On Mon, Apr 28, 2014 at 11:07 AM, Karl Denninger <k...@denninger.net> wrote:
> > 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? Only if you have some sort of non-volatile intermediary, or are willing to risk your data integrity. Otherwise, the fsync nature trumps the sequential nature. Cheers, Jeff
