what is causing the reads in this configuration?

i have a setup here where we use a Solaris 10 x86, zpool for oracle12c-redo-log using mirrored disks and mirrored ZIL. logbias="latency", recordsize=128: pool: xxxxxx01_redo00 state: ONLINE scan: resilvered 25.1G in 0h1m with 0 errors on Thu Jan 22 15:24:41 2015 config: NAME STATE READ WRITE CKSUM xxxxxx01_redo00 ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 c5t60000970000295700789533031393742d0 ONLINE 0 0 0 c5t60000970000295700860533033424537d0 ONLINE 0 0 0 logs mirror-1 ONLINE 0 0 0 c5t60000970000295700789533032304344d0 ONLINE 0 0 0 c5t60000970000295700860533033423844d0 ONLINE 0 0 0 When i now check what the lgwr process does in terms of IO it is after all mostly writes to the redo-logs (when not switching logs). Here is what i do not get: The lgwr-process does writes in different sizes (which is okay) and i expect to see them actually happening on the ZIL-Devices until the txg_timeout is getting hit, then i expect to see the flush to the data-disks. Instead the picture is different. Here is the iostat: 0.0 133.3 0.0 4067.1 0.0 0.1 0.0 0.5 0 7 c5t60000970000295700789533032304344d0 0.0 133.8 0.0 4077.1 0.0 0.3 0.0 2.2 0 29 c5t60000970000295700860533033423844d0 44.6 0.0 5708.7 0.0 0.0 0.1 0.0 2.0 0 9 c5t60000970000295700789533031393742d0 33.6 0.0 4297.6 0.0 0.0 0.1 0.0 3.3 0 11 c5t60000970000295700860533033424537d0 as you can see, the first two disks are the ZILs, the other two are the actual disks with the Data. checking the lgwr from syscall-level with: ./rwsnoop -p UID PID CMD D BYTES FILE 708 16559 oracle W 512 /export/zones/xxxxxx01/root/opt/app/oracle/redo00/XXXX0/XXXX0_redo07.rdo 708 16559 oracle W 58368 /export/zones/xxxxxx01/root/opt/app/oracle/redo00/XXXX0/XXXX0_redo07.rdo 708 16559 oracle W 2560 /export/zones/xxxxxx01/root/opt/app/oracle/redo00/XXXX0/XXXX0_redo07.rdo 708 16559 oracle W 26112 /export/zones/xxxxxx01/root/opt/app/oracle/redo00/XXXX0/XXXX0_redo07.rdo 708 16559 oracle W 69632 /export/zones/xxxxxx01/root/opt/app/oracle/redo00/XXXX0/XXXX0_redo07.rdo here we can see that the lgwr-process does its io with different blocksizes. now from a devices-pov (of the Data-Devices not the ZIL) ./iosnoop -d UID PID D BLOCK SIZE COMM PATHNAME 708 16559 R 29073760 131072 oracle 708 16559 R 29074016 131072 oracle 708 16559 R 29073504 131072 oracle 708 16559 R 29073760 131072 oracle 708 16559 R 29073504 131072 oracle 708 16559 R 29073760 131072 oracle 708 16559 R 29074784 131072 oracle 708 16559 R 29074784 131072 oracle 708 16559 R 29074784 131072 oracle 708 16559 R 29074272 131072 oracle 708 16559 R 29074272 131072 oracle this seem to be clearly zfs 128k reads but why so they happen? is it just because the recordsize=128 and zfs collecting the writes and because of the c-o-w it needs to read? I would appreciate to dig deeper with you all or just be enlightened ;-) Cheers B.