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Old 12-25-2013, 12:32 AM
fsluck
 
Default how to know ext cache hit rate?

how to know ext cache hit rate?*thanks

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Old 12-25-2013, 05:00 PM
"Theodore Ts'o"
 
Default how to know ext cache hit rate?

On Wed, Dec 25, 2013 at 09:32:50AM +0800, fsluck wrote:
> how to know ext cache hit rate?

Which cache are you referring to, specifically? The page cache? The
inode cache? The dentry cache?

What problem are you trying to solve, at a high level?

- Ted

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Old 02-26-2014, 07:01 AM
fsluck
 
Default how to know ext cache hit rate?

i think* it is page cache.my case is*in virualization environment, because* vm's disk image is much larger than system's cache, i want to know when many vms are running* if system cache hit rate is very* low?** if there is a need to* seek for other way to suit virtualization.*system's default cache mechanism does not care about virtualization. *thanks*




At*2013-12-26*02:00:43,"Theodore*Ts'o"*<tytso@mit.edu>*wrote:
>On*Wed,*Dec*25,*2013*at*09:32:50AM*+0800,*fsluck* wrote:
>>*how*to*know*ext*cache*hit*rate?
>
>Which*cache*are*you*referring*to,*specifically?** The*page*cache?**The
>inode*cache?**The*dentry*cache?
>
>What*problem*are*you*trying*to*solve,*at*a*high*l evel?
>
>***** ***** ***** ******* ******-*Ted


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Old 02-26-2014, 02:50 PM
"Theodore Ts'o"
 
Default how to know ext cache hit rate?

On Wed, Feb 26, 2014 at 04:01:00PM +0800, fsluck wrote:
> i think it is page cache. my case is in virualization environment,
> because vm's disk image is much larger than system's cache, i want
> to know when many vms are running if system cache hit rate is very
> low? if there is a need to seek for other way to suit
> virtualization.

Why not simply measure the I/O rate on the disk? If there are cache
misses, they will show up as I/O requests, and it's the cache misses
that you care about. Or more to the point, what's important is
determining whether I/O is the bottleneck or whether something else
like memory pressure.

Also note that with Linux, any available memory will be used for
caching. If the cache is very small, it's likely the real problem is
that your system doesn't have enough memory --- which is very often
the problem with virtualization environments, and also why Amazon
charges $$$$$ for VM's that have a large amount of memory.

- Ted

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Old 03-04-2014, 01:16 AM
fsluck
 
Default how to know ext cache hit rate?

*virtualization is to make best use of resources,memory and disk over-providing. so i guess cache hit rate is very low in such case,because cache is very small and vm's disk is very large.* im virtualization environment , my original idea is to cache part of backing file into memory(for example 1G as cache), all vms that use that backing file can access cache directly and performance improved.do you have a idea?



At*2014-02-26*23:50:40,"Theodore*Ts'o"*<tytso@mit.edu>*wrote:
>On*Wed,*Feb*26,*2014*at*04:01:00PM*+0800,*fsluck* wrote:
>>*i*think*it*is*page*cache.**my*case*is*in*virual ization*environment,
>>*because*vm's*disk*image*is*much*larger*than*sys tem's*cache,*i*want
>>*to*know*when*many*vms*are*running*if*system*cac he*hit*rate*is*very
>>*low?**if*there*is*a*need*to*seek*for*other*way* to*suit
>>*virtualization.
>
>Why*not*simply*measure*the*I/O*rate*on*the*disk?**If*there*are*cache
>misses,*they*will*show*up*as*I/O*requests,*and*it's*the*cache*misses
>that*you*care*about.**Or*more*to*the*point,*what' s*important*is
>determining*whether*I/O*is*the*bottleneck*or*whether*something*else
>like*memory*pressure.
>
>Also*note*that*with*Linux,*any*available*memory*w ill*be*used*for
>caching.**If*the*cache*is*very*small,*it's*likely *the*real*problem*is
>that*your*system*doesn't*have*enough*memory*---*which*is*very*often
>the*problem*with*virtualization*environments,*and *also*why*Amazon
>charges*$$$$$*for*VM's*that*have*a*large*amount*o f*memory.
>
>***** * -*Ted


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