Two questions --- SSD block sizes and buffering
Ronald F. Guilmette
rfg at tristatelogic.com
Thu Apr 12 03:09:43 UTC 2018
Two rather simple questions:
1) I don't know much, generally speaking, but I have certainly read that
the underyling hardware of flash memory products (which I guess
includes both USB sticks and also SSDs) all effectively have
"physical" block sizes on the order of 128 KiB.
So anyway, I'm just curious to know to what extent, if any, FreeBSD,
when running with, on, or from any such (flash-memory based) mass
storage device does things specifically with these larger physical
block sizes (of flash memory) in mind.
Does FreeBSD automagically sense that it is dealing with an SSD, and
does it then adjust the way it operates on the relevant filesystem(s)
accordingly, for maximal performance? Or must the system administrator
tweek something explicitly (e.g. tunefs parameters, or perhaps newfs
parameters) in order to get optimal performance on/with an SSD?
2) I have written some modest C programs that output lots and lots of
very short little tidbits of data, one per line. In some cases these
programs output millions of lines.
For reasons that I can explain, these programs explicitly call setvbuf()
on stdout during startup, and they set the buffering type for stdout
to _IOLBF (i.e. line buffering).
My question is just this: Assme that one of these programs is called
"xyz". Now, if I run the program thusly:
xyz > xyz.output
i.e. so that stdout is redirected to a file, will there be one actual
write to disk for each and every line that is written to stdout by xyz?
In other words, will my act of explicitly setting line buffering (for
stdout) in a case like this cause the xyz program to beat the living
hell out of my disk drive?
I hope not, but I'd like to know if it will, or know why it won't, if
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