Hi Amos,
Amos Jeffries wrote:
Is there such a thing as too much disk cache? Presumably squid has to
have some way of checking this cache, and at some point it takes longer
to look for a cached page than to serve it direct. At what point do you
hit that sort of problem, or is it so large no human mind should worry?
:)
Paul
IT Systems Admin
Disk cache is limited by access time and ironically RAM.
Squid holds an in-memory index of 10MB-ram per GB-disk. With large disk
caches this can fill RAM pretty fast, particularly if the cache is full of
small objects. Large objects use less index space more disk.
Some with smaller systems hit the limit at 20-100GB, others in cache farms
reach TB.
As for the speed of lookup vs DIRECT. If anyone has stats, please let us
know.
I can't understand under what circumstances the cache Lookup will be
slower than DIRECT lookup unless one has a net connection faster than
the disks!
For a 20 GB cache with 1175539 on-disk objects:
Median Service Times (seconds) 5 min 60 min:
HTTP Requests (All): 1.24267 1.38447
Cache Misses: 1.54242 1.71839
Cache Hits: 0.00919 0.00865
Near Hits: 1.38447 1.62803
Not-Modified Replies: 0.00179 0.00091
DNS Lookups: 0.04237 0.04433
ICP Queries: 0.00102 0.00096
The cache Lookup is 170 times faster than DIRECT lookups!
MAYBE, if I use a bigger cache say, 100-300 GB, the results could be
different. But I believe that running multiple Squid boxes with smaller
caches (10-30 GB) is always better than running 1 single Squid box with
a (100-300 GB) cache.
The benefits of running multiple smaller caches far outweigh running a
single large cache.
But this is only my opinion.
From my guess and experience, to run a 300 GB cache, one needs about 6
GB of memory! But I can't imagine how to manage a 300 GB cache if it
gets corrupted!
Thanking you...
Amos
--
With best regards and good wishes,
Yours sincerely,
Tek Bahadur Limbu
System Administrator
(TAG/TDG Group)
Jwl Systems Department
Worldlink Communications Pvt. Ltd.
Jawalakhel, Nepal
http://www.wlink.com.np
http://teklimbu.wordpress.com
