A persistent message

This message is about -spersistent and why you should not use it, even though it is still present in Varnish 4.x.

TL;DR:

Under narrow and ill defined circumstances, -spersistent works well, but in general it is more trouble than it is worth for you to run it, and we don’t presently have the development resources to fix that.

If you think you have these circumstances, you need to specify

-sdeprecated_persistent

in order to use it.

The long story

When we added -spersistent, to Varnish, it was in response to, and sponsored by a specific set of customers who really wanted this.

A persistent storage module is an entirely different kettle of vax than a non-persistent module, because of all the ugly consistency issues it raises.

Let me give you an example.

Imagine a cluster of some Varnish servers on which bans are used.

Without persistent storage, if one of them goes down and comes back up, all the old cached objects are gone, and so are, by definition all the banned objects.

With persistent storage, we not only have to store the still live bans with the cached objects, and keep the two painfully in sync, so the bans gets revived with the objects, we also have to worry about missing bans during the downtime, since those might ban objects we will recover on startup.

Ouch: Straight into database/filesystem consistency territory.

But we knew that, and I thought I had a good strategy to deal with this.

And in a sense I did.

Varnish has the advantage over databases and filesystems that we can actually loose objects without it being a catastrophe. It would be better if we didn’t, but we can simply ditch stuff which doesn’t look consistent and we’ll be safe.

The strategy was to do a “Log Structured Filesystem”, a once promising concept which soon proved very troublesome to implement well.

Interestingly, today the ARM chip in your SSD most likely implements a LFS for wear-levelling, but with a vastly reduced feature set: All “files” are one sector long, filenames are integers and there are no subdirectories or rename operations. On the other hand, there is extra book-keeping about the state of the flash array.

A LFS consists of two major components: The bit that reads and writes, which is pretty trivial, and the bit which makes space available which isn’t.

Initially we didn’t even do the second part, because in varnish objects expire, and provided they do so fast enough, the space will magically make itself available. This worked well enough for our initial users, and they only used bans sporadically so that was cool too.

In other words, a classic 20% effort, 80% benefit.

Unfortunately we have not been able to find time and money for the other 80% effort which gives the last 20% benefit, and therefor -spersistent has ended up in limbo.

Today we decided to officially deprecate -spersistent, and start warning people against using it, but we will leave it in the source code for now, in order to keep the interfaces necessary for a persistent storage working, in the hope that we will get to use them again later.

So you can still use persistent storage, if you really want to, and if you know what you’re doing, by using:

-sdeprecated_persistent

You’ve been warned.

Poul-Henning, 2014-05-26