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kern/52273: filesystems don't take advantage of VFS read-ahead

>Number:         52273
>Category:       kern
>Synopsis:       filesystems don't take advantage of VFS read-ahead
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jun 05 03:25:00 +0000 2017
>Originator:     coypu
>Release:        NetBSD 7.99.75
NetBSD loggy 7.99.75 NetBSD 7.99.75 (GENERIC) #62: Sun Jun  4 02:48:22 IDT 2017  fly@loggy:/home/fly/obj/sys/arch/amd64/compile/GENERIC amd64

I've tested with LFS but I believe FFS is the same.

# dtrace -x nolibs -n 'fbt::breadn:entry /arg5 != 0/ { printf("%d \n", arg5); }'

Will print the argument whenever the nrablks argument is non-zero.

It is almost always 0, but sometimes 1.

1178 #define ULFS_DIRRABLKS 0
1179 int ulfs_dirrablks = ULFS_DIRRABLKS;

Most likely this code will be beneficial to use, but that claim should probably be checked.
Filed as a bug for increased awareness.

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