So I've been running with sync=disabled on my vsphere NFS datastore. I've been willing to do so because I have a big-ass UPS, and do hourly backups. But, I'm thinking of going to an active/passive connection to my JBOD, using Saso's blog post on zfs zfs-create.blogspot.com. Here's why I think I can't keep using sync=disabled (I would love to have my logic sanity checked.) If you switch manually from host A to B, all is well, since before host A exports the pool, any pending writes will be completed (so even though we lied to vsphere, it's okay.) On the other hand, if host A crashes/hangs and host B takes over, forcibly importing the pool, you could end up with the following scenario: vsphere issues writes for blocks A, B, C, D and E. A and B have been written. C and D were sent to host A, and ACKed, so vsphere thinks all is well. Host A has not yet committed blocks C and D to disk. Host B imports the pool, assumes the virtual IP for the NFS share and vsphere reconnects to the datastore. Since it thinks it has written blocks A-D, it then issues a write for block E. Host B commits that to disk. vsphere thinks blocks A-E were written to disk, when in fact, blocks C and D were not. Silent data corruption, and as far as I can tell, no way to know this happened, so if I ever did have a forced failover, I would have to rollback every single VM to the last known, good snapshot. Anyway, I decided to see what would happen write-wise with an SLOG SSD. I took a samsung 840PRO used for l2arc and made that a log device. I ran crystaldiskmark before and after. Prior to the SLOG, I was getting about 90MB/sec (gigabit enet), which is pretty good. Afterward, it went down to 8MB/sec! I pulled the SSD and plugged it into my windows 7 workstation, formatted it and deleted the partition, which should have TRIM'ed it. I reinserted it as SLOG and re-ran the test. 50MB/sec. Still not great, but this is after all an MLC device, not SLC, and that's probably 'good enough'. Looking at open-zfs.org, it looks like out of illumos, freebsd and ZoL, only freebsd has TRIM now. I don't want to have to re-TRIM the thing every few weeks (or however long it takes). Does over-provisioning help?
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