Hello
This email is for people who are looking to use cachefs in a scenario
where there are lots of small clients accessing the storage and where
files sizes are small( smaller than 100MB) or non streaming IO patterns.
These are my experiences and may not be the same for everyone.
1. No way of monitoring the benefit of the caching external to your
application. The stats that are provided don't include any reads from
memory, only from the disk cache. This means that it greatly
underreports the cache hits. There is no way to determine the data churn
in your cache.
2. Metadata is not cached. When you are accessing files particularly
application/executable files, reads are quite small. Every read
generates a metadata read on the original filer. Meta data operations
for small read type IO patterns can keep the filers very busy.
3. Doesn't handle Dynamic IP clustering. In the case of EMC isilon, the
client is given a dynamic IP. This means that even if the same location
is mounted it invalidates the cache if a different IP is given.
We were trying to use it in a very controlled scenario, accessing RO
data where files are never modified. Files are only created. Even in
this situation the benefits were minimal.
We have been excited about this feature since it was a tech preview in
RHEL5.1 in November 2007. It has only become stable for us in RHEL6.5,
released Nov 2013 still as a tech preview.
If anyone would like clarification on the above please feel free to email.
I hope this helps others.
Grant
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