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Comment by ara24 | Hacker News Reader

This is great! In my case, only reason for choosing Redis over Memcached was persisting to disk. With memcached being multi-threaded compared to Redis being single-threaded, I see a big win in simple use-cases for Memcached.

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The way i understand it, it is not crash safe. It can now persist data in some cases.

However redis is crash safe to about 1 second before crash or so, if not even better.

With fsync_always it's crash safe for all but the most pathological scenarios.

How? It's persisting to a RAM disk. It only properly cleans up when sent a specific shutdown signal.

Sorry fsync_always is a redis setting, not memcached. The fsync is to a real disk.

The parent poster's comment about redis being crash safe to 1s is just the default.

But it's performance is way worst, almost voiding the case for choosing Redis.

I have not found this to be true. I suppose it depends on your use case and deployment.

I do wonder what happens when a process writes to a memory mapped file, but crashes before the page is synced to disk. Does the write disappear?

If the OS is still alive, everything is ok (but the OS may sync different pages at different times and in any order if you don't fsync specific ranges with special calls). If the whole machine crashes (power issue, kernel panic, ...), what you find on disk can be a mess.

> what you find on disk can be a mess

Is this "can be" determined by the backing filesystem? E.g., would you expect to get a clean result from a log-structured or copy-on-write filesystem?

I prefer to architect to use redis in an ephemeral way. Redis isn't exactly safe in the same way postgresql was up until very recently on newer linux. The semantics of fsync on Linux have been esoteric and poorly understood in the error cases. I would try to cause fsync to fail in another process, while memcached is shutting down and immediately recover. I wonder if the authors checked this scenario. Redis kindof does the right thing and will eventually put the right thing on disk but why do it?

What issue caused postgres to not be safe recently?

Fsync doesn't work the way you think it does

https://www.google.com/url?sa=t&source=web&rct=j&url=https:/...

PostgreSQL › wiki › Fsync_Errors Web results Fsync Errors - PostgreSQL wiki

LWN.net › Articles PostgreSQL's fsync() surprise [LWN.net]

Note that that only was an issue in cases the storage system itself was failing (i.e. IO errors were generated). In contrast to protection against power failures etc, which was/is working correctly.

Uhm the first sentence is accurate. The second is not

I'm fairly sure I was accurate. What exactly are you referring to?

Sharding is a fairly easy path to scale redis writes out across multiple processes/cores.

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