Varnish intermittently returns incomplete images

[Guillaume Quintard]

No no, with Varnish open-source you really want all your stuff in memory
anyway. What I meant is really to have two storages based on object size.

Imagine you have a 50MB object that needs to be stored, and your nuke_limit
is 50.

If you only have one store, then you could easily evict 50 100KB objects,
forcing you to fail the transaction.

But if you have a pool that is dedicated to bigger objects, you know that
each object you evict from it is at least 1MB big, so you cannot fail the
transaction due to nuke_limit
-- 
Guillaume Quintard


On Fri, May 8, 2020 at 11:50 AM Batanun B <batanun at hotmail.com> wrote:

> ok. Interesting. ��
>
> 99.99% of the time that this is happening (after adjusting the cache
> sizes), I would say that it would be only a single user requesting the
> image. So it would make sense it it was possible to configure Varnish so it
> could handle that scenario in a better way. What happens when multiple
> users request the same resource, and this nuke problem happens is less of a
> problem, and those times a broken could be acceptable (but preferably it
> would start serving each request separately, uncached, fetching from the
> backend each time).
>
> I will increase the cache size, and look into splitting it into two
> storages. But, I'm guessing you mean that the small objects should be
> cached in-memory, and the larger ones on disk? It would make much more
> sense if it cached less "popular" objects on disk, and more "popular"
> objects in memory, and only considering the object size when the in-memory
> cache starts to get full. Is it possible to configure Varnish to handle
> that in a smart and dynamic way?
> ------------------------------
> *From:* Guillaume Quintard <guillaume at varnish-software.com>
> *Sent:* Friday, May 8, 2020 8:33 PM
> *To:* Batanun B <batanun at hotmail.com>
> *Cc:* varnish-misc at varnish-cache.org <varnish-misc at varnish-cache.org>
> *Subject:* Re: Varnish intermittently returns incomplete images
>
> Good question. This is because by default varnish streams the response, so
> it starts sending what it has, even though it's unsure it can actually
> deliver. When the eviction strikes, it just aborts the transaction.
>
> The problem with "just" passing the data to the user is that there may be
> more than one and things get really complicated.
>
> Getting a bigger cache would help, and segregating the storages (smaller
> than 1MB, and bigger than 1MB for example) would too
> --
> Guillaume Quintard
>
>
> On Fri, May 8, 2020 at 11:28 AM Batanun B <batanun at hotmail.com> wrote:
>
> also... could you explain this part for me? "so it had to truncate it and
> throw it away"
> Why does it have to truncate it? Why not avoid caching it, and returning
> it as is, from the backend, untouched?
> ------------------------------
> *From:* Guillaume Quintard <guillaume at varnish-software.com>
> *Sent:* Friday, May 8, 2020 7:34 PM
> *To:* Batanun B <batanun at hotmail.com>
> *Cc:* varnish-misc at varnish-cache.org <varnish-misc at varnish-cache.org>
> *Subject:* Re: Varnish intermittently returns incomplete images
>
> Hi,
>
> Do you have objects that are sensibly smaller that your images in your
> cache?
>
> What you are describing sounds like LRU failure (check nuke_limit in
> "varnishadm param.show"), basically, on a miss, varnish couldn't evict
> enough objects and make room for the new object, so it had to truncate it
> and throw it away.
>
> If that's the issue, you can increase nuke_limit, or get a bigger cache,
> or segregate small and large objects into different storages.
>
> --
> Guillaume Quintard
>
>
> On Fri, May 8, 2020 at 10:14 AM Batanun B <batanun at hotmail.com> wrote:
>
> Our Varnish (test environment) intermittently returns incomplete images.
> So the binary content is not complete. When requesting the image from the
> backend directly (using curl), the complete image is returned every time (I
> tested 1000 times using a script).
>
> This happens intermittently. Sometimes Varnish returns the complete image,
> sometimes half of it, sometimes 20% etc... The incomplete image is returned
> quickly, so I don't think there is a timeout involved (we have not
> configured any specific timeout in varnish).
>
> I see nothing special in varnishlog when this happens. But I don't know
> how to troubleshoot this in a good way. Any suggestions?
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