It occurs to me that your filesystems may not be in the same state.

That is, destroy both pools. Recreate them, and run the tests. This will eliminate any possibility of allocation issues.


On 10/27/2011 10:37 AM, weiliam.hong wrote:

Thanks for the replies. In the beginning, I only had SAS drives installed when I observed the behavior, the SATA drives were added later for comparison and troubleshooting.

The slow behavior is observed only after 10-15mins of running dd where the file size is about 15GB, then the throughput drops suddenly from 70 to 50 to 20 to <10MB/s in a matter of seconds and never recovers.

This couldn't be right no matter how look at it.


On 10/27/2011 9:59 PM, Brian Wilson wrote:
On 10/27/11 07:03 AM, Edward Ned Harvey wrote:
From: [mailto:zfs-discuss-] On Behalf Of weiliam.hong

3. All 4 drives are connected to a single HBA, so I assume the mpt_sas
is used. Are SAS and SATA drives handled differently ?
If they're all on the same HBA, they may be all on the same bus. It may be *because* you're mixing SATA and SAS disks on the same bus. I'll suggest separating the tests, don't run them concurrently, and see if there's any

Also, the HBA might have different defaults for SAS vs SATA, look in the HBA
to see if write back / write through are the same...

I don't know if the HBA gives you some way to enable/disable the on-disk
cache, but take a look and see.

Also, maybe the SAS disks are only doing SATA. If the HBA is only able to do SATA, then SAS disks will work, but might not work as optimally as they
would if they were connected to a real SAS HBA.

And one final thing - If you're planning to run ZFS (as I suspect you are, posting on this list running OI) ... It actually works *better* without any
HBA.  *Footnote

*Footnote: ZFS works the worst, if you have ZIL enabled, no log device, and
no HBA.  It's a significant improvement, if you add a battery backed or
nonvolatile HBA with writeback. It's a signfiicant improvement again, if you get rid of the HBA, add a log device. It's a significant improvement
yet again, if you get rid of the HBA and log device, and run with ZIL
disabled (if your work load is compatible with a disabled ZIL.)

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First, ditto everything Edward says above. I'd add that your "dd" test creates a lot of straight sequential IO, not anything that's likely to be random IO. I can't speak to why your SAS might not be performing any better than Edward did, but your SATA's probably screaming on straight sequential IO, where on something more random I would bet they won't perform as well as they do in this test. The tool I've seen used for that sort of testing is iozone - I'm sure there are others as well, and I can't attest what's better or worse.


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