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List:       zfs-discuss
Subject:    Re: [zfs-discuss] zvol (slow) vs file (fast) performance snv_130
From:       Richard Elling <richard.elling () gmail ! com>
Date:       2009-12-31 5:49:11
Message-ID: 6C8C138E-C6A6-45C2-8DA4-C77201CACC84 () gmail ! com
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On Dec 30, 2009, at 9:35 PM, Ross Walker wrote:

> On Dec 30, 2009, at 11:55 PM, "Steffen Plotner"  
> <swplotner@amherst.edu> wrote:
>
>> Hello,
>>
>> I was doing performance testing, validating zvol performance in  
>> particularly, and found that zvol write performance to be slow  
>> ~35-44MB/s at 1MB blocksize writes. I then tested the underlying  
>> zfs file system with the same test and got 121MB/s.  Is there any  
>> way to fix this? I really would like to have compatible performance  
>> between the zfs filesystem and the zfs zvols.
>>
> Been there.
>
> ZVOLs were changed a while ago to make each operation synchronous so  
> to provide data consistency in the event of a system crash or power  
> outage, particularly when used as backing stores for iscsitgt or  
> comstar.
>
> While I think that the change is necessary I think they should have  
> made the cooked 'dsk' device node run with caching enabled to  
> provide an alternative for those willing to take the risk, or modify  
> iscsitgt/comstar to issue a sync after every write if write-caching  
> is enabled on the backing device and the user doesn't want to write  
> cache, or advertise WCE on the mode page to the initiators and let  
> them sync.

CR 6794730, need zvol support for DKIOCSETWCE and friends, was
integrated into b113.  Unfortunately, OpenSolaris 2009.06 is b111, where
zvol performance will stink.
http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6794730

This still requires that the client implements WCE (or WCD, as some  
developers
like double negatives :-(. This is optional for Solaris iSCSI clients  
and, IIRC, the
default has changed over time.  See the above CR for more info.
  -- richard

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