Date: Sat, 15 Aug 2009 14:15:07 GMT From: Oles Hnatkevych <don_oles@able.com.ua> To: freebsd-gnats-submit@FreeBSD.org Subject: kern/137797: gmirror split does not improve performance Message-ID: <200908151415.n7FEF7Yk037395@www.freebsd.org> Resent-Message-ID: <200908151420.n7FEK64d025855@freefall.freebsd.org>
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>Number: 137797
>Category: kern
>Synopsis: gmirror split does not improve performance
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Sat Aug 15 14:20:06 UTC 2009
>Closed-Date:
>Last-Modified:
>Originator: Oles Hnatkevych
>Release: FreeBSD 7.2
>Organization:
>Environment:
>Description:
Mirror is created with gmirror on ad8 and ad10
ad8: 476940MB <Seagate ST3500630NS 3.AEK> at ata4-master SATA150
ad10: 476940MB <Seagate ST3500630NS 3.AEK> at ata5-master SATA150
Geom name: ar1
State: COMPLETE
Components: 2
Balance: split
Slice: 4096
Flags: NONE
GenID: 0
SyncID: 1
mirror/ar1 COMPLETE ad8
ad10
The split algorithm does not improve any performance. Even simple dd with block size 1M has the same speed as a single drive, while two dd's on singe drive run simultaneously proove that SATA throughput is not a bottleneck.
>How-To-Repeat:
gmirror label ar1 ad8 ad10
dd if=/dev/ad8 of=/dev/null bs=1M count=500
dd if=/dev/mirror/ar1 of=/dev/null bs=1M count=500
both give the same speed (around 80MB/s for Seagate 500GB SATA drives)
but
dd if=/dev/ad8 of=/dev/null bs=1M count=500 & dd if=/dev/ad10 of=/dev/null bs=1M count=500
show that in parallel they do provide data faster.
BTW, the performance of a RAID1 created with the atacontrol is a bit better then gmirror.
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted:
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