HEADS UP: Major CAM performance regression
Tom Evans
tevans.uk at googlemail.com
Fri Feb 13 05:21:21 PST 2009
On Fri, 2009-02-13 at 03:55 -0700, Scott Long wrote:
> All,
>
> A major performance regression was introduced to the CAM subsystem in
> FreeBSD 7.1. The following configurations are known to be affected:
>
> VMWare ESX
> VMWare Fusion
> (using bt or lsilogic controller options)
> HP CISS RAID
> Some MPT-SAS combinations with SATA drives attached
> (Includes Dell SAS5/ir, but not PERC5/PERC6).
>
> Pure SCSI and SAS subsystems likely are NOT affected. Any hardware
> that uses the 'ata' driver is also definitely NOT affected. To
> determine if your installation is affected, run the following command as
> root:
>
> camcontrol tags da0
>
> Substitute 'da0' with another appropriate drive device number, if
> needed. Note that this ONLY AFFECTS 'da' DEVICES. If your disks are
> 'ad' devices, they are NOT affected.
>
> The result from running this command should be an output similar to the
> following:
>
> (pass0:mpt0:0:8:0): device openings: 255
>
> If, instead, it reports a value of '1', you are likely affected. Note
> that it may be normal for USB memory devices to report a low number.
> Also, many legacy SCSI disks, and devices that are not disks, may also
> be expected to report a low number.
>
> The effect of this problem is that only one I/O command will be issued
> to the controller and disk at a time, instead of overlapping multiple
> commands in parallel. This causes significantly higher latency in
> servicing moderate and heavy I/O workloads, leading to very poor
> performance. Performance can be easily compared by downgrading to
> FreeBSD 7.0.
>
> I have committed a fix for this problem for FreeBSD 8-CURRENT as of SVN
> revision 188570. FreeBSD 7-STABLE will be updated with the fix in a few
> days once I've gotten confirmation that the fix works and doesn't cause
> any adverse side-effects. Anyone wanting to help in this validation
> effort should apply the attached patch to their kernel source tree and
> recompile. Please contact me directly by email to report if the problem
> is fixed for you.
>
> If the validation process goes smoothly, I will work with the release
> engineering team to turn this fix into an official errata update for
> FreeBSD 7.1.
>
> Thanks in advance for your help.
>
> Scott
>
Hi Scott
I have one da0 device, a USB attached hard disk:
umass0: <LaCie LaCie Hard Drive USB, class 0/0, rev 2.00/0.00, addr 2>
on uhub6
da0 at umass-sim0 bus 0 target 0 lun 0
da0: <SAMSUNG SP2514N VF10> Fixed Direct Access SCSI-2 device
da0: 40.000MB/s transfers
da0: 238475MB (488397168 512 byte sectors: 255H 63S/T 30401C)
camcontrol shows:
> $ sudo camcontrol tags da0
(pass0:umass-sim0:0:0:0): device openings: 1
Is that to be expected? This is RELENG_7 from October '08:
FreeBSD strangepork.mintel.co.uk 7.1-PRERELEASE FreeBSD 7.1-PRERELEASE
#0: Wed Oct 22 02:25:56 BST 2008
root at sweetpork.pc.mintel.co.uk:/usr/FreeBSD/RELENG_7/obj/usr/FreeBSD/RELENG_7/src/sys/STRANGEPORK i386
Thanks
Tom
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