sftp prefetch disabled

[Robey Pointer]

On 17 Jul 2006, at 10:30, John Arbash Meinel wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
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> Robert disabled sftp prefetch a while ago, and I'm trying to figure  
> out why.
>
> Specifically revision id:
> robertc at robertcollins.net-20060419233208-2ed6906796994316
> date: Thu 2006-04-20 09:32:08 +1000
>
> Has the commit message:
> ... Disable all SFTP prefetching for known paramikos - direct readv
> support is now a TODO.

There were bugs in the early implementations of prefetch.  It looks  
like Robert just pushed the version requirement up; he didn't  
completely disable it.  As far as I can tell from the current code,  
prefetch is enabled for all recent versions of paramiko.

The version check could be changed to 1.6.  There wasn't a 1.5.5 and  
before 1.6 there were interaction issues if you used both readv() and  
prefetch() (which bzr often tries to do, unfortunately).  But the  
version check should probably stay; I don't think it's safe to assume  
that everyone has upgraded yet.


> I will probably do a little bit of testing to see what effect prefetch
> has. (I thought I was, until I dug deep enough to see this). My gut
> feeling is that for readv() it would be a net loss, but for a plain
> 'get()' it would be good.

I agree with that assessment.  Currently readv() is implemented in  
paramiko as a prefetch() over a limited range.  That is, prefetch()  
is effectively readv(everything)  without waiting for the results.

robey