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by magicalhippo 630 days ago
I've done some testing with using ZFS on top of LVM with dm-writecache.

Worked well enough on the small scale, but sadly haven't had the time or hardware to test it in a more production-like environment.

Also, it was starting to feel a bit like a Jenga tower, increasing the chances of bugs and other weird issues to strike.
1 comments
kaliszad 630 days ago
I wouldn't recommend combining those two. It's only begging for problems.
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magicalhippo 630 days ago
Yeah that's my worry. Still, got 6x old 3TB disks that still work and a few spare NVMEs, so would be fun to try it for teh lulz.
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kaliszad 629 days ago
Rather build a L2ARC or metadata special device hybrid setup using ZFS or skip ZFS and go for lvmcache/ mdadm style RAID with XFS or something.
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magicalhippo 629 days ago
But L2ARC only helps read speed. The idea with dm-writecache is to improve write speed.

I started thinking about this when considering using a SAN for the disks, so that write speed was limited by the 10GbE network I had. A local NVMe could then absorb write bursts, maintaining performance.

That said, it's not something I'd want to use in production that's for sure.

There was some work being done on writeback caching for ZFS[1], sadly it seems to have remained closed-source.

[1]: https://openzfs.org/wiki/Writeback_Cache

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kaliszad 629 days ago
That's what SLOG is for if the writes are synchronous or if you have many small files/ want to optimize the metadata speed look at the metadata special device, which can also store small files of configurable size.

ZFS of course has its limits too. But in my experience I feel much more confident (re)configuring it. You can tune the real world performance well enough especially if you can utilize some of the advanced features of ZFS like snapshots/ bookmarks + zfs-send/recv for backups. Because with LVM/ XFS you can certainly hack something together which will work pretty reliably too but with ZFS it's all integrated and well tested (because it is a common use case).

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magicalhippo 629 days ago
As I mentioned in my other[1] post, the SLOG isn't really a write-back cache. My workloads are mostly async, so SLOG wouldn't help unless I force sync=always which isn't great either.

I love ZFS overall, it's been rock solid for me in the almost 15 years I've used it. This is just that one area where I feel could do with some improvements.

[1]: https://news.ycombinator.com/item?id=41670945

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