svn commit: r344188 - in head: lib/libc/sys sys/vm
ian at freebsd.org
Sat Feb 16 17:08:37 UTC 2019
On Sun, 2019-02-17 at 02:58 +1100, Bruce Evans wrote:
> Slowness is relative. In FreeBSD-1, floppy disk devices were still in
> use and were especially slow. Now hard disks are slow relative to fast
> SSDs. But the number of buffers was unchanged. It is still essentially
> unchanged except for vn pager pbufs. The hard disks can complete 128
> i/o's for a full queue much faster than a floppy disk, so the relative
> slowness might be similar, but now there are more subsystems and some
> systems have many more disks.
The modern replacement for a floppy disk in this regard is an sdcard.
When doing large numbers of random writes, such as untarring a snapshot
of rootfs to a ufs filesystem on sdcard, gstat will show ms/w values
anywhere from 30,000 to 90,000 depending on the card. It stays that way
throughout the operation, and IO to all other disks on the system
essentially comes to a standstill. This is true whether the card is in
a native sdhci controller or a usb-attached multiformat reader/burner.
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