Linux tmpfs (Was: [OpenAFS] Solaris 10u6: ZFS cache?)
Chas Williams (CONTRACTOR)
Fri, 14 Nov 2008 07:18:57 -0500
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,
"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).
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.
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.