3 ms·

> Huh? What are you talking about? ZFS has had a write cache from day 1: ZIL. ZFS Intent Log, with SLOG which is a dedicated device. Back in the day we'd use RA

> Huh? What are you talking about? ZFS has had a write cache from day 1: ZIL. ZFS Intent Log, with SLOG which is a dedicated device. Back in the day we'd use RAM based devices, now you can use optane (or any other fast device of your choosing including just a regular old SSD). You are spreading incorrect information, please stop. You know enough to be dangerous, but not enough to actually be right. I've benchmarked ZIL SLOG in all configurations. It doesn't speed up writes generally. It speeds up acknowledgements on sync writes only. But it doesn't act as a write through cache in my readings and in my testing. What it does is it allows a sync acknowledgements to be sent once a sync write is written to the ZIL SLOG device. But it doesn't actually use the ZIL SLOG for reading at all in normal operation, instead it uses the in-memory RAM to cache the actual write to the HDD-based Pool. Thus you are still limited to RAM size when you are doing large writes -- you may get acknowledges quicker on small sync writes, but your RAM limits the size of large write speeds because it will fill up and have to wait for it to be saved to the HDD to accept new data. Here is more data on this: https://www.reddit.com/r/zfs/comments/bjmfnv/comment/em9lh1i/?utm_source=reddit&utm_medium=web2x&context=3 https://www.reddit.com/r/zfs/comments/bjmfnv/comment/em9lh1i...

tw04 5y ago
You've modified this response 4 times and counting to try to twist my response into something it wasn't and isn't so I'll respond once and be done with the conversation. I never said ZIL is a write-back cache and neither did you. Sync writes are the only writes you should be doing if you care about your data in almost all circumstances.
- bhouston 5y ago
  A ZIL SLOG doesn’t even speed up sync writes if your writes do not fit into RAM. It isn’t a write cache either, it is a parallel cache for integrity purposes so sync acknowledgements happen faster. But you are still limited by RAM size for large write speed as I said in my original post.

tw04 5y ago

You've modified this response 4 times and counting to try to twist my response into something it wasn't and isn't so I'll respond once and be done with the conversation. I never said ZIL is a write-back cache and neither did you. Sync writes are the only writes you should be doing if you care about your data in almost all circumstances.

bhouston 5y ago
A ZIL SLOG doesn’t even speed up sync writes if your writes do not fit into RAM. It isn’t a write cache either, it is a parallel cache for integrity purposes so sync acknowledgements happen faster. But you are still limited by RAM size for large write speed as I said in my original post.