zfs code - Jan 2008 - vdev_cache.c: documented 128k readahead, looks like only 64k

In vdev_cache.c:

76 int zfs_vdev_cache_max = 1<<14;
77 int zfs_vdev_cache_size = 10ULL << 20;
78 int zfs_vdev_cache_bshift = 16;
79 
80 #define	VCBS (1 << zfs_vdev_cache_bshift)

So VCBS is 64k, but:

39  * the result.  In the best case, this can turn 256 back-to-back 512-byte
40  * reads into a single 128k read followed by 255 cache hits; this reduces

Says the readahead is 128k.

IIRC, ZFS is documented to do 128k readahead in higher level documentation as
well, but it looks like it''s doing 64k readahead to me.
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