[OpenAFS] Re: Reducing cache scan time

Simon Wilkinson sxw@inf.ed.ac.uk
Sun, 30 Jan 2011 20:36:23 +0000

On 27 Jan 2011, at 16:14, Andrew Deason wrote:

>> Is there a way to measure how effective the chunksize is on a live
>> system?
> You can look at the cache performance metrics, and see if your miss
> ratios seem worse than before you changed it.

Chunksize does affect more than cache performance though. In particular, =
larger chunksizes will slow down non-chunk aligned file access. When we =
fetch files from the fileserver, we fetch by chunk - so if you are =
reading data at the end of a chunk, then the whole chunk will be fetched =
before your read completes. If you have a large chunksize, and a slow =
link, then this will impact performance.

On Unix, the same is true when writing back to the server. Even if you =
only change a byte of data, we'll write back the whole chunk containing =
that byte. Fixing this so that the cache manager maintains information =
about the range of bytes that have been dirtied would be a relatively =
straightforward performance optimisation.