Re: HEADS UP: Major CAM performance regression

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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.

Any estimate on when this will be MFC'ed down to RELENG_7 yet?

//Svein

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