[cthalupa]

In a thread that is about the perils of ZFS fragmentation, you are replying to a link saying that a ZIL seriously reduces the risk of fragmentation, and saying that someone worried about fragmentation does not need to use a ZIL.

Why? If there's a legitimate reason, please expand.

[gnoway]

I think he meant that they might not have one.

It's been a while since I looked at using ZFS for anything meaningful, but at the time (~6 years ago), while losing L2ARC was no big deal, losing dedicated ZIL was catastrophic. I think that's still true today.

So you need at least two ZIL devices in a mirror. On top of that, you really need something faster and lower latency for your ZIL vs. the ARC or main pool; people were trying to use SSDs but most commonly-available drives at the time would either degrade or fail in a hurry under load. So the options were RAM-based, e.g. STEC ZeusRAM on the high end, or some sort of PCI-X/PCIe RAM device. The former was not easy or cheap to acquire for testing stuff, and the latter made failover configs impossible.

I think that ZIL is also not soaking up all writes, just most writes meeting a certain criteria. Some just stream through to the pool. So I was always thinking of it as a protection device that also converted random writes to sequential. Some people don't think they need that.

I remember the fragmentation issue being a problem at the time, but also thinking it was probably going to get solved soon because there was so much interest and a whole company behind it. Then Oracle happened. My guess is that if it were still Sun and all the key people were still there, this would be a solved problem right now. As it is, Oracle probably wants you to buy all the extra storage anyway, and would love to offer professional services to get you out of the fragmentation bind you're in.

[leonroy]

A lot has changed. Well - one thing actually: you no longer lose your ZFS pool if your dedicated ZIL log (called a SLOG) dies.

Here is some info on ZIL vs SLOG: http://www.freenas.org/blog/zfs-zil-and-slog-demystified/

[ryao]

Your information is out of date. Losing s SLOG device while the system is running is fine. As far as I know, it has always been fine (unless someone goofed on the initial implementation long before I became involved). All data in ZIL is kept in memory, regardless of whether it is written to the main pool or to a SLOG device. The data is written to the main pool in a permanent fashion with the transaction group commit. If a SLOG device dies, that write out still happens and the pool harmlessly stops using it. If the SLOG device dies on an exported pool, you need to set the zil_replay_disable kernel module parameter to allow the pool to be imported. The same might be true if you reboot (although I doubt it, but need to check).

You can test these things for yourself.

[sneak]

> Anyone using ZFS in a serious capacity would have both dedicated ARC and ZIL.

I contend that most people using ZFS in a serious capacity do not have a dedicated ZIL.