> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of Anatoly
> The speed of send/recv is around 30-60 MBytes/s for initial send and
> 17-25 MBytes/s for incremental. I have seen lots of setups with 1 disk

I suggest watching zpool iostat before, during, and after the send to
/dev/null.  Actually, I take that back - zpool iostat seems to measure
virtual IOPS, as I just did this on my laptop a minute ago, I saw 1.2k ops,
which is at least 5-6x higher than my hard drive can handle, which can only
mean it's reading a lot of previously aggregated small blocks from disk,
which are now sequentially organized on disk.  How do you measure physical
iops?  Is it just regular iostat?  I have seriously put zero effort into
answering this question (sorry.)

I have certainly noticed a delay in the beginning, while the system thinks
about stuff for a little while to kick off an incremental... And it's
acknowledged and normal that incrementals are likely fragmented all over the
place so you could be IOPS limited (hence watching the iostat).

Also, whenever I sit and watch it for long times, I see that it varies
enormously.  For 5 minutes it will be (some speed), and for 5 minutes it
will be 5x higher...

Whatever it is, it's something we likely are all seeing, but probably just
ignoring.  If you can find it in your heart to just ignore it too, then
great, no problem.  ;-)  Otherwise, it's a matter of digging in and
characterizing to learn more about it.

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