On 4/28/2014 1:04 PM, Heikki Linnakangas wrote:
On 04/28/2014 06:47 PM, Karl Denninger wrote:
What I am curious about, however, is the xlog -- that appears to suffer
pretty badly from 128k record size, although it compresses even
more-materially; 1.94x (!)

The files in the xlog directory are large (16MB each) and thus "first
blush" would be that having a larger record size for that storage area
would help.  It appears that instead it hurts.

The WAL is fsync'd frequently. My guess is that that causes a lot of extra work to repeatedly recompress the same data, or something like that.

- Heikki

It shouldn't as ZFS re-writes on change, and what's showing up is not high I/O *count* but rather percentage-busy, which implies lots of head movement (that is, lots of sub-allocation unit writes.)

Isn't WAL essentially sequential writes during normal operation?

--
-- Karl
k...@denninger.net


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