Hi Scott,
This seems to significantly help for reading through large files (argus
/ netflow files in my case) on my zfs server. Doing some quick tests
and setting the size to 2 makes a difference from the default. In your
case, did you find some optimal settings ? Are there some tradeoffs /
caveats to setting this value to a non default value ?
---Mike
On 5/22/2013 4:44 AM, Scott Long wrote:
> Author: scottl
> Date: Wed May 22 08:44:21 2013
> New Revision: 250906
> URL: http://svnweb.freebsd.org/changeset/base/250906
>
> Log:
> MFC r250327
>
> Add a sysctl vfs.read_min to complement the exiting vfs.read_max. It
> defaults to 1, meaning that it's off.
>
> When read-ahead is enabled on a file, the vfs cluster code deliberately
> breaks a read into 2 I/O transactions; one to satisfy the actual read,
> and one to perform read-ahead. This makes sense in low-latency
> circumstances, but often produces unbalanced i/o transactions that
> penalize disks. By setting vfs.read_min, we can tell the algorithm to
> fetch a larger transaction that what we asked for, achieving the same
> effect as the read-ahead but without the doubled, unbalanced transaction
> and the slightly lower latency. This significantly helps our workloads
> with video streaming.
>
> Submitted by: emax
> Reviewed by: kib
> Obtained from: Netflix
>
> Modified:
> stable/9/sys/kern/vfs_cluster.c
> Directory Properties:
> stable/9/sys/ (props changed)
>
> Modified: stable/9/sys/kern/vfs_cluster.c
> ==============================================================================
> --- stable/9/sys/kern/vfs_cluster.c Wed May 22 07:52:41 2013
> (r250905)
> +++ stable/9/sys/kern/vfs_cluster.c Wed May 22 08:44:21 2013
> (r250906)
> @@ -75,6 +75,10 @@ static int read_max = 64;
> SYSCTL_INT(_vfs, OID_AUTO, read_max, CTLFLAG_RW, &read_max, 0,
> "Cluster read-ahead max block count");
>
> +static int read_min = 1;
> +SYSCTL_INT(_vfs, OID_AUTO, read_min, CTLFLAG_RW, &read_min, 0,
> + "Cluster read min block count");
> +
> /* Page expended to mark partially backed buffers */
> extern vm_page_t bogus_page;
>
> @@ -169,6 +173,7 @@ cluster_read(vp, filesize, lblkno, size,
> } else {
> off_t firstread = bp->b_offset;
> int nblks;
> + long minread;
>
> KASSERT(bp->b_offset != NOOFFSET,
> ("cluster_read: no buffer offset"));
> @@ -176,6 +181,13 @@ cluster_read(vp, filesize, lblkno, size,
> ncontig = 0;
>
> /*
> + * Adjust totread if needed
> + */
> + minread = read_min * size;
> + if (minread > totread)
> + totread = minread;
> +
> + /*
> * Compute the total number of blocks that we should read
> * synchronously.
> */
> _______________________________________________
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> http://lists.freebsd.org/mailman/listinfo/svn-src-stable-9
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>
>
--
-------------------
Mike Tancsa, tel +1 519 651 3400
Sentex Communications, m...@sentex.net
Providing Internet services since 1994 www.sentex.net
Cambridge, Ontario Canada http://www.tancsa.com/
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