zfs very poor performance compared to ufs due to lack of cache?

[Steven Hartland]

----- Original Message ----- 
From: "Andriy Gapon" <avg at freebsd.org>
>> From my tests it seems that to serve a file to a client using sendfile without
>> having to
>> read it from disk you need said file in ARC. See sendfile on and primarycache set to
>> metadata results.
> 
> Yeah, I am puzzled by that.
> But, OTOH, I don't know that feature of ZFS well enough to say what additional
> pessimizations may have happened.

Anyone else care to comment on this?

>> So to use your caching analogy it seems that sendfile cant use the L1 cache unless
>> its also present in L2 for whatever reason.
> 
> It's possible that this is how it works for you because of some bug.
> But I don't see anything in the code that would lead to that behavior _after_ the
> change that was committed in r212650.  Change in r212782 might be useful too.
> 
> In VM theory the data should be just taken from "L1" aka "Inactive" aka page cache.

That's what I thought you where saying. Is there a test you would suggest to confirm
either way more accurately?

What springs to mind is setting primarycache to metadata and requesting a large
single file multiple times and see if disk access is zero on the repeat requests.

If this turns out to be the case does anyone have an idea where the issue would be?

    Regards
    Steve

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