[moved off osol-discuss] Zhu Han wrote:
Hi, Erik,I've always wondered what the benefit (and difficulty to add to ZFS) would be to having an async write cache for ZFS - that is, ZFS currently buffers async writes in RAM, until it decides to aggregate enough of them to flush to disk. I think it would be interesting to see what would happen if an async SSD cache was available, since the write pattern is "large, streaming", which means that the same devices useful for L2ARC would perform well as an async write cache. In essence, use the async write SSD as an extra-large buffer, in the same way that L2ARC on SSD supplements main memory for the read cache.I think HDD can handle those "large and streaming like" workload quite well. You can observe SSD doesn't play very much better than HDD on streaming write/read. So it could be good enough to flush those async aggreated write data back to HDD instead of bringing another level of write cache.
This is true. SSDs and HDs differ little in their ability to handle raw throughput. However, we often still see problems in ZFS associated with periodic system "pauses" where ZFS effectively monopolizes the HDs to write out it's current buffered I/O. People have been complaining about this for quite awhile. SSDs have a huge advantage where IOPS are concerned, and given that the backing store HDs have to service both read and write requests, they're severely limited on the number of IOPs they can give to incoming data.
You have a good point, but I'd still be curious to see what an async cache would do. After all, that is effectively what the HBA cache is, and we see a significant improvement with it, and not just for sync write.
I also don't know what the threshold is in ZFS for it to consider it time to do a async buffer flush. Is it time based? % of RAM based? Absolute amount? All of that would impact whether an SSD async cache would be useful.
-- Erik Trimble Java System Support Mailstop: usca22-123 Phone: x17195 Santa Clara, CA _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
