There's a lot of discussion on the sysctl settings for vfs
disk
cache/buffers,
but very little consolidated or comprehensive explanations.
I have a NAS box with 1GB of memory, but rarely see over
~40MB of
utilization
during file transfers. When writing a large file to the NAS,
I see the
transfer
rate throttle and pause in regular intervals. This indicates
to me the dirty
buffers are flushing and writing to the disk, and the file
transfer is
interrupted
during the disk write process.
Therefore, I would like to (1) increase the memory
utilization, so that a
large
file write (500MB) will buffer entirely to memory for
fastest transfer
speeds,
and (2) then take as long as needed to write the dirty
buffers to the disk,
after
the transfer is concluded.
For part (1), I looked at the following sysctl settings, but
I don't fully
understand the interactions:
vfs.maxbufspace
vfs.lobufspace
vfs.hibufspace
vfs.hidirtybuffers
vfs.hirunningspace
also /boot/loader.conf kern.nbuf
I understand some rules: hirunningspace should be between
1MB to 4MB.
lobufspace should be 25% to 75% of hibufspace.
What I don't understand is the relationship between
maxbufspace and
hibufspace,
and what are good values.
I also don't understand hidirtybuffers.
For part (2), I need to slow down the occurrence of sync()
or fsync().
Ideally,
a sync would not occur for 300 seconds, which is about 10
times as long as
(apparently) the default setting. (obviously, if the buffer
space is full, a
sync would need to occur to make more room.) I've seen
references of a
daemon
to periodically run a sync, or is it controlled by init?
Yes, this increases vulnerability when the data is in buffer
before
committed
to disk, but in my application, this is reasonable.
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