningful without context.  memcache does use quite
>> a bit of host memory.  if your system is short on memory, memcache is
>> not for you.  however, memcache outperforms diskcache is most cases
>> (ignoring say rereads over a link with high latency).
> In this case "efficient" was in terms of memory/space usage.  For the
> same memory usage the memcache will be able to hold fewer files
> and less file data than a similarly-sized memdisk cache.
>> the afs cache is handled by a seperate thread.  the filesystem client
>> requests a fetch from the cache manager.  the cache manager does the
>> fetch and writes the data into the cache.  it signal the filesystem
>> thread all is ready and the filesystem read reads the data.  if you
>> use a disk cache this is written to the disk and then read from the
>> disk.  memcache speeds this up a bit since you dont have the disk
>> latency during this copy.
> Yes, it is definitely FASTER to use a memcache.  However it is (was?)
> more space efficient to use a ramdisk.
>> i thought it might be interesting to make the filesystem thread give
>> the cache manager a hint pointer, and the cache manager could memcpy
>> the data to the filesystem thread buffers and write it to the disk.
> -derek
Does the disk cache take advantage of linux's disk cache? I'm thinking
that is a freebie if we use tmpfs or some other linux-provided fs-backed