Two questions --- SSD block sizes and buffering
Ronald F. Guilmette
rfg at tristatelogic.com
Fri Apr 13 08:45:03 UTC 2018
In message <1cc45af6-bd4b-3854-4d37-8e9343786ce6 at qeng-ho.org>,
Arthur Chance <freebsd at qeng-ho.org> wrote:
>... man newfs says
> The fragment size of the file system in bytes. It must be a
> power of two ranging in value between blocksize/8 and blocksize.
> The default is 4096 bytes.
>so it's been fixed for 4k disks.
Swell. But that may not really do much (in the way of improving performance)
if the underlying mass storage device has a "native" block size of, say,
128 KiB. And as I noted, it is my understanding that essentially all
flash-based mass storage devices -do- have a native block size of 128 KiB,
or perhaps even larger.
So.... When installing FreeBSD onto an SSD, would one be well advised
to perform all newfs operations with an explicit "-f 131072" option?
More to the point, has anyone run any benchmarks to see if that would
either help or hurt anything?
(There -are- a number of "cloud" providers these days who offer FreeBSD
together with SSD mass storage, so it isn't as if my question is merely
academic. Modern SSDs have certainly been tuned up in ways so that
they can gracefully accomodate almost whatever is thrown at them, even
while maintaining decent performance, but that's not to say that even
a little better performance couldn't be eeked out of them, with a little
help from an accomodating OS/filesystem.)
(If the answer to that question is "no", I'd quite frankly be shocked.
I've come to expect outstanding quality from FreeBSD generally, and
that certainly includes all performance aspects.)
>>> 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?
>> Probably not. The actual write operationg are being issued
>> somewhere "down the line" through the file system driver
>> down to the disk driver. Even a "sync" command issued by
>> the OS will not _immediately_ cause the drive to act.
>write(2) calls, which underlie stdio, add the data to the disk block
>image in the VM cache. Unless your machine is under extreme memory
>pressure or you call fsync or sync the buffers eventually get written
>out by a kernel task. See man syncer for details.
Thank you. That certanly answers my question, and I am greatly relieved
to know that I will probably not be materially shorting the life of my
hardrive by doing the exact sort of silly thing I am doing.
P.S. There actually -is- a good reason for me to be explicitly setting
stdio line buffering on my stdout stream in my program. You see, my
program actually spaws a number of child processes and it is those
children who are actually writing to stdout. I've found that unless
line buffering (for stdout) is set for all of them, sometimes bits and
pieces of lines from the different children can get mangled together
in the output, creating one big uselessly mangled blob.
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