new class / geom_cache / request for comments

[Eric Anderson]

R. B. Riddick wrote:
> --- Eric Anderson <anderson at centtech.com> wrote:
>> R. B. Riddick wrote:
>>> --- Eric Anderson <anderson at centtech.com> wrote:
>>>> Just curious, how is this different/better than the regular buffer
>>>> cache?
>>> Do you mean this cache of those file systems?
>>> If yes: The answer should be: "It just makes everything worse."
>>> (there is not even some read ahead...)
>> Not sure what I'm missing here, please clue me in.  I guess I am indeed 
>> talking about the cache normally used for filesystems.  If I read a 
>> 500MB file in, first time it comes from disk, second time it comes from 
>> memory cache, correct?  How is geom_cache different?
>>
> Yes, I think so...
> But geom_cache is just useful, when file system's buffer cache cannot help.
> 
> E. g.:
> A degraded RAID5 on 4 consumers (3 good plus 1 failed).
> When we want to get a data block, that resides on the failed consumer, we have
> to read all corresponding blocks (2+1) in order to rebuild the missing block.
> When we do a sequential read, we would have to read the consumers, that hold
> the data blocks twice (2 x 2).
> So the geom_cache could help here (2+1 real reads plus 2 from the cache), if
> the provider is not too busy.

Ok, I understand now.  I could see this being useful for fsck's also.  I 
wonder if it's possible to have the cache distributed across multiple 
machines? (just out of curiosity)


>>> It is just useful, if you dont have any other caches (e. g. a ufs on a
>>> geom_raid5 (I think I should have it tomorrow... :-) ) on some geom_cache
>>> providers)...
>> I suppose I just need to play with it to completely understand..
>>
> :-)
> Maybe it is useless... and I dont see it... and nobody dares to say it...
> I saw, that the CVS tree does not have a sys/geom/cache directory...

Not useless!  I think it's a good addition.. I've even thought about a 
geom_cache before, but then wrote it off to 'oh wait, the filesystem 
caches would handle all that', but you've thought further and realized a 
good use for it..

Thanks for writing it!

Eric



-- 
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Eric Anderson        Sr. Systems Administrator        Centaur Technology
Anything that works is better than anything that doesn't.
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