On Wed, 19 May 1999 jaharkes_at_cs.cmu.edu wrote:

> 
> cytseng_at_CCCA.NCTU.edu.tw said:
> | Diego Carvalho wrote:
> | > I tried to fetch (by ftp) a 400 MB file. The venus starts to loop
> | > on a cache overflow,
> |
> | hi...
> |   I also had this problem with the venus having cache overflow.
> 
> Hi,
> 
> It is pretty obvious what happens if you know how the kernel and venus 
> are interacting. Venus is only told about a few VFS-events, this saves
> kernel-user space context switches, which is a must for any user-space
> filesystem implementation.
> 
> So the kernel tells venus that someone creates a file (CODA_CREAT/OPEN)
> In response Venus allocates a filesystem object, but it's still zero
> length (no cache space is reclaimed, we don't know how big the file is 
> going to be).
> 
> Then the user application can read and write all it wants, until it 
> closes the file. The kernel informs venus of this event (CODA_CLOSE).
> 
> Now venus does an fstat on the container file, and readjusts the cache 
> usage, and starts throwing things out if we went over the cache-size
> limit. And because the 400MB file is very large (and expensive to
> refetch) it is less likely to be thrown out!! So Venus tries to shrink
> the cache a couple of times, but fails to get the cache usage below the 
> limit, so it will print a Cache Overflow message, until the object has
> aged enough to be purged.
> 
> To avoid this, we will need to modify the kernel-Venus protocol so that
> the kernel asks venus for permission to enlarge a file, that way Venus
> can make sure that we do not overflow the cache, and that quotas are
> honoured.

I would suggest that any files over some threshold.. lets say 1/3 the
cache size would be accessed either completely over the network, or by
caching large blocks of the file instead of the whole file (4 MB chunks,
maybe?). I suspect this would require a decent amount of work to support
block caching, however.

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| Troy Benjegerdes    |       troy_at_microux.com     |    hozer_at_drgw.net   |
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