Skip site navigation (1)Skip section navigation (2)
Date:      Wed, 11 Apr 2018 20:03:07 -0700
From:      "Ronald F. Guilmette" <>
Subject:   Two questions --- SSD block sizes and buffering
Message-ID:  <>

Next in thread | Raw E-Mail | Index | Archive | Help

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
     it won't.

Want to link to this message? Use this URL: <>