Relax zfs_vnops_read_chunk_size limitations

It makes no sense to limit read size below the block size, since
DMU will any way consume resources for the whole block, while the
current zfs_vnops_read_chunk_size is only 1MB, which is smaller
that maximum block size of 16MB.

Also while there, we don't need to align reads to the chunk size,
but only to a block size, which is smaller and so more forgiving.

My profiles show ~4% of CPU time saving when reading 16MB blocks.

Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.

Author: amotin

module/zfs/zfs_vnops.c

@@ -401,7 +401,8 @@ zfs_read(struct znode *zp, zfs_uio_t *uio, int ioflag, cred_t *cr)
 #if defined(__linux__)
 	ssize_t start_offset = zfs_uio_offset(uio);
 #endif
-	ssize_t chunk_size = zfs_vnops_read_chunk_size;
+	uint_t blksz = zp->z_blksz;
+	ssize_t chunk_size;
 	ssize_t n = MIN(zfs_uio_resid(uio), zp->z_size - zfs_uio_offset(uio));
 	ssize_t start_resid = n;
 	ssize_t dio_remaining_resid = 0;
@@ -432,11 +433,14 @@ zfs_read(struct znode *zp, zfs_uio_t *uio, int ioflag, cred_t *cr)
 		if (dio_remaining_resid != 0)
 			n -= dio_remaining_resid;
 		dflags |= DMU_DIRECTIO;
+	} else {
+		chunk_size = MIN(MAX(zfs_vnops_read_chunk_size, blksz),
+		    DMU_MAX_ACCESS / 2);
 	}
 
 	while (n > 0) {
 		ssize_t nbytes = MIN(n, chunk_size -
-		    P2PHASE(zfs_uio_offset(uio), chunk_size));
+		    P2PHASE(zfs_uio_offset(uio), blksz));
 #ifdef UIO_NOCOPY
 		if (zfs_uio_segflg(uio) == UIO_NOCOPY)
 			error = mappedread_sf(zp, nbytes, uio);