Crazy ZFS ZIL options: md(4) umass(4)

[Daniel Kalchev]

In short, don't.

ZIL is not a cache. ZIL is there for you to recover transactions in case of a crash. It is your safety net.
If you don't have low latency SSD, put it on separate spinning disk, but by all means never on RAM and USB only as a last resort, because USB is slow for most uses and it is also extremely unreliable.

Use the RAM for ARC, it will provide more performance.

Daniel

On Feb 15, 2013, at 10:00 AM, grarpamp <grarpamp at gmail.com> wrote:

> I have thousands of small files being written most under 8KiB, they
> either end up being removed or combined in various ways to
> produce a set of data that is stored long term. I also have
> tens of 10-50MiB files similarly, but rarely. It's not fully clear
> to me the benefits of a split ZIL. Some say a split ZIL will ward
> off some fragmentation, which pushing over 80% I'm sure to see
> otherwise. Plus a speed boost if on faster media. And maybe
> even no need to commit some ZIL to disk as small files are
> removed before ZFS decides to aggregate?
> 
> Anyway, use case aside, I can put 1GiB of ram as ZIL.Same for 32GiB USB.
> RAM is obviously fast and power fail prone.
> USB is slow and power safe.
> Either could be mirrored, 2xRAM, 2xUSB.
> 
> - If I lose power on RAM, will the disk still be consistent?
> - What data integrity does ZIL have? None? ZFS dataset's sha256?
> - Any production experiences with this crazy ideas?
> 
> Thx.
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